Chapter VI: The Leaf (2)

=194. The light relation.=—The principal external conditions to which leaves have to adjust themselves are light, air, moisture, gravity, temperature, and the attacks of animals. From the knowledge of their work and function gained in the preceding sections, it will be clear that the primary relation of the leaf is a light relation, and to this, first of all, it must adjust itself.

It was shown in Exps. 56 and 57 how promptly leaves respond to changes in the direction of light, and a little observation (Exp. 74) will convince us that they are equally sensitive to changes in intensity and periodicity of illumination.

=195. Phototropism.=—The movement of plants in response to light is called _phototropism_—a word that means “turning toward or away from light.” It includes all kinds of light adjustments, and examples of it are to be met with everywhere in the disposition of leaves with reference to their light exposure.

=196. Horizontal and vertical adjustment.=—Take two sprigs, one upright, the other horizontal, from any convenient shrub or tree—and notice the difference in the position of the leaves. Examine their points of attachment and see how this is brought about, whether by a twist of the petiole or of the base of the leaf blades, or by a half twist of the stem between two consecutive leaves, or by some other means.

(_From_ Mo. Botanical Garden Rep’t.)]

Observe both branches in their natural position; what part of the leaf is turned upward, the edge or the surface of the blade? Change the position of the two sprigs, placing the vertically growing one horizontal, and the horizontal one vertical. What part of the leaves is turned upward in each?

=197. Leaf mosaics.=—Trees with horizontal or drooping branches, like the elm and beech, and vines growing along walls or trailing on the ground, generally display their foliage in flat, spreading layers, each leaf fitting in between the interstices of the others like the stones in a mosaic, whence this has been called the _mosaic_ arrangement. (Plate 10.) In plants of more upright or bunchy habit, the leaves are placed at all angles, giving the appearance of a rosette when viewed from above, whence this is called the _rosette_ arrangement.

A variety of the same disposition is seen in the pyramidal shape assumed by plants with large, undivided leaves like the mullein and burdock (Fig. 237), in which access of light is secured by a mutual adjustment between the size and position of leaves, the upper ones becoming successively smaller.

=198. Heliotropism=—“turning with the sun”—is the name given to the daily movement of plants like the cotton and sunflower in turning their leaves or their blossoms to face the sun. If you live where cotton is grown, notice the leaves in a field about ten o’clock on a bright sunny morning, and again from the same point of view at about four or five in the afternoon. Do you perceive any difference in their general disposition? Watch on a cloudy day and see if any change takes place. Find out by observation whether the “heliotrope” of the hothouses is really heliotropic.

=199. Adjustment against too great intensity of light.=—Plants frequently have to protect themselves against excess of light and heat. An interesting example of this kind of adjustment is furnished by the rosinweed, or compass plant (_Silphium laciniatum_, Figs. 238, 239), which grows in the prairies of Alabama and westward, where it is exposed to intense sunlight. The leaves not only stand vertical, but have a tendency to turn their edges north and south so that the blades are exposed only to the gentler morning and evening rays. The prickly lettuce manifests the same habit in a less marked degree.

=200. Night and day adjustments.=—These are movements in response to changes in the degree of illumination and temperature, as evidenced by the fact that they become feeble and soon cease altogether if the plant is kept a sufficient time under uniform conditions as to these two factors. (Exp. 74.) They are called “nyctitropic” or sleep movements, because they are most obvious in certain plants that undergo periodic adjustments to the alternations of day and night suggestive of an imaginary likeness to the sleep of animals. Examples are most frequently met with among members of the pea family (_Leguminosæ_), the spurges (_Euphorbiaceæ_), and the sorrel (_Oxalis_) family. They are found among other species also, and indeed are much more general than is usually supposed, most plants showing signs of them if carefully tested. A simple way of doing this is by attaching bristles about two inches long to the tips of two leaves on opposite sides of the stem, as in Figs. 240, 241, and comparing the divergence of the bristles during the day and at nightfall. In this way a change of position in the leaves, too slight to attract attention otherwise, will be made apparent. The positions assumed vary in different plants, and even in the parts of the same compound leaf; in the kidney bean, for instance, the common petiole turns up at night, while the individual leaflets turn down. One of the common pigweeds (_Amaranthus Palmeri_, Figs. 242-244) is heliotropic in the day time and nyctitropic at night.

