Chapter X: Cryptogams (2)

=377. The spores.=—Notice the color of the spores as shown in the print. This is a matter of importance in distinguishing gill-bearing fungi, which are divided into five sections according to the color of the spores. One source of danger, at least, to mushroom eaters would be avoided if this difference was always attended to, for the deadly amanita (_Amanita phalloides_) and the almost equally dangerous fly mushroom (_A. muscaria_) both have white spores, while the favorite edible kind (_Agaricus campestris_), though white-gilled when young, produces dark, purple-brown spores that cannot fail to distinguish it clearly for any one who will take the trouble to make a print.

=378. Economic properties.=—Most of the wood-destroying fungi belong to this and allied orders. They are among the worst enemies the forester has to deal with (140), and millions of feet of lumber are destroyed every year by them.

Over seven hundred kinds of fungi growing in the United States have been described as edible, but the evil repute into which the whole class has been brought by the poisonous qualities of a few species, and the difficulty, to any but an expert, of distinguishing between these and the harmless kinds, has caused them to be generally neglected as articles of diet. While they are pleasant relishes and furnish an agreeable variety to our daily fare, their food value has been greatly exaggerated. They contain a large proportion of water, often over 90 per cent, and the most valued of them, the _Agaricus campestris_, is about equivalent to cabbage in nutrient properties.

Practical Questions

1. Why are mushrooms generally grown in cellars? (186, 343.)

2. Name any fungi you know of that are good for food or medicine or
any other purpose.

3. Name the most dangerous ones you know of.

4. Do you find fungi most abundant on young and healthy trees, or
on old, decrepit ones? Account for the difference. (141, 343, 378.)

5. Do you ever find them growing on perfectly sound wood anywhere?

6. Are they ever beneficial to a tree? (86.)

7. Is it wise to leave old, unhealthy trees and decaying trunks in
a timber lot?

IV. LICHENS

MATERIAL.—Specimens can be found almost everywhere, growing on
rocks, walls, logs, stumps, and trees. Some of the more common
kind are: _Parmelia_, recognizable by the shallow spore cups borne
on the upper surface of the thallus; _Cladonia_, by the little
stalked receptacles, like goblets, in which its spores are held;
_Physcia_, by its bright orange color. Where practicable, it is
well to have several different kinds for comparison. Iceland moss
(_Cetraria islandica_) can generally be obtained from the grocers,
and is a good example of an intermediate form between foliaceous
and fruticose lichens.

If the specimens are very dry, they will be too brittle to handle
conveniently, and should be moistened by soaking a short time in
water. This will render them quite flexible and also bring out the
green color more clearly.

=379. Examination of a typical specimen.=—The commonest kind of lichens, and generally the most easily obtained, are those that grow on rocks and tree trunks in flat, spreading patches. Their margins are much dented and curled, giving them a somewhat leaflike appearance, whence they are called “foliaceous” lichens. This broad, expanded body is the thallus, or vegetative part, as distinguished from its reproductive part. Examine carefully the thallus of your specimen. Note the size and shape of the indentations. Is there any order or regularity about them, such as was observed in the lobing of leaves? Is there any difference in color between the upper and under sides? What other differences do you notice? Do you see anything like hairs, or rootlets, on the under side? Mount one of them in water and place under the microscope. What does it look like? Compare with one of the hairs from a leaf of mullein, gromwell, blueweed, or other hairy plant, with the hypha of a fungus mycelium, and with your study of the root hair in 67 (_a_). Is it a hair or a root? These rootlike hairs are called _rhizoids_, and serve to anchor the lichen to its substratum. Look on the upper side for little cup-shaped or saucer-shaped receptacles. On what part of the thallus are they situated? Examine with a lens and see if you can make out what they contain. These cups are the spore cases. The lichen fungus belongs to the division of sac fungi, which produce their spores in closed sacs, or cups.

=380. Structure of the thallus.=—Make a thin section through a thallus and place under the microscope. Notice the small green bodies enveloped in the hyphæ of the fungus. Are they most abundant near the upper or the lower epidermis? Has their green color anything to do with this, and with the difference in color between the two surfaces of the thallus? (184.) Do they look like chlorophyll granules? Can you tell what they are? Compare with your study of the unicellular algæ (337) and with Fig. 429. Does this throw any light on their real nature?

=381. The lichen thallus a composite body.=—You will probably have no difficulty in making out that these small round bodies are green algæ of some kind, but of what species will depend upon the kind of lichen with which it is associated. In Cladonia and the bearded lichen (Fig. 473), it is a protococcus; in other forms, a pleurococcus or a nostoc—and so on, each species of lichen fungus being specialized to a certain form of alga. The great botanist, De Bary, showed that it is even possible to produce a lichen thallus artificially by sowing the spores of a fungus among the cells of the particular alga with which it is able to unite. The spores will germinate without the alga, but soon perish unless they come in contact with the right one. It is thus made clear that the lichen plant as a whole is a combination of elements belonging to two distinct orders, the algæ and fungi, but so closely associated as to constitute practically a single individual.

=382. Slavery, or partnership?=—Now, what can be the object of this peculiar association? Is it a symbiosis, or a case of enslavement? The fungi, as we know, are all parasites, unable to manufacture their own food or to exist at all except at the expense of other organisms, living or dead. But the lichens have refined upon the gross rapacity of their order, and instead of indiscriminately destroying the hosts that furnish their nourishment, have used their victims to better purpose by converting them into contented, well-fed slaves! The imprisoned algæ perform for them the same service that the chlorophyll bodies do for the higher plants, and so the lichen fungi have the advantage of other parasites in getting their food manufactured at home, so to speak. And while the algæ have to do double work in order to feed both themselves and their masters, the fungus, in return, shelters them against cold and drought, and prolongs their growing period by giving them a more continuous supply of moisture and food materials, which it draws from the substratum by means of its rhizoids. In this way both plants are enabled to live in situations that neither could occupy without the other.

