Chapter VI: GYMNOGENS { Cotyledons, two or (2)

A flower is said to be hermaphrodite or monoclinous when, as in the elm, both stamens and pistils are present in the same blossom. With insect-fertilized flowers this is mostly the case, though there are some exceptions, such as the cucumber and begonia, which are unisexual or diclinous, stamens and pistils being produced in separate blossoms. The diclinous condition is exceedingly common in the wind-fertilized class. The staminate or male, and the pistillate or female, flowers are sometimes found growing on the same individual plant, which is then termed monœcious, as in the oak, hazel, birch, pine, etc. The poplar, willow, yew, juniper, nettle, and dog’s-mercury, on the other hand, are diœcious; their staminate and pistillate flowers grow on separate plants. This separation of the sexes renders self-fertilization impossible, and secures whatever benefit may arise from the physiological division of labor. Anemophilous species in general show a marked tendency in the direction of separation. Self-fertilization may be prevented in monoclinous flowers by the stamens and stigmas maturing at different times. This arrangement, known as dichogamy, occurs in both insect and wind-fertilized blossoms, but while the former usually have the stamens in advance of the stigmas, in the latter the reverse order is much more frequent. There are thus two kinds of dichogamy--protandrous, when the stamens are in advance; protogynous, if the pistils are first developed. Protogyny is characteristic of wind-fertilized flowers, and may be easily observed in the rush and plantain. In the first or female stage of the flower of the rush, the thread-like stigma protrudes from the top of the still unopened perianth, while the stamens, as yet immature, are completely concealed. In the second stage, the pollinated stigmas have begun to shrivel, the perianth has now spread out, disclosing the six stamens which are ready to discharge their pollen. The same two stages are equally apparent in plantago. All our readers must be familiar with the black heads of this plant, which are to be seen in every pasture, bending and waving in the wind. In the first stage, the head appears black, but on looking into it we see projecting from each little unopened floret a white thread-like stigma. Later on, the lower part of the spike or head is seen to be encircled by a wreath of tiny white bodies, and closer inspection shows that these are the stamens, four of which project like little banners from each of the newly opened florets. The protogynous character belongs in the bur-reed to the plant itself rather than the individual flowers. Its pistillate flowers, which are lowermost, expand first; only when their stigmas have withered do the male florets higher up begin discharging their pollen. In this case, it is evident that the flowers on any plant must be fertilized with pollen from another in more advanced condition. A social habit is highly characteristic of wind-fertilized plants--pines, grasses, sedges, nettles, etc., usually grow together in considerable numbers. Entomophilous plants have a much more sporadic character, and admit of a greater degree of isolation; their guests, doubtless, maintain the necessary communication between members of the species. This social habit partly explains the tendency toward the diœcious condition, for a complete separation of the sexes is hardly possible, except in plants of social habit. From the gymnosperms, the oldest flowering plants, being all wind-fertilized, it has been inferred that such must also have been the case with the primitive angiosperms. It is not certain, however, that any of their representatives remain, for many of our existing wind-fertilized flowers appear to be merely degraded forms. Anemophilous species appear in families, the rest of which are highly specialized in relation to insects. Some species of plantago are adapted to insects; others, as we have seen, to the wind. Most of the sub-classes with incomplete flowers, from which so many of our examples are taken, also exhibit striking marks of degeneration, and the same may be said of the grasses and other anemophilous monocotyledons. We also find some flowers in an intermediate condition, such as the vine and certain willows, which secrete honey and are visited by insects. Facts of this description are held by some to show that all existing anemophilous species, with the exception of the gymnosperms, are descended from bright-colored, insect-fertilized ancestors.

Wind-fertilization has, in some instances, been rendered highly efficient, but in any case it is far from economical, for the vast amount of pollen miscarried represents an enormous loss to plants; neither does this method admit of the same certainty and precision as the other. A wind-fertilized bears to an insect-fertilized blossom very much the relation which an æolian harp bears to a pianoforte.

MOVEMENTS OF PLANTS
--DAVID ROBERTSON

Scarcely any one can have failed to notice that many plants close their flowers when evening approaches, others again at various periods of the day, while some close their flowers when the sky is overcast; foliage leaves also are in many cases subject to periodic movements.

The movements of different plants are dependent on various causes.

Some of these movements are solely mechanical, and caused by the tissues being affected, owing to the condition of the surrounding air and to varying states of turgidity and exhaustion.

Other movements are apparently due to physical causes, but can not be fully explained by attributing them to these causes.

Movements in plants also depend upon the contractile quality of the protoplasm in the cells, and on the passage of the protoplasm from cell to cell. The property of the protoplasm gives rise to movements caused by the plant itself, which are not at least directly due to any external exciting cause. These movements can be compared with the movements of the lower animals, and to the ciliary motion found in certain tissues belonging to the most highly organized animals.

The periodic movements, such as the “waking” and “sleeping” condition of leaves, the closing of flowers, etc., are manifested only when the organs are fully matured, and when the peculiarity of their internal structure which gives rise to the phenomena of periodic movements is fully developed.

These movements are to be carefully distinguished from those due to unequal growth, such as movements of nutation. In this case there is no special structure upon which the movements depend.

The bursting of seed-vessels, anthers, etc., is due partly to the fact that the condition of the tissues, as regards the amount of liquid they contain from their possessing unequal power of imbibing moisture, is not equally elastic. For this reason, when the less elastic portions of tissue are subjected to strain they are torn apart or bent in various ways, owing to unequal contractions and expansions, caused by an access or withdrawal of moisture.

