Elements of Structural and Systematic Botany Part 7

Besides zoospores there are also resting spores developed. Oogonia like those of _Vaucheria_ or the _Peronosporeae_ are formed usually after the formation of zoospores has ceased; but in many cases, perhaps all, these develop without being fertilized. Antheridia are often wanting, and even when they are present, it is very doubtful whether fertilization takes place.[5]

[5] The antheridia, when present, arise as branches just below the oogonium, and become closely applied to it, sometimes sending tubes through its wall, but there has been no satisfactory demonstration of an actual transfer of the contents of the antheridium to the egg cell.

The oogonia (Fig. 36, _G_, _H_) arise at the end of the main filaments, or of short side branches, very much as do the sporangia, from which they differ at this stage in being of globular form. The contents contract to form one or several egg cells, naked at first, but later becoming thick-walled resting spores (_H_).

CHAPTER IX.

THE TRUE FUNGI (_Mycomycetes_).

The great majority of the plants ordinarily known as _fungi_ are embraced under this head. While some of the lower forms show affinities with the _Phycomycetes_, and through them with the algae, the greater number differ very strongly from all green plants both in their habits and in their structure and reproduction. It is a much-disputed point whether s.e.xual reproduction occurs in any of them, and it is highly probable that in the great majority, at any rate, the reproduction is purely non-s.e.xual.

Probably to be reckoned with the _Mycomycetes_, but of doubtful affinities, are the small unicellular fungi that are the main causes of alcoholic fermentation; these are the yeast fungi (_Saccharomycetes_).

They cause the fermentation of beer and wine, as well as the incipient fermentation in bread, causing it to "rise" by the giving off of bubbles of carbonic acid gas during the process.

If a little common yeast is put into water containing starch or sugar, and kept in a warm place, in a short time bubbles of gas will make their appearance, and after a little longer time alcohol may be detected by proper tests; in short, alcoholic fermentation is taking place in the solution.

If a little of the fermenting liquid is examined microscopically, it will be found to contain great numbers of very small, oval cells, with thin cell walls and colorless contents. A careful examination with a strong lens (magnifying from 500-1000 diameters) shows that the protoplasm, in which are granules of varying size, does not fill the cell completely, but that there are one or more large vacuoles or s.p.a.ces filled with colorless cell sap. No nucleus is visible in the living cell, but it has been shown that a nucleus is present.

If growth is active, many of the cells will be seen dividing. The process is somewhat different from ordinary fission and is called budding (Fig. 37, _B_). A small protuberance appears at the bud or at the side of the cell, and enlarges rapidly, a.s.suming the form of the mother cell, from which it becomes completely separated by the constriction of the base, and may fall off at once, or, as is more frequently the case, may remain attached for a time, giving rise itself to other buds, so that not infrequently groups of half a dozen or more cells are met with (Fig. 37, _B_, _C_).

[Ill.u.s.tration: FIG. 37.--_A_, single cells of yeast. _B_, _C_, similar cells, showing the process of budding, 750.]

That the yeast cells are the princ.i.p.al agents of alcoholic fermentation may be shown in much the same way that bacteria are shown to cause ordinary decomposition. Liquids from which they are excluded will remain unfermented for an indefinite time.

There has been much controversy as to the systematic position of the yeast fungi, which has not yet been satisfactorily settled, the question being whether they are to be regarded as independent plants or only one stage in the life history of some higher fungi (possibly the _s.m.u.ts_), which through cultivation have lost the power of developing further.

CLa.s.s I.--THE s.m.u.tS (_Ustillagineae_).

The s.m.u.ts are common and often very destructive parasitic fungi, living entirely within the tissues of the higher plants. Owing to this, as well as to the excessively small spores and difficulty in germinating them, the plants are very difficult of study, except in a general way, and we will content ourselves with a glance at one of the common forms, the corn s.m.u.t (_Ustillago maydis_). This familiar fungus attacks Indian corn, forming its spores in enormous quant.i.ties in various parts of the diseased plant, but particularly in the flowers ("ta.s.sel" and young ear).

The filaments, which resemble somewhat those of the white rusts, penetrate all parts of the plant, and as the time approaches for the formation of the spores, these branch extensively, and at the same time become soft and mucilaginous (Fig. 38, _B_). The ends of these short branches enlarge rapidly and become shut off by part.i.tions, and in each a globular spore (Fig. 38, _C_) is produced. The outer wall is very dark-colored and provided with short spines. To study the filaments and spore formation, very thin sections should be made through the young kernels or other parts in the vicinity, before they are noticeably distorted by the growth of the spore-bearing filaments.

[Ill.u.s.tration: FIG. 38.--_A_, "ta.s.sel" of corn attacked by s.m.u.t (_Ustillago_). _B_, filaments of the fungus from a thin section of a diseased grain, showing the beginning of the formation of the spores, 300. _C_, ripe spores, 300.]

As the spores are forming, an abnormal growth is set up in the cells of the part attacked, which in consequence becomes enormously enlarged (Fig. 38, _A_), single grains sometimes growing as large as a walnut.