=383. Reproduction.=—The multiplication of the lichen algæ is exclusively vegetative. The fungus, on the other hand, reproduces normally by spores, and the fruiting bodies found on the thallus originate from the fungus mycelium.

=384. Classification.=—To be strictly accurate, the two kinds of vegetable bodies that make up the lichen thallus would probably have to be classified separately, as algæ or fungi, respectively, but as fructification is the generally accepted basis of classification, and the plant body is too intimately permeated with both kinds of tissue to be divided, each lichen body as a whole is classed with its particular kind of fungus. The entire group, on account of the distinctive characters that mark it, is placed in a separate order of its own. This includes three principal divisions, distributed according to the shape of the thallus, and its habit of growth: (1) _Crustaceous_, those that adhere closely to the substratum, as if glued or inscribed on it; (2) _Foliaceous_, with a broad, more or less lobed and leaflike thallus that adheres loosely to the substratum by means of rhizoids springing from its under surface; (3) _Fruticose_, with branching, stemlike thallus attached at the base like a regularly rooting plant (Figs. 473, 474). Among these are the Iceland moss, used as an article of food by man, and the reindeer moss (_Cladonia rangiferina_), which is the chief sustenance of the reindeer.

Practical Questions

1. Have lichens any economic value? (384.)

2. In what way are they most useful? (320.)

3. Do you find them, as a general thing, on healthy young trees and
boughs, or on old ones, and those showing signs of decay?

4. Do you ever find them growing on trees or other objects in
densely inhabited areas,—cities, large towns, and manufacturing
centers?

5. Do they grow more thickly on the shady (northern) side of
rocks, walls, and trees growing in the open, than on the sunny and
(presumably) warmer sides?

6. Mention some ways in which a growth of lichens might be
beneficial to a tree.

7. In what ways could it be harmful?

V. LIVERWORTS

MATERIAL.—Liverworts can generally be found growing with mosses
in damp, shady places, and are easily recognized by their flat,
spreading habit, which gives them the appearance of green lichens.
_Marchantia polymorpha_ (Fig. 475), one of the largest and best
specimens for study, is common in shady, damp ground throughout
the states. It is diœcious, and specimens bearing both male and
female organs should be provided. _Lunularia_, a smaller species
that can be recognized by the little crescent-shaped receptacles on
some of the divisions of the thallus, is abundant in greenhouses on
the floor, or on the sides of pots and boxes kept in damp places;
but the spore-bearing receptacles are seldom or never present, the
species being an introduced one and possibly rendered sterile by
changed conditions. _Anthoceros_ (Fig. 426) and leafy liverworts,
such as that shown in Fig. 484, also make good examples for study.

EXPERIMENT 97. WHY ARE THE UPPER AND UNDER SIDES OF A LIVERWORT
DIFFERENT?—Plant a growing branch of marchantia, or of any flat,
spreading liverwort, in moist earth so that the upper side will lie
next the soil, and watch for a week or two, noting the changes that
take place. What would you infer from these as to the cause of any
differences that may have been observed between the two surfaces?

=385. Examination of a typical liverwort=—The thallus.—The broad, flat, branching organ that forms the body of the plant is the thallus. Examine the end of each branch; what do you find there? Are the two forks into which the apex of the branches divides equal or unequal? Compare the growing end with the distal one; does it proceed from a true root? Notice that as the lower end dies, the growing branches go on increasing and reproducing the thallus.

Do you find anything like a midrib? If so, trace it through the branches and body of the thallus; where does it end? Does it seem to be formed like the midrib of a leaf? Hold a piece of the thallus up to the light and see if you can detect any veins. Is it of the same color in all parts, and if there is a difference, can you give a reason for it? Examine the upper surface with a lens. Peel off a piece of the epidermis, place it under a low power of the microscope, or between two moistened bits of glass, and hold up to the light, keeping the upper surface toward you; what is its appearance? Observe a tiny dot near the center of the rhomboidal areas into which the epidermis is divided and compare it with your drawings of stomata (181, 183). What would you judge that these dots are for? While differing in structure from the stomata of leaves, they serve the same purposes and may be regarded a more rudimentary form of the same organ.

=386. Rhizoids.=—Wash the dirt from the under side of a thallus and examine with a lens; how does it differ from the upper surface? Do you see anything like roots? Place one in a drop of water under the microscope. Compare with similar organs found on the lichen (379). What are they? Would rhizoids be of any use on the upper side? stomata on the under side?

=387. Gemmæ.=—Look along the upper surface for little saucer-shaped (in lunularia, crescent-shaped) cupules (_g_, _g_, Fig. 476). Notice their shape and position, whether on a midrib or near the margin. Examine the contents with a lens and see if you can tell what they are. These little bodies, called _gemmæ_, are of the nature of buds, by which the plant propagates itself vegetatively somewhat as the onion and the tiger lily do by means of bulblets. Sow some of the gemmæ on moist sand, cover them with a tumbler to prevent evaporation, and watch them develop the thalloid structure.

=388. The fruiting receptacles.=—Procure, if possible, thalli with upright pedicels bearing flattened enlargements at the top (Figs. 475, 476). These are thallus branches modified into receptacles containing the reproductive organs, which, in marchantia, are diœcious, the two kinds growing on separate thalli. Notice their difference in shape, one kind being slightly lobed or scalloped, the other rayed like the spokes of a wheel. The first kind are known as _antheridial_, or male, receptacles; the second as _archegonial_, or female.