These cases can scarcely be regarded as vital phenomena, but should rather come under the category of what is in ordinary language named “warping.” They are simply caused by particular modes of the destruction of dead tissue due to conditions brought about by variations in the structure of the tissues in question.

Movements in plants which take place periodically, such as sleeping and waking, or those movements that take place when they are touched or otherwise affected by certain kinds of exciting stimulus, can not be attributed to mechanical causes. The slightest mechanical stimulus on the sensitive plant Mimosa pudica causes the leaflets to fold together. Such movements are not proportional to the external stimulus, but depend on the internal structure of the plant.

To this class of movements have been added the very remarkable movements which give rise to the twining condition of certain stems.

Another class of movements may be mentioned, viz., movements of the protoplasm in cells, or movements of free bodies, such as zoospores (Greek, _zoon_, animal, and _spora_, seed), antherozoids (Greek, _anthos_, flower; _zoon_, animal; _eidos_, form), and sometimes even perfect individuals, such as Desmediæ, etc., which may have the power of temporary or permanent locomotion.

The rotation of the protoplasm of cells is attributed to causes similar to those which produce locomotion in the simpler plants, and these movements are strikingly like some of the movements of the protozoa in the animal kingdom. The movements of the products of cell contents having no cell-wall, such as zoospores and antherozoids, are generally caused by the rapid movement of cilia (plural of the Latin word _cilium_, an eyelid) or small filaments which cover the surface. The locomotion of certain plants, such as Diatomaceæ, is apparently not due to cilia.

Sensitive plants, such as the Mimosa pudica, are strongly affected by any mechanical stimulus, and thus afford us examples of the phenomenon named “irritability.”

The sleep of plants is most probably a case of irritability, and differs only in degree, not in kind.

Sensitiveness in plants is affected both by light and heat. It has been experimentally proved that sensitive plants, if kept in the dark, lose their sensibility after a period of seven days, and actually die after twelve days.

We know that white light is composed of light of different colors. Light is propagated in waves, and each color is distinguished by having a different wave-length from that of any other color. Red light differs, for example, from violet light in the length of its waves, and violet light differs from blue, etc.

It is, therefore, not surprising to find that the different colored rays are capable of producing different effects. It has been ascertained that under the influence of green light sensitive plants die after sixteen days’ exposure, though they retain their sensibility for twelve days.

When the plants were exposed to violet and blue light, their growth completely ceased. They, however, retained their vitality as well as their sensibility for three months. The effect of heat on sensitive plants has also been ascertained.

The sensitiveness and periodical movements of Mimosa do not begin till the temperature of the surrounding air exceeds 15° C. The periodical movements of the lateral leaflets of the Indian telegraph plant (Desmodium gyrans) can only occur when the temperature exceeds 22° C.

When the temperature of the air is 40° C., the leaves become stiff in less than an hour, and at 48° C. to 50° C. rigidity takes place within a few minutes; but when the temperature falls, the sensitiveness may again be manifested.

A temperature of 52° C. not only causes loss of permanent motion, but also the death of the plant.

The mechanism to which the periodic movements of plants is due is not by any means fully known.

The particular circumstances which regulate the turgidity have not been, so far, determined with precision.

It has, however, been clearly ascertained that this turgid state is associated with the passage of fine threads or filaments of protoplasm from one cell to another, and at the same time with an accumulation of a soluble chemical compound named glucose, a kind of sugar, in fact. This substance possesses great osmotic power; that is, it can pass very rapidly through the flexible cell-walls of the pulvinus forming the so-called springs. These movements are, therefore, closely connected with the rapid absorption and expulsion of liquid.

Contrary to the habit of most plants, the sensitive plant raises its leaves at night and closes them by day.

The most usual kind of movement in these plants is that in which the leaves as well as the floral envelopes assume the position they occupied before the buds opened.

Compound leaves, such as the leaves of the Leguminosæ, or pea-family, exhibit a simple or compound movement.

The leaves of the bean fold upward, those of the Lupinus fold downward. In Tamarinds the leaves fold to the side. In some other plants the common petiole of the compound leaves become raised or depressed, while the leaflets turn downward or sidewise. This is the case in Amorpha fruticosa and Gleditschia tracanthus.

In the well-known Mimosa pudica, which is a hothouse plant in temperate regions, the leaflets fold together, the small stalks of the leaflets of the compound leaves of this plant approach each other, and the main petiole becomes depressed.

In one exceedingly sensitive species of Oxalis, the pinnate leaves fold upward. A footfall is said to be sufficient to cause it to close its leaves.

When these movements of leaves or leaf-organs take place at stated hours, and when the leaves remain in the new position after the movement has ceased until a particular period of time recur, the closing up is called the _sleep_ of plants. This condition is observed both in seed-leaves and true leaves, as well as in the petals of flowers.

So far as can be made out, the object of this closing of the leaves seems to be to prevent the chilling effect due to radiation from being injurious to the plant. This folding up causes a smaller extent of surface to be exposed. Radiation of heat during a clear night goes on rapidly from all surfaces such as those of expanded leaves. The closing of the leaves may be supposed to form a protective covering, which prevents the heat passing away into space, and thus saves the plant from the injurious effects of cold.

This is only true of the foliage leaves, which expand during the day and close during the night.