The very striking nyctitropic adjustments of the wild senna (_Cassia tora_) photographed by Professor Francis E. Lloyd of the Alabama Polytechnic Institute (Figs. 245-250), though obviously influenced by the sun, are not directed toward it as in those of truly heliotropic plants.

These movements are common also among flowers, many of them having regular hours for opening and closing, as indicated by such names as “morning-glory” and “four-o’clock.” In these cases, however, other causes (277, 280) than the light relation must be taken into account.

=201. Irritability= is a general term applied to the power in plants of receiving and responding by spontaneous movements to impressions from without. In its widest acceptation, irritability includes, besides the various forms of adjustment described in this section and the next, all movements due to geotropism, those of roots seeking air and moisture, the revolution of twining stems and tendrils, the circulation of protoplasm in the cell—any movement, in short, that is made in response to an impression from the environment is a manifestation of irritability. It may be of various degrees, but is possessed to some extent by every living vegetable organism.

The term is usually applied, however, more especially to those obvious and pronounced responses made by plants to their surroundings, as exemplified in the cases just given. Still more marked instances are to be found in the movements of the tentacles of insectivorous plants, and the sensitive leaflets of the mimosa that close at the slightest touch. The tendrils of the passion flower are said to appreciate and respond to a pressure that cannot be distinguished even by the human tongue, and many plants will detect and respond to the ultra-violet rays of light, which are entirely invisible to man.

This faculty of irritability among plants corresponds, in an imperfect, rudimentary way, to what we recognize in animals as nervous excitability. By this it is not meant to imply that the two things are identical in their ultimate manifestations, though we may regard them as fundamentally the same in that they are both to be referred to the property inherent in protoplasm of responding to stimuli. There is no indication, however, that irritability in the vegetable kingdom is accompanied by anything like consciousness or volition, or that plants possess any power of initiative. While the movements in response to stimuli are in many cases eminently adapted to a purpose, we have no evidence of a controlling power behind them. The movement comes automatically in response to the stimulus, whether the effect at the moment be advantageous or the reverse.

=202. Adjustments in relation to moisture.=—These adjustments may be—(1) To guard against excess of moisture; _e.g._ glands for excreting water and salts; scales, wax, down, etc., on the surface of leaves. These may serve also for protection against cold, insects, excess of light and heat. (2) For the conservation of moisture; _e.g._ the revolute leaf margins of grasses and sand plants growing along the seashore; the fleshy leaves of stonecrops and purselanes; the hard epidermis of yuccas and aloes; the scales, scurf, and down, by which the moisture absorbed from the soil by plants growing in dry and barren places is prevented from escaping too rapidly through the stomata; the leaf cups and holders sometimes formed by winged petioles and clasping leaf bases for retaining dew or rain water. (3) For leaf drainage, or the conduction of moisture, by means of grooves, channels, and taper-pointed leaves, which act as natural gutters and drain pipes.

=203. The fall of the leaf.=—This is, in effect, an adjustment to change of temperature, but that it is not directly due to cold is shown by Exp. 75, and also by the fact that leaves in the tropics and those of evergreens, while they do not fall at stated periods like the bulk of the foliage in the temperate zones, are cut off just the same and replaced by new ones, whenever, for any reason, they are unable to perform their function. In cold climates they fall at the approach of winter, not because the frost loosens them, but because the roots are not able to absorb enough moisture to supply them with material for making food. The needles and the scale-leaves characteristic of evergreens in cold regions are enabled to persist indefinitely by reason of their contracted surface. This prevents the dissipation of moisture and affords no lodging for the accumulations of sleet and snow that would otherwise cumber and perhaps break the boughs with their weight. Trees and shrubs that shed their leaves in winter are said to be _deciduous_, from a Latin word meaning “to fall.” Can you mention some advantages of the deciduous habit to a plant with broad, expanded leaves, growing in a cold climate?