=389. The antheridia.=—Examine one of the male receptacles on both surfaces and in vertical section. Notice the tiny egg-shaped bodies sunk in little cavities between the lobes just under the upper epidermis (Fig. 478). These are antheridia. When mature, they rupture at the apex, and multitudes of extremely small bodies, called _antherozoids_, or _spermatozoids_, are discharged from them.

=390. Archegonia.=—Next examine one of the female receptacles. Look on the under surface, between the narrow divisions of the receptacle, for radiating rows of flask-shaped bodies with their necks turned downward, and all surrounded by a toothed sheath or involucre (Fig. 479). These bodies are the archegonia, or female organs, and correspond, loosely speaking, to the ovaries of flowering plants. If the receptacle is a mature one, the archegonia will be replaced by the ripe spore cases (_sporangia_), as at _f_, Fig. 479.

Make enlarged drawings of the upper surface of a male and a female receptacle, and of a vertical section of each, passing through an antheridium in the male, and an archegonial row in the female receptacle. Label the parts observed in each.

=391. Minute study of an archegonium.=—Place under the microscope a very thin, longitudinal section through a ray of a receptacle containing a young archegonium, and observe that the latter consists of a lower portion, the _venter_, _v_, Fig. 480, and an upper part, the neck, which is perforated by the _neck canal_, _ca_. The venter contains the _egg cell_, _o_, and the ventral canal cell, _vc_. The neck canal is filled with small cells which, at maturity, dissolve into a mucilaginous substance that swells on being wet and discharges itself through the top of the neck, leaving an open passage to the venter, where the egg cell is ready to be fertilized.

Make a drawing of the section as seen under the microscope, labeling all the parts.

=392. Fertilization.=—In the liverworts, and in cryptogams generally, this process has to take place under water, as the antherozoids are motile only in a liquid, but the amount required is so small that a few drops of rain or dew will enable them to make their journey to the archegonium. The mucilaginous substances discharged from the neck canal attract them to the mouth of the opening, one or more of them penetrates to the egg cell, and fertilization is accomplished. Do you see any analogies between this and the same function among flowering plants? (250, 251.)

=393. The spore case.=—After fertilization the egg becomes an _oöspore_, capable of producing a new plant. Instead, however, of separating from the mother plant and giving rise to an independent growth, it germinates within the archegonium and produces there a small, stalked body, called a _sporogonium_, or _sporophyte_, which at length ripens into a spore case, as shown at _f_, Fig. 479. At maturity this capsule-like sporophyte ruptures at the apex, and discharges a mass of spores, mingled with elongated filaments called _elators_, which, by their elastic movements, assist in disseminating the spores. These latter, on germinating, produce, not a simple sporophyte like that which bore them, but the thallus of the liverwort with all its complicated arrangement of antheridia and archegonia and vegetative organs that seem to foreshadow, by the analogies they suggest, the coming of the higher plants.

=394. Sexual and asexual reproduction.=—We find here a very marked change from the simple reproductive processes observed in the algæ and fungi. In the forms thus far considered, this function was carried on mainly by simple vegetative fission or budding, with a more or less irregular intervention of resting spores. If only one kind of spore is concerned, reproduction is said to be _asexual_. When two different kinds of cells, the egg and sperm cell, unite to form an oöspore, as in the liverworts, reproduction is said to be _sexual_. While sexual reproduction takes place to some extent among both algæ and fungi, the prevailing method among thallophytes is asexual, and may be carried on in three different ways: by fission (and budding), by resting spores, and by conjugation.

Representing the plant body by _A_ and the resting spores by _a_, the primitive asexual processes may be expressed to the eye by the accompanying formulas:—

(1) Fission and budding: _A_ → _A_ → _A_ → _A_ →
(2) Resting spores: _A a_ → _A a_ → _A a_ →
(3) Conjugation: _A_ + _A_ → _a_ → _A_ + _A_ → _a_ →

In (3), as was seen in the conjugating cells of the spirogyra (342), the method is a little more complicated, showing an approach toward the sexual process. In each of these cases, however, there is only one kind of cell concerned, while in the liverworts there are not only different kinds, technically known as _gametes_, but specialized organs, archegonia and antheridia, for producing them. The thallus body bearing these organs is termed the _gametophyte_, because it bears the gametes, or sexual organs,—the suffix _phyte_ meaning a plant; for example, _epiphyte_, on or upon plants; _spermophyte_, or _spermatophyte_, seed plant; _sporophyte_, spore plant. The _sporophyte_, produced within the archegonium, bears simple nonsexual spores that are capable of germinating independently. Structurally it is a separate, individual organism, though it does not appear as such in this class, but lives inclosed in the archegonium, as a parasite on the mother plant.

=395. Alternation of generations.=—If we represent the sporophyte by _S_, the thallus, or gametophyte, by _G_, the female gamete, or egg cell, by _fg_, the antherozoids (male gametes) by _mg_, the fertilized egg cell, or oöspore, resulting from their union by _oös_, and the asexual spores discharged from the sporophyte by _o_, this complicated mode of reproduction may be expressed diagrammatically as follows:—

_fg_ _fg_
╱ ╲ ╱ ╲
_G_ _oös_→_s_→_o_→_G_ _oös_→_s_→_o_→_G_→etc.
╲ ╱ ╲ ╱
_mg_ _mg_

A glance at the diagram will show a continual interchange of the sexual and asexual modes of reproduction, in which each generation gives rise to its _opposite_, the asexual sporophyte producing the sexual gametophyte, and this in turn, through its gametes, giving rise to the asexual sporophyte. This regular recurrence in genealogical succession of two differing forms is what is meant by the expression “alternation of generations.” Analogous processes occur also among some of the thallophytes, but as there is no well-defined differentiation of sporophyte and gametophyte, alternation proper may be regarded as beginning with the bryophytes. The subject is a complicated one and somewhat difficult to grasp, but it is important to form a correct idea of it and to fix clearly in mind the different modes of reproduction as we proceed from the lower to the higher forms of vegetation, since in this way alone can their biological relationships and their order of succession in the evolutionary scale be made intelligible.