The period at which the movement of closing and opening of flowers takes place is very varied. Ordinary leaves, as has been stated, close toward evening and open in the day. The periods of opening and closing in the case of flowers vary considerably, being affected, no doubt, by the visits of insects, which carry the pollen from plant to plant belonging to the same species. By this means flowers are fertilized, and the seeds resulting from plants that are so fertilized are much more numerous than those resulting from self-fertilized plants. Some plants, such as the pimpernel, close their petals when the sky is overcast. This is doubtless to protect the pollen from the injurious effects of rain. This kind of closing, however, is not to be confounded with the regular and periodic closing and opening of flowers.

The diversity in the regular and periodic opening and closing of flowers in regard to time is so great that Linnæus was able to arrange flowers in a list in accordance with their times of opening and closing.

This list he named a _Horologium floræ_, or floral clock, the time of opening or closing representing each succeeding hour.

Some closing flowers open under the influence of strong artificial light, such, for example, as Crocus and Gentiana verna; on others, however, such as Convolvulus, artificial light has no effect.

The closing of flowers is usually a slow process, as may easily be observed, but there are exceptions to this.

“In Desmodium gyrans” (the Indian telegraph-plant) “the trilobate compound leaf has a large terminal leaflet and a smaller one on each side. When the plant is exposed to bright sunlight in a hothouse, the end leaflet stands horizontally, and it folds downward in the evening, but the lateral leaflets move constantly during the heat of the day, advancing, edgewise, first toward the end leaflet, and then returning and moving toward the base of the common petiole alternately on each side, in a manner very well compared to the movements of the arm of the old semaphore telegraphs.”

Such are some of the more striking movements of plants. Even in cases where the precise advantage, as far as regards the economy of plant life, is not fully ascertained, it can not be doubted that such movements are advantageous. In strict accordance with the accepted theory of evolution, no peculiarity would be continued from generation to generation of either plants or animals, if it possessed no essential characteristic which helped the plant or animal to hold its own in “the struggle for existence.”

Cacti--1 and 3, Mamillaria; 2, Echinocactus; 4, Cereus. Fuci--5, Sargassum; 6, Agarum; 7, Thalassophyllum. The Wool Tree (Bombax) and the Rafflesia Arnoldi]

MOVEMENT IN PLANTS
--CHARLES DARWIN

Plants become climbers in order, it may be presumed, to reach the light and to expose a large surface of leaves to its action and to that of the free air. This is effected by climbers with wonderfully little expenditure of organized matter, in comparison with trees, which have to support a load of heavy branches by a massive trunk. Hence, no doubt, it arises that there are in all quarters of the world so many climbing plants belonging to so many different orders. These plants are here classed under three heads. First, hook-climbers, which are, at least in our temperate countries, the least efficient of all, and can climb only in the midst of an entangled vegetation. Secondly, root-climbers, which are excellently adapted to ascend naked faces of rock: when they climb trees, they are compelled to keep much in the shade; they can not pass from branch to branch, and thus cover the whole summit of a tree, for their rootlets can adhere only by long-continued and close contact with a steady surface. Thirdly, the great class of spiral climbers, with the subordinate divisions of leaf-climbers and tendril-bearers, which together far exceed in number and in perfection of mechanism the climbers of the two previous classes. These plants, by their power of spontaneously revolving and grasping objects with which they come in contact, can easily pass from branch to branch, and securely wander over a wide and sunlit surface. I have ranked twiners, leaf and tendril-climbers as subdivisions of one class, because they graduate into each other, and because nearly all have the same remarkable power of spontaneously revolving. Does this gradation, it may be asked, indicate that plants belonging to one subdivision have passed, during the lapse of ages, or can pass, from one state to the other; has, for instance, a tendril-bearing plant assumed its present structure without having previously existed either as a leaf-climber or a twiner? If we consider leaf-climbers alone, the idea that they were primordially twiners is forcibly suggested. The internodes of all, without exception, revolve in exactly the same manner as twiners; and some few can twine as well, and many others in a more or less imperfect manner. Several leaf-climbing genera are closely allied to other genera which are simple twiners. It should be observed that the possession by a plant of leaves with their petioles or tips sensitive, and with the consequent power of clasping any object, would be of very little use, unless associated with revolving internodes, by which the leaves could be brought into contact with surrounding objects. On the other hand, revolving internodes, without other aid, suffice to give the power of climbing, so that, unless we suppose that leaf-climbers simultaneously acquired both capacities, it seems probable that they were first twiners, and subsequently became capable of grasping a support, which, as we shall presently see, is a great additional advantage.

From analogous reasons, it is probable that tendril-bearing plants were primordially twiners--that is, are the descendants of plants having this power and habit. For the internodes of the majority revolve, like those of twining plants; and, in a very few, the flexible stem still retains the capacity of spirally twining round an upright stick. With some the internodes have lost even the revolving power. Tendril-bearers have undergone much more modification than leaf-climbers; hence it is not surprising that their supposed primordial revolving and twining habits have been lost or modified more frequently than with leaf-climbers. The three great tendril-bearing families in which this loss has occurred in the most marked manner are the Cucurbitaceæ, Passifloraceæ, and Vitaceæ. In the first the internodes revolve; but I have heard of no twining form, with the exception of Mormodica balsamina, and this is only an imperfect twiner. In the other two families I can hear of no twiners; and the internodes rarely have the power of revolving, this power being confined to the tendrils; nevertheless, the internodes of Passiflora gracilis have this power in a perfect manner, and those of the common vine in an imperfect degree: so that at least a trace of the supposed primordial habit is always retained by some members of the larger tendril-bearing groups.