The mechanical means by which the leaf fall is accomplished is through the growth of a corky layer of loose cells that forms at the base of the petiole and cuts it away from the stem, leaving a smooth, clean scar. Tear some fresh young leaves from a growing twig and compare the scars with those on a winter bough. Do you see any difference? This corky layer can be made to form in some plants artificially, by depriving them of working material. (Exp. 75.)

=204. The protection of wintergreen leaves.=—A great many, perhaps the majority of broad-leaved evergreens, bear no obvious protection against cold, while a large proportion, such as chickweed, violet, fumitory, groundsel (_Senecio_), and dead nettle (_Lamium_), would seem peculiarly unfitted, by their delicate structure, to withstand it. But recent investigations by the Swedish botanist, Lidforss, have shown that all wintergreen leaves, with the exception of those on submerged water plants, which are sufficiently protected by the medium in which they live, lose their starch in winter and contain instead an increased percentage of sugar. The same is true of other vegetable structures also, where starch is present, such as roots, stems, tubers, and winter fruits—nuts, haws, persimmons, and the like, which, as every schoolboy knows, become perceptibly sweeter after frost.

The presence of certain substances, of which sugar is the most frequent, enables plants to withstand a greater degree of cold than they could otherwise endure (Exp. 76). This effect, as shown by Lidforss’s experiments, is due to the action of sugar in counteracting, or retarding, the “salting out” of proteins by cold, as explained in 33.

As sugar is readily reconverted into starch by exposure to a moderately high temperature for even a few days, we may find here an explanation of the fact that plants which have survived the prolonged cold of winter are often killed by a single sharp night frost following a few warm days in early spring, before the tender new growth has appeared. The plant suffers, not from the direct effects of cold, but from the warmth preceding it, which stimulated the transformation into starch of the sugar that would have prevented the loss of proteins. On the same principle we may account for the puzzling fact that the sunny southern side of trees and shrubs usually suffers more from the effects of sudden frost than the shaded and colder northern face.

In apparent conflict with this reasoning is the fact that sugar cane and the sugar beet are peculiarly susceptible to cold. This, however, does not invalidate the premises established by Lidforss’s researches, but merely emphasizes the need of further investigation, which may either reconcile all the facts, or modify their interpretation.

=205. The colors of autumn leaves.=—These are due to the breaking up and disappearance of the chlorophyll when the leaf factory has to “shut down” for want of raw material to work with (203). It is closely connected with the appearance of frost, since the same changes of temperature which produce frost cause the cessation of sap flow that brings about the disorganization of the chlorophyll and the formation of various pigments derived from it. Besides these, leaves may contain other coloring matters that are perceptible only when the chlorophyll disappears; and in the sap there is a reddish pigment which becomes either a very bright red, or a dark purplish maroon, from the effect of chemicals that combine with it in the leaves. With these coloring materials at command it is easy to see how the autumn woods can assume such splendid hues.

Practical Questions

1. How would you explain the fact that the outer twigs of trees
generally are the most leafy? (99, 194; Exps. 57, 74.)

2. Is the common sunflower a compass plant? Is cotton?

3. Are there any such plants in your neighborhood?

4. Compare the leaves of half a dozen shade-loving plants of your
neighborhood with those of as many sun-loving ones; which, as a
general thing, are the larger and less incised?

5. Give a reason for the difference. (169.)

6. Why do most leaves—notably grasses—curl their edges backward in
withering? (182.)

7. What advantage is gained by doing this? (202.)

8. Observe such of the following plants as are found in your
neighborhood, and report any changes of position that may take
place in their leaves and the causes to which such changes should
be ascribed: wood sorrel, mimosa, honey locust, wild senna,
partridge pea, wild sensitive plant, redbud, bush clover, Japan
clover, Kentucky coffee tree, sensitive brier (_Schrankia_),
peanut, kidney bean.

9. Which of the trees named below shed their leaves from base to
tip of the bough (centripetally), and which in the reverse order:
ash, beech, hazel, hornbeam, lime, willow, poplar, pear, peach,
sweet gum, elm, sycamore, mulberry, China tree, sumac, chinquapin?