VI. MOSSES

MATERIAL.—One of the most widely distributed of mosses is the
Sphagnum, or peat moss, so generally used by florists in packing
plants for shipment, and it can be obtained from them at almost all
times. It is rather difficult, however, to find specimens with the
fruiting organs, since they are rarely to be met with except in
late autumn or early spring. Other common forms are _Polytrichum_,
_Funaria_, and _Mnium_, any of which will meet all essential
conditions of the study outlined in the text.

=396. The protonema or thallus stage.=—In mosses the sexual, or gametophyte generation differs from that of liverworts in undergoing two phases. The germinating cells of the sporophyte do not develop immediately into the leafy stem, which is the typical gametophyte of true mosses, but produce first a filamentous, creeping structure called the _protonema_ (Fig. 483), that spreads over the ground and forms the tangled green felt usually observed where mosses are growing. Place a few of these filaments on a slide in water, and examine under the microscope. Do they remind you of any of the forms of algæ? Look near the base of the branches for knots or enlargements, like those seen at _kn_, Fig. 483. These are buds from which the leafy moss stems will develop. Do they correspond to anything observed among the thallophytes? Notice the rootlike filaments that extend under ground; how do they differ from the ones above ground? Why are they colorless? How do you know that they are not true roots? [67 (_a_), 379.] Sketch one of each kind of filament sufficiently enlarged to show the cells composing it.

A protonema that arises directly from the spore is said to be _primary_, while those which sometimes spring from rhizoids above ground, or from stems or leaves, are _secondary_. The fact that a protonema can bud from parts of the fruiting stems shows that the two do not belong to different generations, but are merely successive stages of a single generation, and both together compose the gametophyte.

=397. The leafy stage.=—In their fully developed state the true mosses show a marked advance in organization over the liverworts. There is a distinct differentiation of the growing axis into stem and leaves, though no true roots are formed. The leaves are arranged spirally, on upright stems, while in the liverworts the vegetative body is either a flat, spreading thallus, or the leaves are arranged horizontally on opposite sides of a prostrate, or more or less inclined, axis. Sometimes a second set occurs, on the upper side of the axis, but in this case the leaves are usually much smaller and inclined to the horizontal arrangement, as shown in Fig. 484.

=398. The reproductive organs.=—The antheridia and archegonia are borne in groups at the end either of the main axes, or of lateral branches (Figs. 485, 486), but as a rule only one archegonium is fertilized, so the mature sporogonia are solitary. The plants may be either diœcious or monœcious, as in Fig. 485; and in the latter case, the reproductive organs may be borne on the same, or on different, receptacles. The antheridia and the archegonia are both mixed with club-shaped hairs called paraphyses (Fig. 485).

=399. The sporophyte.=—An examination of the fruiting capsule of any of the true mosses will show that it consists of a long footstalk, the _seta_, _s_, Fig. 486, bearing a capsule, or ripened sporogonium, _f_, which is at first surmounted by a cap or hood, known as the _calyptra_, _c_. The hood represents the excessively developed and often highly specialized wall of the archegonium. It falls away at maturity, and the spores are discharged through an opening made by the removal of the _operculum_, or lid, _d_. The spores and the capsule are both developed from the fertilized egg (oöspore), within the archegonium, in much the same manner as in the liverworts, and together constitute the sporophyte, or asexual generation. It never leads a completely independent existence, but remains a partial parasite on the mother plant, though the lower part of the young sporogonium is usually provided with stomata and chlorophyll so that it is capable of manufacturing food. In this respect it shows a distinct advance on the corresponding phase of the liverworts—if we except the single genus _Anthoceros_, which alone among the liverworts has the cells of the sporogonium provided with chlorophyll.

=400. Alternation of generations.=—The process of reproduction in mosses is so closely similar to that of liverworts that it is unnecessary to repeat the details. There are some minor variations, but in all essentials the processes are the same and may be represented to the eye by the same formula.

=401. Relative position of mosses and liverworts in the line of evolution.=—Though mosses, as a rule, show a higher degree of organization than liverworts, in both generations, their development has been _away_ from the general course of evolution followed by the higher plants. This, as will be seen later, tends towards a decreasing complexity of the gametophyte with increasing complexity of the sporophyte, while the mosses show increasing complexity of _both_. Like the order of birds in the animal kingdom, they form a highly specialized and somewhat isolated group. While they may be regarded as descendants from a common ancestral stock with the ferns and club mosses, they have been switched off, so to speak, on a side track of the great evolutionary trunk line, and their advance on this side track has carried them to a point more remote from the course along which the higher forms of plant life have traveled than the distant junction at which they branched off from their less progressive kindred, the humble liverworts.