On the view here given, it may be asked, Why have nearly all the plants in so many aboriginally twining groups been converted into leaf-climbers or tendril-bearers? Of what advantage could this have been to them? Why did they not remain simple twiners? We can see several reasons. It might be an advantage to a plant to acquire a thicker stem, with short internodes bearing many or large leaves; and such stems are ill fitted for twining. Any one who will look during windy weather at twining plants will see that they are easily blown from their support; not so with tendril-bearers or leaf-climbers, for they quickly and firmly grasp their support by a much more efficient kind of movement. In those plants which still twine, but at the same time possess tendrils or sensitive petioles, as some species of Bignonia, Clematis, and Tropæolum, we can readily observe how incomparably more securely they grasp an upright stick than do simple twiners. From possessing the power of movement on contact, tendrils can be made very long and thin; so that little organic matter is expended in their development, and yet a wide circle is swept. Tendril-bearers can, from their first growth, ascend along the outer branches of any neighboring bush, and thus always keep in the full light; twiners, on the contrary, are best fitted to ascend bare stems, and generally have to start in the shade. In dense tropical forests, with crowded and bare stems, twining plants would probably succeed better than most kinds of tendril-bearers; but the majority of twiners, at least in our temperate regions, from the nature of their revolving movement, can not ascend a thick trunk, whereas this can be effected by tendril-bearers, if the trunks carry many branches or twigs; and in some cases they can ascend by special means a trunk without branches, but with a rugged bark.

The object of all climbing plants is to reach the light and free air with as little expenditure of organic matter as possible; now, with spirally ascending plants, the stem is much longer than is absolutely necessary; for instance, I measured the stem of a kidney-bean which had ascended exactly two feet in height, and it was three feet in length: the stem of a pea, ascending by its tendrils, would, on the other hand, have been but little longer than the height gained. That this saving of stem is really an advantage to climbing plants I infer from observing that those that still twine, but are aided by clasping petioles or tendrils, generally make more open spires than those made by simple twiners. Moreover, such plants very generally, after taking one or two turns in one direction, ascend for a space straight, and then reverse the direction of the spire. By this means they ascend to a considerably greater height, with the same length of stem, than would otherwise be possible; and they can do it with safety, as they secure themselves at intervals by their clasping petioles.

Tendrils consist of various organs in a modified state, namely, leaves and flower-peduncles, and perhaps branches and stipules. The position alone generally suffices to show when a tendril has been formed from a leaf; and in Bignonia the lower leaves are often perfect, while the upper ones terminate in a tendril in place of a terminal leaflet; in Eccremocarpus I have seen a lateral branch of a tendril replaced by a perfect leaflet; and in Vicia sativa, on the other hand, leaflets are sometimes replaced by tendril-branches; and many other such cases could be given. But he who believes in the slow modification of species will not be content simply to ascertain the homological nature of different tendrils; he will wish to learn, as far as possible, by what steps parts acting as leaves or as flower-peduncles can have wholly changed their function, and have come to serve as prehensile organs.

In the whole group of leaf-climbers abundant evidence has been given that an organ, still subserving its proper function as a leaf, may become sensitive to a touch, and thus grasp an adjoining object. In several leaf-climbers true leaves spontaneously revolve; and their petioles, after clasping a support, grow thicker and stronger. We thus see that true leaves may acquire all the leading and characteristic qualities of tendrils, namely, sensitiveness, spontaneous movement, and subsequent thickening and induration. If their blades or laminæ were to abort, they would form true tendrils. And of this process of abortion we have seen every stage; for in an ordinary tendril, as in that of the pea, we can discover no trace of its primordial nature; in Mutisia clematis, the tendril in shape and color closely resembles a petiole with the denuded midribs of its leaflets; and occasionally vestiges of laminæ are retained or reappear. Lastly, in four genera in the same family of the Fumariaceæ we see the whole gradation; for the terminal leaflets of the leaf-climbing Fumaria officinalis are not smaller than the other leaflets; those of the leaf-climbing Adlumia cirrhosa are greatly reduced; those of the Corydalis claviculata (a plant which may be indifferently called a leaf-climber or tendril-bearer) are either reduced to microscopical dimensions or have their blades quite aborted, so that this plant is in an actual state of transition; and, finally, in the Dicentra the tendrils are perfectly characterized. Hence, if we were to see at the same time all the progenitors of the Dicentra, we should almost certainly behold a series like that now exhibited by the above-named four genera. In Tropæolum tricolorum we have another kind of passage; for the leaves which are first formed on the young plant are entirely destitute of laminæ, and must be called tendrils, while the later formed leaves have well-developed laminæ. In all cases, in the several kinds of leaf-climbers and of tendril-bearers, the acquirement of sensitiveness by the midribs of the leaves apparently stands in the closest relation with the abortion of their laminæ or blades.