10. Account for the fact that evergreen trees and shrubs have
generally thick, hard, and shiny leaves, like those of the holly
and magnolia, or scales and needles, as the cedar and pine. (203.)

11. Why do many plants which are deciduous at the North tend to
become evergreen at the South? (203.)

12. Why are evergreens more abundant in cold than in warm climates?
(203.)

13. There is an apparent inconsistency between questions 11 and 12;
can you reconcile it? (203.)

14. Why is it more important to protect the south side of trees
against exposure to frost than the northern side? (33, 204.)

15. Explain why peach orchards on the tops and northern slopes of
elevated areas are less liable to have their fruit destroyed by
late frost than those in the valleys and on the southern slopes.
(33, 204.)

VIII. MODIFIED LEAVES

MATERIAL.—Get from a florist a potted plant of sundew,
Venus’s-flytrap, sarracenia, or, if possible, one of all three, and
keep in the schoolroom for observation. The subject can be studied
best in a well-stocked greenhouse, if one is accessible.

=206. Modification and adaptation.=—Modification is structural adjustment, or adaptation, carried so far as to obscure the original form of an organ. Its true nature, however, can generally be determined by some of the tests mentioned in 100.

Examples of the modification of leaves to do the work of other organs have already been noticed, as also their entire disappearance in certain cases (97, 101, 149) and replacement by other parts; it is unnecessary, therefore, to revert to this branch of the subject here.

=207. Protective modifications.=—The most general protective modifications that leaves undergo are (1) for the conservation of moisture, as explained in 202, and (2) for protection against animals. Many of the adaptations for the former purpose serve incidentally for defense against animals also. Spines, hairs, scales, sticky exudations, water holders, clasping and perfoliate leaves bar the way to crawling insects; horny cuticles, as well as offensive odors, bitter secretions, and poisonous juices warn leaf-eating cattle and bugs away. These devices are merely protective, however, and adapted to a passive attitude of self-defense.

=208. Insectivorous leaves.=—But sometimes a plant becomes the aggressor, and instead of standing on the defensive or suffering itself to be quietly devoured, proceeds to capture and devour small game on its own account, and in this case, the leaf sometimes becomes a deadly weapon of destruction.

=209. Pitcher plants.=—The sarracenia, or trumpet leaf, is a familiar example of this class. The lower part of the leaf blade is transformed into a hollow vessel for holding water, and the top is rounded into a broad flap called the _lamina_. Sometimes the lamina stands erect, as in the common yellow trumpets of our coast regions, and when this is the case, it is brilliantly colored and attracts insects (Fig. 259). Sometimes, as in the parrot-beaked and the spotted trumpet leaf, it is bent over the top of the water vessel like a lid, and the back of the leaf, near the foot of the lamina, is dotted with transparent specks that serve to decoy foolish flies away from the true opening and tempt them to wear themselves out in futile efforts to escape, as we often see them do against a window pane.

If the contents of one of these leaves are examined with a lens, there will generally be found mixed with the water at the bottom the remains of the bodies of a large number of insects. The hairs on the outside all point up, toward the rim of the pitcher, while those on the inside turn down, thus smoothing the way to destruction, but making return impossible to a small insect when once it is ensnared. When we remember that these plants are generally found in poor, barren soil, we can appreciate the value to them of the animal diet thus obtained.

=210. Flytraps.=—The most remarkable examples of insect-catching leaves are the Venus’s-flytrap, found in the seacoast region of North Carolina, and the sundew (_Drosera rotundifolia_), common on the margins of sandy bogs and ponds. The latter is a delicate, innocent-looking little plant, and owes its poetic name to the dewlike appearance of a shining, sticky fluid exuded from glands on its leaves, which glitter in the sun like dewdrops. It is, however, a most voracious carnivorous plant, the sticky leaves acting as so many bits of fly paper by means of which it catches its prey. When a fly has been trapped, the tentacles close upon it, the edges of the leaf curve inward, making a sort of stomach, from the glands of which an acid juice exudes and digests the meal. After a number of days, varying according to the digestibility of the diet, the blades slowly unfold again and are ready for another capture.