VII. FERN PLANTS

MATERIAL.—Any kind of fern in the fruiting stage. Several different
varieties should be cultivated in the schoolroom for observation.
While gathering specimens, look along the ground under the fronds,
or in greenhouses where ferns are cultivated, among the pots and
on the floor, for a small, heart-shaped body like that represented
in Figs. 501, 502, called a _prothallium_. It is found only in
moist and shady places, and care should be taken in collecting
specimens, as in their early stages the prothallia bear a strong
resemblance to certain liverworts found in the same situations. The
best way is for each class to raise its own specimens by scattering
the spores of a fern in a glass jar, on the bottom of which is a
bed of moist sand or blotting paper. Cover the jar loosely with a
sheet of glass and keep it moist and warm, and not in too bright a
light. Spores of the sensitive ferns (_Onoclea_) will germinate in
from two to ten days, according to the temperature. Those of the
royal fern (_Osmunda_) germinate promptly if sown as soon as ripe,
but if kept even for a few weeks are apt to lose their vitality.
The spores of sensitive fern can be kept for six months or longer,
while those of the bracken (_Pteris_) and various other species
require a rest before germinating, so that in these cases it is
better to use spores of the previous season.

=402. Study of a typical fern.=—Observe the size and general outline of the fronds, and note whether those of the same plant are all alike, or if they differ in any way, and how. Observe the shape and texture of the divisions or pinnæ composing the frond, their mode of attachment to the rachis, and whether they are simple, or notched, or branched in any way. Hold a pinna up to the light and notice the veining. Is it like any of the kinds described in 171, 172? In what respect is it different? This forked venation is a very general characteristic of ferns. When the forks do not reticulate or intercross, the veins are said to be free; are they free in your specimen, or reticulated? Make a sketch, labeling the primary branches of the frond, _pinnæ_ (sing., _pinna_), the secondary ones, if any, _pinnules_, and the common stalk that supports them, _stipe_. Note the color, texture, and surface of the stipe. If any appendages are present, such as hairs, chaff, or scales (in Pteris, nectar glands), notice whether they are equally distributed. If not, where are they most abundant?

Examine the mode of attachment of the stipes to their underground axis. Break one away and examine the scar. Compare with your drawings of leaf scars and with Fig. 105. Do the stipes grow from a root or a rhizome? How do you know? Do you find any remains of leafstalks of previous years? How does the rootstock increase in length? Measure some of the internodes; how much did it increase each year? Cut a cross section and look for the ends of the fibrovascular bundles. Trace their course through several internodes. Do they run straight, or do they turn or bend in any way at the nodes? If so, where do they go? Do you see anything like roots? Where do they originate? Put one of them under the microscope and find out whether they are roots or hairs.

True roots are first developed in the pteridophytes. Since those of the fern spring from an underground stem, to what class of roots do they belong? (83.)

=403. Minute study of a fern stem.=—Place a very thin section of a fern rhizoma, or of the stipe of a frond, under the microscope. Except in very young stems the vascular bundles are arranged in a ring, or sometimes in two or more rings (Fig. 492), with plates of strengthening tissue, _l_, _l_, between the inner and outer rings. Notice the inner epidermal layer of hard brown tissue, and within that, the soft parenchyma, which fills the rest of the interior. Test it with iodine and observe how rich in starch it is. If the section of a petiole is under observation, the details will be somewhat different; would you expect to find as much starch in the stipe as in the rootstock? Why, or why not?

Make a longitudinal section of a rhizome through the point where a leafstalk is attached and trace the course of the bundles. This will be facilitated if the specimen has stood in eosin solution a few hours. Make enlarged drawings of both sections, labeling all the parts.

Clearly differentiated conducting bundles occur in the mosses, but they are of much simpler structure than in the pteridophytes, consisting usually of a single central strand, and are found more frequently in the leaves than in the stems. A true vascular structure appears first in the pteridophytes, whence these plants are distinguished as _vascular cryptogams_.

=404. Fructification.=—Examine the back of a fruiting frond; what do you find there? These dots are the _sori_ (sing., _sorus_), or spore clusters, and the fronds or pinnæ bearing them are said to be _fertile_. Are there any differences of size, shape, etc., between the fertile and the sterile fronds of your specimen? between the fertile and the sterile pinnæ? On what part of the frond are the fertile pinnæ borne? Notice the shape and position of the sori, and their relation to the veins, whether borne at the tips, in the forks, on the upper side (toward the margin), or the lower (toward the midrib). Look for a delicate membrane (_indusium_) covering the sori, and observe its shape and mode of attachment. If the specimen under examination is a polypodium, there will be no indusium; if a maidenhair, or a bracken, it will be formed of the revolute margin of the pinna. In lady fern and Christmas fern (_Aspidium_), the sori frequently become confluent, that is, so close together as to appear like a solid mass. Sketch a fertile pinna as it appears under the lens, bringing out all the points noted.

=405. The spore cases.=—Look under the indusium at the cluster of little stalked circular appendages (Fig. 490). These are the _sporangia_, or spore cases, in which the reproductive bodies are borne. Place one of them under the microscope, and it will be found to consist of a little stalked circular body like a tennis racket (Fig. 491), surrounded by a jointed ring called the _annulus_. Watch a few moments and see if you can find out the use of the annulus. If not, warm the slide and you will probably see the ring straighten itself with a sudden jerk, rupturing the wall of the sporangium and discharging the spores with considerable force. If this does not happen, add a drop of strong glycerine to a specimen mounted in water; the rupture will be apt to follow quickly. What causes it, in either case? [56, (1); Exp. 19.]

=406. The sporophyte.=—The spores found in such abundance on the fertile pinnæ; are all alike, and each one is capable of germinating and continuing the work of reproduction as effectually as the sexual spores of the bryophytes. The fertile frond, or part of a frond, on which they are borne is called a _sporophyll_ (spore-bearing leaf), and the entire plant is the _sporophyte_, which, with its crop of spores, makes up one generation.