On the view here given, leaf-climbers were primordially twiners, and tendril-bearers (of the modified leaf division) were primordially leaf-climbers. Hence leaf-climbers are intermediate in nature between twiners and tendril-bearers, and ought to be related to both. This is the case: thus the several leaf-climbing species of the Antirrhineæ, of Solanum, of Cocculus, of Gloriosa are related to the other genera in the same family, or even to other species in the same genus, which are true climbers. On the other hand, the leaf-climbing species of Clematis are very closely allied to the tendril-bearing Naravelia: the Fumariaceæ include closely allied genera which are leaf-climbers and tendril-bearers. Lastly, one species of Bignonia is both a leaf-climber and a tendril-bearer, and other closely allied species are twiners.

Tendrils of the second great division consist of modified flower-peduncles. In this case likewise we have many interesting transitional states. The common vine (not to mention the Cardiospermum) gives us every possible grade from finely developed tendrils to a bunch of flower-buds, bearing the single usual lateral flower-tendril. And when the latter itself bears some flowers, as we know is not rarely the case, and yet retains the power of clasping a support, we see the primordial state of all these tendrils which have been formed by the modification of flower-peduncles.

According to Mohl and others, some tendrils consist of modified branches. I have seen no such case, and, therefore, of course, know nothing of any transitional states, if such occur. But Lophospermum, at least, shows us that such a transition is possible; for its branches spontaneously revolve, and are sensitive to contact. Hence, if the leaves of some of the branches were to abort, they would be converted into true tendrils. Nor is it so improbable as may at first appear that certain branches alone should become modified, the others remaining unaltered; for with certain varieties of Phaseolus some of the branches are thin and flexible and twine, while other branches on the same plant are stiff and have no such power.

If we inquire how the petiole of a leaf, or the peduncle of a flower, or a branch first becomes sensitive and acquires the power of bending toward the touched side, we get no certain answer. Nevertheless, an observation by Hofmeister well deserves attention, namely, that the shoots and leaves of all plants, while young, move after being shaken; and it is almost invariably young petioles and young tendrils, whether of modified leaves or flower-peduncles, which move on being touched; so that it would appear as if these plants had utilized and perfected a widely distributed and incipient capacity, which capacity, as far as we can see, is of no service to ordinary plants. If we further inquire how the stems, petioles, tendrils, and flower-peduncles of climbing plants first acquired their power of spontaneously revolving or, to speak more accurately, of successively bending to all points of the compass, we are again silenced, or at most can only remark, that the power of movement, both spontaneous and from various stimuli, is far more common with plants, as we shall presently see, than is generally supposed to be the case by those who have not attended to the subject. There is, however, one remarkable case of the Maurandia semperflorens, in which the young flower-peduncles spontaneously revolve in very small circles, and bend themselves, when gently rubbed, to the touched side; yet this plant certainly profits in no way by these two feebly developed powers. A rigorous examination of other young plants would probably show some slight spontaneous movement in the peduncles and petioles, as well as that sensitiveness to shaking observed by Hofmeister. We see at least in the Maurandia a plant which might, by a little augmentation of qualities which it already possesses, come first to grasp a support by its flower-peduncles (as with Vitis or Cardiospermum) and then, by the abortion of some of its flowers, acquire perfect tendrils.

There is one interesting point which deserves notice. We have seen that some tendrils have originated from modified leaves, and others from modified flower-peduncles; so that some are foliar and some axial in their homological nature. Hence it might have been expected that they would have presented some difference in function. This is not the case. On the contrary, they present the most perfect identity in their several remarkable characteristics. Tendrils of both kinds spontaneously revolve at about the same rate. Both, when touched, bend quickly to the touched side, and afterward recover themselves and are able to act again. In both the sensitiveness is either confined to one side or extends all round the tendril. They are either attracted or repelled by the light. The tips of the tendrils in these two plants become, after contact, enlarged into disks, which are at first adhesive by the secretion of some cement. Tendrils of both kinds, soon after grasping a support, contract spirally; they then increase greatly in thickness and strength. When we add to these several points of identity the fact of the petiole of the Solanum jaspinoides assuming the most characteristic feature of the axis, namely, a closed ring of woody vessels, we can hardly avoid asking whether the difference between foliar and axial organs can be of so fundamental a nature as is generally supposed to be the case.

We have attempted to trace some of the stages in the genesis of climbing plants. But, during the endless fluctuations in the conditions of life to which all organic beings have been exposed, it might have been expected that some climbing plants would have lost the habit of climbing. In the cases of certain South African plants belonging to great twining families, which in certain districts of their native country never twine, but resume this habit when cultivated in England, we have a case in point. In the leaf-climbing Clematis flammula, and in the tendril-bearing vine, we see no loss in the power of climbing, but only a remnant of that revolving power which is indispensable to all twiners, and is so common, as well as so advantageous, to most climbers. In Tecoma radicans, one of the Bignoniaceæ, we see a last and doubtful trace of the revolving power.

With respect to the abortion of tendrils, certain cultivated varieties of Cucurbita pepo have, according to Naudin, either quite lost these organs or bear semi-monstrous representatives of them. In my limited experience I have met with only one instance of their natural suppression, namely, in the common bean. All the other species of Vicia, I believe, bear tendrils; but the bean is stiff enough to support its own stem, and in this species, at the end of the petiole where a tendril ought to have arisen, a small pointed filament is always present, about a third of an inch in length, and which must be considered as the rudiment of a tendril. This may be the more safely inferred, because I have seen in young, unhealthy specimens of true tendril-bearing plants similar rudiments. In the bean these filaments are variable in shape, as is so frequently the case with all rudimentary organs, being either cylindrical or foliaceous, or deeply furrowed on the upper surface. It is a rather curious little fact that many of these filaments when foliaceous have dark-colored glands on their lower surfaces, like those on the stipules, which secrete a sweet fluid; so that these rudiments have been feebly utilized.