The bladderwort, common in pools and still waters nearly everywhere, has its petioles transformed into floats, while the finely dissected, rootlike blades bear little bladders which, when examined under the microscope, are found to contain the decomposed remains of captured animalculæ.

Practical Questions

1. Can you find any kind of leaf that is not preyed upon by
something? If so, how do you account for its immunity?

2. Make a list of some of the most striking of the protected leaves
of your neighborhood.

3. What is the nature of the protective organ in each case?

4. For protection against what does it seem to be specially adapted?

5. Are the plants in your list for the most part useful ones, or
troublesome weeds?

6. Examine the leaves of the worst weeds that you know of and see
if these will help in any way to account for their persistency.

Field Work

(1) In connection with Sections I and II, observe the effect of the
lobing and branching of leaves in letting the sunlight through.
Notice any general differences that may appear as to shape, margin,
and texture in the leaves of sun plants, shade plants, and water
plants, and account for them. Study the arrangement of leaves on
stems of various kinds, with reference to the size and shapes of
leaves and their light relations. Consider the value of the various
kinds of foliage for shade; for ornament; as producers of moisture;
as food; as insect destroyers, etc.

Make a special study of the twelve principal deciduous trees of
your neighborhood. Compare the leaves, bark, and branches of the
same trees so that you will be able to recognize them by any one of
these means alone.

(2) In connection with Sections III and V, consider the effects
upon soil moisture of transpiration from the leaves of forest
trees and from those of shallow-rooted herbs and weeds that draw
their water supply from the surface. Consider the value of forests
in protecting crops from excessive evaporation by acting as wind
breaks. Study the effect of the fall of leaves upon the formation
of soil. In any undisturbed forest tract turn up a few inches of
soil with a garden trowel and see what it is composed of. Notice
what kind of plants grow in it. Note the absence of weeds and
account for it. Compare the appearance of trees scattered along
windy hillsides, where the fallen leaves are constantly blown away,
or in any position where the soil is unrenewed, with those in an
undisturbed forest, and then give an opinion as to the wisdom of
hauling away the leaves every year from a timber lot.

(3) In Section VII, observe, in different kinds of leaf mosaics,
the means by which the adjustment has been brought about and the
purpose it subserves. Make a list of plants illustrating the two
habits. Notice the form and position of petioles of different
leaves, and their effect upon light exposure, drainage, etc., and
the behavior of the different kinds in the wind. Look for compass
plants in your neighborhood, and for other examples of adjustment
to heat and light. Study the position of leaves at different times
of day and in different kinds of weather and note what changes
occur and to what they are due.

Make a list of ten plants that seem to you to have best worked out
the problem of leaf adjustment, giving the reasons for your opinion.

Study the drainage system of different plants and observe whether
there is any general correspondence between the leaf drainage and
the root systems. This will lead to interesting questions in
regard to irrigation and manuring. Where plants are crowded, the
growth of both roots and leaves is complicated with so many other
factors that it is best to select for observations of this sort
specimens growing in more or less isolated situations.

Notice the time of the expansion and shedding of the leaves of
different plants, and whether the early leafers, as a general
thing, shed early or late; in other words, whether there seems to
be any general time relation between the two acts of leaf expansion
and leaf fall.

(4) Under Section VIII, look for instances of modified leaves;
study the nature of the different modifications you find, and try
to understand their meaning and object. Make a collection (_a_)
of all the leaves you can find modified to serve other than their
normal purposes; (_b_) of all the organs of other kinds that
have been modified to serve as leaves; (_c_) of all the modified
parts of leaves—stipules and petioles—that you can find. Keep the
collections separate, labeling each specimen with the name of the
plant it belongs to, what part it is, what use it serves, when and
where found. These collections need not be made individually, but
by the class as a whole and kept for the use of the school.

Observe also (_d_) the differences between young and old leaves of
the same kind, and the leaves of young and old plants or parts of
plants of the same kind; (_e_) resemblances between young leaves
belonging to plants of different species; (_f_) between young
leaves of one species and mature ones of one or more different
species. Make a collection of all the specimens you can find
illustrating the three points mentioned, referring each to its
proper head, and giving the name and relative age—old or young—of
all specimens collected.