It is important to observe that in the ferns and in all pteridophytes the sporophyte is the conspicuous and highly organized body that is commonly recognized as the normal growing plant; while with the bryophytes just the reverse holds true,—the sexual generation, or gametophyte, represents the normal plant structure, while the sporophyte is an insignificant appendage which never attains an independent existence. Broadly speaking, in bryophytes, it is a spore fruit; in the pteridophytes and spermatophytes a highly developed plant.

=407. The gametophyte.=—When one of these asexual spores germinates, it produces, not a fern plant like the one that bore it, but a small, heart-shaped body like that shown in Fig. 501. Examine one of these bodies carefully with a lens. Observe that there are no veins nor fibrovascular bundles, and the whole body of the plant seems to consist of one uniform tissue. Compare it with the forked apex of a branching thallus of a liverwort. Do you perceive any points of similarity? The two are, in fact, morphologically the same. This heart-shaped body is called a _prothallium_, and is the gametophyte of the fern. It may be of different shapes, and in some species is branching and filamentous, like the protonema of a moss. Generally, however, it is flat and more or less two-lobed, as shown in Fig. 501. It is small and inconspicuous and very short-lived, being of importance only in connection with the work of reproduction.

Look with your lens for a cluster of small, bottle-shaped bodies just below the deep cleft in the heart. If you cannot make out what they are, put a thin section through a part of the prothallium containing one under the microscope, and you will see that they are the archegonia. Lower down among the rhizoids, near the pointed base, will be found the antheridia. In some species the prothalli are diœcious, one kind bearing antheridia, the other archegonia, but this is rare among the true ferns.

=408. Fertilization.=—This process is the same in all essentials as in the bryophytes. As in other cryptogams, it can take place only under water,—a circumstance which points to an aquatic origin for this sub-kingdom, and through them to the entire flora of the globe. The archegonia differ somewhat in shape from those of the liverworts and mosses, but a section under the microscope will show that they consist of essentially the same parts. On account of the similarity of these organs, the pteridophytes and bryophytes are often classed together as _Archegoniates_.

=409. Alternation of generations.=—Among the section of ferns that we have been considering, the order of alternation corresponds in all essentials to that prevailing among the bryophytes, and may be represented by the same formula. The chief difference is in the relatively much greater importance of the sporophyte, which may be expressed by putting it first:—

_fg_ _fg_
╱ ╲ ╱ ╲
_S_→_o_→_G_ _oös_→_S_→_o_→_G_ _oös_→_S_→_o_→_G_ etc.
╲ ╱ ╲ ╱
_mg_ _mg_

But some of the pteridophytes—of which the Selaginella offers a conspicuous example—have differentiated their asexual spores (_o_ of the formula) into two kinds, large and small, known respectively as _megaspores_ and _microspores_. The prothallia developed by the former bear archegonia containing female gametes only; those by the latter, antheridia containing male gametes—while in the diœcious bryophytes, the archegonial and antheridial thalli are produced by spores of the same kind.

The differentiation of the asexual spores in the higher pteridophytes gives rise to corresponding changes in the sporangia that bear them, and even in the sporophylls themselves, one kind bearing microsporangia only, the other megasporangia. In this way the differentiation of sex is pushed back, step by step, until it virtually begins with the sporophyte, or asexual generation.

Using the same terms as before, and representing the microspores by the abbreviation _mo_, the megaspores by _Mo_, the archegonial gametophyte by _arG_, the antheridial by _anG_, the formula may be modified to express this more complicated process of alternation, as follows:—

_Mo_→_arG_→_fg_ _Mo_→_arG_→_fg_
╱ ╲ ╱ ╲
_s_ _oös_→_S_ _oös_→_S_ etc.
╲ ╱ ╲ ╱
_mo_→_anG_→_mg_ _mo_→_anG_→_mg_

Comparing this formula with the preceding, it will be seen that the increased complexity affects the sporophyte at the expense of the gametophyte, which has now become a mere dependent on the former.

=410. Advantages of alternation.=—This roundabout mode of reproduction would hardly have been developed unless it had been of some benefit to the plants in which it occurs. The chief advantage seems to be in more rapid multiplication and consequently better chance to propagate the species, as compared with the slow process of sexual reproduction were the plant confined to that method alone. Only one plant is produced by each oöspore, and if this were a gametophyte with its limited number of archegonia, multiplication would be slow; but the sporophyte with its millions of spores, each capable of producing a new individual, enables the species to multiply indefinitely. At the same time the interposition of a gametophyte, or sexual generation, secures the introduction of a new strain with effects analogous to those of cross fertilization.

=411. Classification of pteridophytes.=—In our study of this group, the ferns have been taken as the type because they are the most familiar and most widely distributed of all the vascular cryptogams. But while they exceed in numbers, both of individuals and species, all the other orders combined, they form only one division of three great groups that make up the class Pteridophyta. These groups are: (1) ferns, under which are included, besides the true ferns, two widely differing orders, with the grape ferns and adder’s-tongue in one, and the water ferns in the other; (2) the club mosses, embracing the two subdivisions of _Lycopodium_ and _Selaginella_; (3) the horsetail family, including horsetails and scouring rushes. Orders (2) and (3) are grouped together as cone-bearing (strobilaceous) pteridophytes, because their sporangia are clustered in oblong heads, or _strobiles_ (Fig. 509), somewhat like the cones of the pine. The orders of pteridophytes differ greatly among themselves, but agree in possessing certain characteristics that point to their derivation from a common ancestry.

=412. Distinction between pteridophytes and bryophytes.=—In passing from the Thallophytes and Bryophytes to the vascular cryptogams, we cross the widest chasm in the vegetable kingdom—a gap relatively as great as that between vertebrates and invertebrates among animals. The most important modifications that discriminate the two groups are: (1) the presence in Pteridophytes of a highly organized vascular system accompanied by a well-marked differentiation of the plant body into root and stem; (2) increased importance and complexity of the sporophyte with proportionate diminution of the gametophyte.