One other analogous case, though hypothetical, is worth giving. Nearly all the species of Lathyrus possess tendrils; but L. nissolia is destitute of them. This plant has leaves which must have struck every one who has noticed them with surprise, for they are quite unlike those of all common papilionaceous plants, and resemble those of a grass. In L. aphaca the tendril, which is not highly developed (for it is unbranched, and has no spontaneous revolving power), replaces the leaves, the latter in function being replaced by the large stipules. Now, if we suppose the tendrils of L. aphaca to become flattened and foliaceous, like the little rudimentary tendrils of the bean, and the large stipules, not being any longer wanted, to become at the same time reduced in size, we should have the exact counterpart of L. nissolia, and its curious leaves are at once rendered intelligible to us.

It may be added, as it will serve to sum up the foregoing views on the origin of tendril-bearing plants, that if these views be correct, L. nissolia must be descended from a primordial spirally twining plant; that this became a leaf-climber; that first part of the leaf and then the whole leaf became converted into a tendril, with the stipules by compensation greatly increased in size; that this tendril lost its branches and became simple, then lost its revolving power (in which state it would resemble the tendril of the existing L. aphaca), and afterward losing its prehensile power and becoming foliaceous would no longer be called a tendril. In this last stage (that of the existing L. nissolia) the former tendril would reassume its original function as a leaf, and its lately largely developed stipules, being no longer wanted, would decrease in size. If it be true that species become modified in the course of ages, we may conclude that L. nissolia is the result of a long series of changes, in some degree like those just traced.

The most interesting point in the natural history of climbing plants is their diverse power of movement; and this led one on to their study. The most different organs--the stem, flower-peduncle, petiole, midribs of the leaf or leaflets, and apparently aerial roots--all possess this power.

In the first place, the tendrils place themselves in the proper position for action, standing, for instance, in the Cobæa, vertically upward, with their branches divergent and their hooks turned outward, and with the young terminal shoot thrown on one side; or, as in Clematis, the young leaves temporarily curve themselves downward, so as to serve as grapnels.

Secondly, if the young shoot of a twining plant, or of a tendril, be placed in an inclined position, it soon bends upward, though completely secluded from the light. The guiding stimulus to this movement is no doubt the attraction of gravity, as Andrew Knight showed to be the case with germinating plants. If a succulent shoot of almost any plant be placed in an inclined position in a glass of water in the dark, the extremity will, in a few hours, bend upward; and if the position of the shoot be then reversed, the now downward bent shoot will reverse its curvature; but if the stolon of a strawberry, which has no tendency to grow upward, be thus treated, it will curve downward in the direction of, instead of in opposition to, the force of gravity. As with the strawberry, so it is generally with the twining shoots of the Hibbertia dentata, which climbs laterally from bush to bush; for these shoots, when bent downward, show little and sometimes no tendency to curve upward.

Thirdly, climbing plants, like other plants, bend toward the light by a movement closely analogous to that incurvation which causes them to revolve. This similarity in the nature of the movement was well seen when plants were kept in a room, and their first movements in the morning toward the light and their subsequent revolving movements were traced on a bell glass. The movement of a revolving shoot, and in some cases of a tendril, is retarded or accelerated in traveling from or to the light. In a few instances tendrils bend in a conspicuous manner toward the dark. Many authors speak as if the movement of a plant toward the light was as directly the result of the evaporation or of the oxygenation of the sap in the stem, as the elongation of a bar of iron from an increase in its temperature. But, seeing that tendrils are either attracted to or repelled by the light, it is more probable that their movements are only guided and stimulated by its action in the same manner as they are guided by the force of attraction toward the centre of gravity.

Fourthly, we have in stems, petioles, flower-peduncles and tendrils the spontaneous revolving movement which depends on no outward stimulus, but is contingent on the youth of the part and on its vigorous health, which again, of course, depends on proper temperature and the other conditions of life. This is, perhaps, the most interesting of all the movements of climbing plants because it is continuous. Very many other plants exhibit spontaneous movements, but they generally occur only once during the life of a plant, as in the movements of the stamens and pistils, etc., or at intervals of time, as in the so-called sleep of plants.

Fifthly, we have in the tendrils, whatever their homological nature may be, in the petioles and tips of the leaves of leaf-climbers, in the stem in one case and apparently in the aerial roots of the vanilla, movements--often rapid movements--from contact with any body. Extremely slight pressure suffices to cause the movement. These several organs, after bending from a touch, become straight again, and again bend when touched.

Sixthly, and lastly, most tendrils, soon after clasping a support, but not after a mere temporary curvature, contract spirally. The stimulus from the act of clasping some object seems to travel slowly down the whole length of the tendril. Many tendrils, moreover, ultimately contract spontaneously even if they have caught no object; but this latter useless movement occurs only after a considerable lapse of time.

We have seen how diversified are the movements of climbing plants. These plants are numerous enough to form a conspicuous feature in the vegetable kingdom; every one has heard that this is the case in tropical forests; but even in the thickets of our temperate regions the number of kinds and of individual plants is considerable, as will be found by counting them. They belong to many and widely different orders. To gain some crude idea of their distribution in the vegetable series, I marked from the lists given by Mohl and Palm (adding a few myself, and a competent botanist, no doubt, could add many more) all those families in _Lindley’s Vegetable Kingdom_, which include plants in any of our several subdivisions of twiners, leaf-climbers, and tendril-bearers; and these (at least some of each group) all have the power of spontaneously revolving. Lindley divides Phanerogamic plants into fifty-nine alliances; of these, no less than above half, namely, thirty-five, include climbing plants according to the above definition, hook and root-climbers being excluded. To these a few Cryptogamic plants must be added which climb by revolving. When we reflect on this wide serial distribution of plants having this power, and when we know that in some of the largest, well-defined orders, such as the Compositæ, Rubiaceæ, Scrophulariaceæ, Liliaceæ, etc., two or three genera alone, out of the host of genera in each, have this power, the conclusion is forced on our minds that the capacity of acquiring the revolving power on which most climbers depend is inherent though undeveloped in most every plant in the vegetable kingdom.

FLOWER COLORATION
--ALEXANDER S. WILSON

The Prophet-plant (Arnebia echioides) is a native of Persia and Arabia, but has been introduced and grows freely in gardens in England. Its chief interest lies in its variable flowers, which may fairly rank with those of the changeable Hibiscus and other

“Plants divine and strange
That every hour their blossoms change.”

The plant is about two feet in height, and somewhat resembles a cowslip or an auricula. It belongs to the natural order Boraginaceæ, and is nearly allied to the lungwort, viper’s-bugloss, borage, and forget-me-not, all of which exhibit color changes more or less distinct. The various species of Myosotis, or forget-me-not, are also called scorpion grasses, from the upper flower-bearing portion of the stem being curled on itself like a watch-spring. The cluster of flowers, forming the inflorescence of Arnebia, develops in same scorpioid fashion. There is a double row of flower buds on the curled stalk, and as this gradually unwinds pair after pair of the flowers expand in succession. In shape and color the individual flowers are not unlike those of the primrose, though rather smaller. When a flower first opens, five conspicuous jet-black spots are seen upon the yellow rim of the salver-shaped corolla. If the flower be examined the following day, we are surprised to discover that the black spots have vanished as if by magic. The yellow of the corolla is also much paler, and a little later on presents quite a bleached and silvery appearance, the petals becoming almost white. No sooner have the spots disappeared from the first pair of flowers than a second pair expand, and display their sable marks in bold relief upon the yellow enamel of their petals. From this time onward the inflorescence comprises both kinds of flower, those but newly opened having the five conspicuous spots, and the older ones on which no spots are visible. From these dark spots--the so-called finger-marks of Mahomet, Arnebia has received its name--the Prophet-plant. Its flowers seem bewitched, the change is so pronounced and obvious; a day or two after unfolding they differ so much from the newly opened ones beside them, that were they growing on separate plants, we should at once set them down as belonging to another species.

This change of color gives rise to another interesting peculiarity. If Arnebia be examined by daylight, and again in the dim twilight, the observer is struck by a remarkable circumstance. In broad daylight, the golden spotted flowers at once arrest the eye, while their paler companions are hardly observed. The inflorescence owes by far the greater part of its display to the younger flowers. In the dusk this is entirely reversed; the conspicuousness of the inflorescence now depends on the paler flowers, and the others are so obscured that a second glance is needed before they can be discerned. The relative brilliancy of the two sets of flowers can also be tested by gradually retiring from the plant, keeping the eyes still fixed on the blossoms. At dusk the young flowers are lost sight of much sooner than the others; by day the older ones first disappear in the distance. This peculiar transformation imparts to the inflorescence of Arnebia a faint similitude of the pillar of cloud by day and of fire by night--that celestial manifestation of sacred story so closely associated with the native region of this desert flower.

Here, then, we have one of those phenomena which for the naturalist possess all the fascination of a mystery. What can be the explanation of this remarkable change of color, and what advantage does the flower derive from the sudden disappearance of its spots and the blanching of its petals?

With the reader’s permission, we shall now proceed to show why nature has bestowed on Arnebia what she has denied to the leopard--the power of changing its spots. Before we can say why any flower should change its color, we must first know why a flower is colored at all, and why all flowers are not colored alike. Almost all the peculiarities of flowers can be explained as having reference to the visits of insects. The honey is secreted as an inducement, while the secret and brilliant colors serve to attract the attention of the honey-gatherers. The researches of the late Charles Darwin demonstrated the importance of cross-fertilization in the vegetable kingdom. Very many flowers are quite sterile with their own pollen; in other cases, although the flower has the capacity of self-fertilization, the resulting seeds are of very inferior quality compared with those obtained as a result of cross-fertilization. As carriers of pollen, then, insects perform an essential service to plants, and it is in order to secure their services that flowers are brightly colored.

For the variety of color observed among flowers there appear to be two principal reasons. A little reflection will show that, since flowers are so dependent on insects for the conveyance of their pollen, it must be to the advantage of each species of plant to possess flowers distinctively colored and capable of being easily recognized by honey-seeking insects. A bee does not visit all flowers indiscriminately; it would be greatly to the flowers’ disadvantage if it did. In the course of a single journey the bee for the most part restricts itself to the flowers of one species, and has been known to visit as many as thirty dead-nettles in succession, passing over all other flowers. Time is saved by this method, for by keeping to one kind of flower at a time the insect becomes familiar with its outs and ins, and the practice thus acquired enables it to overtake a larger number of blossoms than it could if it did not observe this rule. This constancy in visiting the same kind of flower is of great importance to plants, since it ensures that the pollen will be conveyed to a flower of the same species as that from which it came. But if all flowers were colored and perfumed alike, the winged botanist could not identify the species; the pollen would be constantly transferred to the stigmas of the wrong flowers, where it would be useless, and so the work of cross-fertilization would be seriously impeded.