While vessels for conducting water occur in some of the bryophytes (403), a well-defined vascular system and true roots are met with first in the Pteridophytes. The change in the relative importance of sporophyte and gametophyte is so marked that in Selaginella, the genus which approaches nearest in structure to the seed-bearing plants, the suppression of the gametophyte has proceeded so far that it never leads an independent existence at all and is difficult even to recognize as a distinct individual.

Practical Questions

1. Have ferns any economic use—that is, are they good for food,
medicines, etc.?

2. What is their chief value?

3. Under what ecological conditions do they grow?

4. Are they often attacked by insects, or by blights and disease of
any kind?

5. Of what advantage is it to ferns to have their stems
underground, in the form of rootstocks? (321.)

6. What causes the young frond of ferns to unroll? (54, 98.)

7. Name the ferns indigenous to your neighborhood.

8. Which of these are most ornamental, and to what peculiarities of
structure do they owe that quality?

9. Are cultivated ferns usually raised from the spores or in some
other way? Why?

10. After the great eruption of Krakatoa in 1883, by which the
vegetation of the island was completely destroyed, ferns were the
first plants to reappear. Explain why. (19; Exp. 17.)

VIII. THE RELATION BETWEEN CRYPTOGAMS AND SEED PLANTS

=413. No break in the chain of life.=—The great gap that was once supposed to exist between the cryptogams and phanerogams has been bridged over by the discovery of analogies in the reproductive processes of the two groups that connect them together as successive links in one continuous chain of vegetable life. It is therefore very important to have a clear understanding of the nature and meaning of these processes, for the chief turning points in the life history of the different groups of plants are connected with them, their natural relationships to each other, and their distribution according to their respective places in the evolutionary scale, being determined largely by a comparison of their modes of continuing the life of the group.

=414. Alternation of generations in seed plants.=—This process, so conspicuous among Bryophytes and Pteridophytes, and not unknown among Thallophytes, is universal among seed plants (Spermatophytes) also, though in so masked a form that it is not easy to recognize without a more detailed study than would be practicable within the limits of a book like this. Briefly, we may say that the stamens of spermatophytes, and the pistils, or rather the carpels, which we have seen to be transformed leaves (298), represent the sporophylls (406) of the higher pteridophytes. The pollen sacs and ovules are sporangia, bearing microspores and megaspores (409), represented respectively by the pollen grains in the anther and the embryo sac in the ovule. These go through a series of microscopic changes in the body of the ovule analogous to the production of the oöspore in the archegonia of ferns and liverworts, but the process is so obscure that to an ordinary observer the pollen grains and the ovule appear to be the real gametes, and were long supposed to be such. The fertilized germ cell in the embryo sac (251) corresponds to an oöspore; the embryo sac with the endosperm found in all seeds (previous to its absorption by the cotyledons) is a rudimentary gametophyte; and the embryo in the matured seed is the undeveloped sporophyte, destined, after germination and further growth, to produce a new generation with its recurrent cycle of alternating phases.

In the gymnosperms,—pines, yews, cycads, etc.,—which represent the most ancient and primitive type of existing seed-bearing plants, the similarity of these processes to those of certain of the pteridophytes is very striking, and it was through the study of these that the sequences of the process were traced in the much more obscure form in which they occur among the angiosperms. From the endosperm in the seeds of gymnosperms archegonia were found to be developed (Fig. 510) in much the same way as in Selaginella, from the prothallium, thus showing the endosperm to be a modified and greatly reduced gametophyte. In some cases, it has even been found to protrude a little way out of the embryo sac and to take on a slightly greenish tinge—another reminiscence of its origin. Fertilization, too, takes place in precisely the same manner as in the pteridophytes, except that in all but the ginkgo and the cycads, the fertilizing cells in the pollen grains are non-motile, and find their way to the ovule by growing down into the embryo sac with the pollen tube, instead of swimming to it—an adaptation probably brought about in response to changed condition during the course of evolution from aquatic to terrestrial life.

The analogies between the sequence of alternations in the two classes will be made clearer by a comparison of the accompanying diagrams. The corresponding terms applied to the various organs stand in the same vertical row. Diagram (1) shows the process as it takes place in the more highly developed Pteridophytes; diagram (2) the corresponding phases in angiosperms.

PTERIDOPHYTES

_mospl_→_mic_→_mo_→_anG_→_ant_→_mg_→
╱ ╲
(1) _S_ _öos_→_S_
╲ ╱
_Mospl_→_Mgc_→_Mo_→_arG_→_arc_→_fg_→

_mospl_, microsporophyll; _mic_, microsporangium; _mo_, microspores; _anG_, male gametophyte; _ant_, antheridia; _mg_, antherozoids. The letters in the lower line stand for the corresponding female organs.

SPERMATOPHYTES

_st_→_an_→_pol_→_fc_→ _not_ →_gc_→
╱ _developed_ ╲
(2) _S_ _öos_→_S_
╲ _developed_ ╱
_p_ →_ov_→_em_→_end_→ _only in_ →_ec_→
_gymnosperms_

_st_, stamen; _an_, anther; _pol_, pollen; _fc_, food cells in pollen grain; _gc_, generative cell; _p_, pistil; _ov_, ovules; _em_, embryo sac; _end_, endosperm; _ec_, egg cell.

=415. Disappearance of the gametophyte.=—The seed is a comparatively recent development in plant evolution. It has no counterpart anywhere among the cryptogams, but is strictly characteristic of the three great orders of Spermophytes: Monocotyl, Dicotyl, and Gymnosperms, which compose the greater part of the vegetation of the globe. Structurally, it is a matured sporangium containing a rudimentary sporophyte (the embryo), and a reduced gametophyte (the embryo sac), which, under the form of endosperm, has dwindled to an insignificance that makes it difficult to recognize it as a phase in an alternation of generations.

=416. Significance of the sporophyte.=—The gametophyte is obviously a more ancient and primitive structure than the sporophyte, which first becomes prominent in the ferns and their allies. The sudden and violent break in the succession of vegetable life that accompanies the appearance of the pteridophytes (412) is probably to be explained by the development of a land flora and the necessity of adaptation to life in a new medium. The fact that no living cell, whether vegetable or animal, can absorb nourishment except in a liquid form, seems to point to an aquatic origin more or less remote for all life. This inference is further strengthened, in the case of plants, by the fact that even in so highly organized a group as the pteridophytes, fertilization cannot take place except in water. Such a requirement would manifestly be a great disadvantage to land plants, and one of the first steps in response to the demands of a new habitat would be to get rid, as far as possible, of the primitive gametophyte with its outgrown adaptations to a liquid medium, and to transfer the greater part of the work of reproduction to the asexual generation, in which the problem of fertilization did not have to be directly met, the asexual spores germinating without it. The greater the number of these produced, the better the chance that at least some of the gametes developed from them would meet the difficult conditions of fertilization, and the survival of the species be assured. At the same time, in order to meet the requirements of terrestrial life successfully, and to provide for continuing the sexual generation, correlative changes would have to take place in the gametophyte by which the increasing uncertainty of fertilization due to structural changes in the sporophyte, and the absence of a liquid medium for the conveyance of free swimming antherozoids would be avoided. This necessity has been met by the development of the pollen tube, which bores its way to the egg cell, carrying with it the generative cells, which in seed plants have taken the place of the more primitive antherozoids. With the concomitant reduction of the gametophyte and development of the seed habit, the adaptation to land conditions has been made complete.

Roughly speaking, it may be said: (1) that Thallophytes are predominantly aquatic; (2) Archegoniates (Bryophytes and Pteridophytes), amphibious; (3) Spermophytes, terrestrial; (4) that the seed habit is a response to terrestrial conditions; and (5) that the increased development of the sporophyte was a necessary adaptation to meet those conditions.

IX. THE COURSE OF PLANT EVOLUTION

=417. Plant genealogy.=—It has been shown by a study of existing forms of plant life that there is no hard and fast line of division anywhere between the different groups, but that they are all connected by ties of kinship more or less defined, according to their distance from a common ancestral stock. The geological record points to the same conclusion, and our classification of them into families, orders, and species is merely a very imperfect genealogical table of their supposed pedigrees. This does not mean, however, that we can assert positively that such and such a species is derived from such or such another, but that both are descended from some common intermediate form more or less remote. While we have reason to believe that the flowering plants are derived through pteridophyte and bryophyte types from some of the green algæ, no direct connection has ever been traced between any particular kind of flowering plant and any particular kind of alga,—or between a liverwort and an alga, for that matter,—and probably never will be, because the intermediate forms die out, or pass on by variation into other lines of development. But while this is true, all the evidence we possess does go to show that, since the beginning of life on the globe, there has been a general progressive evolution from lower and simpler to higher and more complex forms.

=418. Retrogressive evolution.=—While the general course of evolution has been upward and onward, the movement has not always followed a straight line, but, like a mountain road, shows many windings and deviations from the direct route. The monocotyls furnish a conspicuous example of this departure from the general law of progression. It was formerly supposed, on account of their greater simplicity of structure, that they were a more ancient type than dicotyls, but recent investigations point to the conclusion that they are a later offshoot, derived from some primitive form of aquatic dicotyl, and represent, not an ancient and primitive stock, but a case of retrogressive evolution from a higher type. Strong presumptions in favor of this view are: (1) that various species of dicotyls show an unequal development of the seed leaves, amounting, in the bryony, to complete abortion of one of them, while some monocotyl seeds show morphological characters that can best be explained as survivals, or inheritances, from a dicotyl ancestor; (2) the structural resemblances between gymnosperms and dicotyls are closer than between gymnosperms and monocotyls, which could hardly be the case if the latter were the more ancient; (3) the geological record does not show them to have appeared before dicotyls; (4) the number of cotyledons furnishes no criterion as to the relative age of any plant group, since all three types are represented among the pteridophytes, where plants are found bearing one, two, or more cotyledons.

The theory of their comparatively recent origin from an aquatic ancestor is further borne out by the many points of similarity between their internal structure and that of hydrophytes (318), and also by the great proportion of aquatic plants among them, amounting to thirty-three per cent, while in dicotyls the proportion is only four per cent. Can you give any reasons, from your examination of their internal structure (113, 114), for believing that the line of development which they have followed is a less effective one for meeting conditions now existing on the globe than that attained by dicotyls?

We should remember, too, that while progressive evolution implies successful adjustment to surroundings, it is possible to conceive of a state, as our planet approaches the period of cosmic debility and decay, when the conditions of existence may become progressively more and more unfavorable. In this case the course of evolution would be reversed, the higher types gradually dying out as the struggle for life became more severe, and the tendency would be constantly toward lower and simpler forms, until finally all life would become extinct on our planet. We have no right, however, to assume that during such a course of retrogressive evolution the same forms would be repeated in reverse order as have already appeared, because there is no reason to believe that the conditions brought about by planetary decline and “old age” would be the same as those attending planetary birth and adolescence.