A second cause contributing to the variety observed among flowers is the desirability of attracting special kinds of insects. As we have just seen, an insect does not visit all kinds of flowers indiscriminately; neither, on the other hand, does a flower attract indiscriminately all kinds of insects. Not only are injurious and unprofitable visitors excluded, but the more specialized insects are in greatest demand. Partiality for particular insects is shown both by the shapes and coloring of flowers. Open shallow flowers, with exposed honey accessible to almost all insects, have, as their most frequent visitors, short-lipped flies and beetles. Many blossoms, again, have become specially adapted to bees. Their honey is placed beyond the reach of short-lipped fliers, and requires the slender proboscis of a bee or butterfly for its extraction. Honeysuckle, habenaria, plumbago, phlox, and narcissus illustrate a third type, with flower-tubes so narrow and deep that their nectar is quite inaccessible even to bees, and is reserved entirely for moths and butterflies, which possess an extremely long and thin proboscis. There is a corresponding adaptation in the colors; the gay tints of the buttercup, poppy, and rose appear to have special attractions for beetles; bees show a decided preference for blue, and this color predominates in flowers whose shapes are adapted to their visits. Deep tubular flowers specialized for Lepidoptera fall into two divisions, according as they solicit the attentions of diurnal butterflies or nocturnal moths. Red and purple are the favorite colors of the former, while nocturnal moths show a preference for white and pale flowers. Thus the carnation and campion (Lychnis diurna), which open by day, have dark tints in comparison with Lychnis respertina, which unfolds its petals toward evening. Almost scentless by day, this white nocturnal flower diffuses a delicious fragrance in the twilight. The evening primrose (Ænothera), which, however, has yellow petals, is another example of this class. But the most remarkable plant of this type is the night-flowering stock (Cereus). Its pale blossoms open about seven in the evening, emit puffs of odor from time to time, and close up again toward midnight; by morning the flowers are withered. It is impossible to doubt that we have in this instance a flower specialized for the visits of nocturnal moths. The reason why nocturnal flowers, like the honeysuckle and evening campion, have pale-colored petals is not far to seek. These pale hues can be more easily distinguished at night than the red or purple of Dianthus or Githago. Among lilies both diurnal and nocturnal flowers occur, and clearly indicate by their colors to which section of the Lepidoptera they are adapted. The Turk’s-cap lily, with its perianth of fiery scarlet, is a characteristic example of a diurnal flower adapted to butterflies which wander abroad in daytime. On the other hand, Lilium Martagon, an L. candidum, with their white bells, are nocturnal lilies fertilized by night-loving moths.

Two flowers, unlike in their coloring, can hardly be equally attractive to the same visitors, even if they grow together on the same plant, as in the case of Arnebia; the presumption, therefore, is that its spotted and pale blossoms are adapted for different insects. Moreover, the stronger colors of the younger flowers correspond with those of the day-blooming class, while the paler tints of those in the second stage will render them more attractive to nocturnal moths; and this view is strongly confirmed by the fact that night-blooming flowers are never variegated, but have their petals uniformly devoid of markings. By night the dark spots tend, in this instance, to conceal the blossoms so much that, if these are to be converted into nocturnal flowers, the removal of the spots is absolutely necessary. We may therefore conclude with tolerable certainty that the flowers of Arnebia in their first stage are adapted to bees and diurnal Lepidoptera, while in their second condition they array themselves in paler hues to attract nocturnal moths.

By the color change, in this instance, a diurnal is converted into a nocturnal flower, and one advantage thereby gained is that the blossoms appeal to a larger class of fertilizing agents. The more restricted the circle of visitors on which any plant depends the greater the risk, in the event of insects being scarce, of its flowers remaining unfertilized and perishing. Here it would seem that Nature proceeds on the same principle as a fisherman in changing his bait. Like some other variable blossoms, Arnebia is in the advantageous position of carrying two strings to her bow.

QUEER FLOWERS
--GRANT ALLEN

If Baron Munchausen had ever in the course of his travels come across a single flower one standard British yard in diameter, fifteen pounds avoirdupois in weight, and forming a cup big enough to hold six quarts of water in its central hollow, it is not improbable that the learned baron’s veracious account of the new plant might have been met with the same polite incredulity which his other adventures shared with those of Bruce, Stanley, Mendez Pinto, and Du Chaillu. Nevertheless, a big blossom of this enormous size has been well known to botanists ever since the beginning of the Nineteenth Century. When Sir Stamford Raffles was taking care of Sumatra during our temporary annexation, he happened one day to light upon a gigantic parasite, which grew on the stem of a prostrate creeper in the densest part of the tropical jungle. It measured nine feet round and three feet across: it had five large petals with a central basin; and it was mottled red in hue, being, in fact, in color and texture surprisingly suggestive of raw beefsteak. One flower was open when Sir Stamford came upon it: the other was in the bud, and looked in that state extremely like a very big red cabbage. Specimens of this surprising find were at once forwarded to England, and it was at last duly labeled after the names of its two discoverers as Rafflesia Arnoldi.