As the spores ripen, the affected parts, which are at first white, become a livid gray, due to the black spores s.h.i.+ning through the overlying white tissues. Finally the ma.s.ses of spores burst through the overlying cells, appearing like ma.s.ses of soot, whence the popular name for the plant.

The remaining _Mycomycetes_ are pretty readily divisible into two great cla.s.ses, based upon the arrangement of the spores. The first of these is known as the _Ascomycetes_ (Sac fungi), the other the _Basidiomycetes_ (mushrooms, puff-b.a.l.l.s, etc.).

CLa.s.s II.--_Ascomycetes_ (SAC FUNGI).

This cla.s.s includes a very great number of common plants, all resembling each other in producing spores in sacs (_asci_, sing.

_ascus_) that are usually oblong in shape, and each containing eight spores, although the number is not always the same. Besides the spores formed in these sacs (ascospores), there are other forms produced in various ways.

There are two main divisions of the cla.s.s, the first including only a few forms, most of which are not likely to be met with by the student.

In these the spore sacs are borne directly upon the filaments without any protective covering. The only form that is at all common is a parasitic fungus (_Exoascus_) that attacks peach-trees, causing the disease of the leaves known as "curl."

All of the common _Ascomycetes_ belong to the second division, and have the spore sacs contained in special structures called spore fruits, that may reach a diameter of several centimetres in a few cases, though ordinarily much smaller.

Among the simpler members of this group are the mildews (_Perisporiaceae_), mostly parasitic forms, living upon the leaves and stems of flowering plants, sometimes causing serious injury by their depredations. They form white or grayish downy films on the surface of the plant, in certain stages looking like h.o.a.r-frost. Being very common, they may be readily obtained, and are easily studied. One of the best species for study (_Podosphaera_) grows abundantly on the leaves of the dandelion, especially when the plants are growing under unfavorable conditions. The same species is also found on other plants of the same family. It may be found at almost any time during the summer; but for studying, the spore fruits material should be collected in late summer or early autumn. It at first appears as white, frost-like patches, growing dingier as it becomes older, and careful scrutiny of the older specimens will show numerous brown or blackish specks scattered over the patches. These are the spore fruits.

[Ill.u.s.tration: FIG. 39.--_A_, spore-bearing filaments of the dandelion mildew (_Podosphaera_), 150. _B_, a germinating spore, 150. _C-F_, development of the spore fruit, 300. _ar._ archicarp. _G_, a ripe spore fruit, 150. _H_, the spore sac removed from the spore fruit, 150. _I_, spore-bearing filament attacked by another fungus (_Cicinn.o.bulus_), causing the enlargement of the basal cell, 150.

_J_, a more advanced stage, 300. _K_, spores, 300.]

For microscopical study, fresh material may be used, or, if necessary, dried specimens. The latter, before mounting, should be soaked for a short time in water, to which has been added a few drops of caustic-potash solution. This will remove the brittleness, and swell up the dried filaments to their original proportions. A portion of the plant should be carefully sc.r.a.ped off the leaf on which it is growing, thoroughly washed in pure water, and transferred to a drop of water or very dilute glycerine, in which it should be carefully spread out with needles. If air bubbles interfere with the examination, they may be driven off with alcohol, and then the cover gla.s.s put on. If the specimen is mounted in glycerine, it will keep indefinitely, if care is taken to seal it up. The plant consists of much-interlaced filaments, divided at intervals by cross-walls.[6] They are nearly colorless, and the contents are not conspicuous. These filaments send up vertical branches (Fig. 39, _A_), that become divided into a series of short cells by means of cross-walls. The cells thus formed are at first cylindrical, but later bulge out at the sides, becoming broadly oval, and finally become detached as spores (_conidia_). It is these spores that give the frosty appearance to the early stages of the fungus when seen with the naked eye. The spores fall off very easily when ripe, and germinate quickly in water, sending out two or more tubes that grow into filaments like those of the parent plant (Fig. 39, _B_).

[6] The filaments are attached to the surface of the leaf by suckers, which are not so readily seen in this species as in some others. A mildew growing abundantly in autumn on the garden chrysanthemum, however, shows them very satisfactorily if a bit of the epidermis of a leaf on which the fungus is just beginning to grow is sliced off with a sharp razor and mounted in dilute glycerine, or water, removing the air with alcohol. These suckers are then seen to be globular bodies, penetrating the outer wall of the cell (Fig. 40).

[Ill.u.s.tration: FIG. 40.--Chrysanthemum mildew (_Erysiphe_), showing the suckers (_h_) by which the filaments are attached to the leaf.

_A_, surface view. _B_, vertical section of the leaf, 300.]

The spore fruits, as already observed, are formed toward the end of the season, and, in the species under consideration at least, appear to be the result of a s.e.xual process. The s.e.xual organs (if they are really such) are extremely simple, and, owing to their very small size, are not easily found. They arise as short branches at a point where two filaments cross; one of them (Fig. 39, _C_, _ar._), the female cell, or "archicarp," is somewhat larger than the other and nearly oval in form, and soon becomes separated by a part.i.tion from the filament that bears it. The other branch (antheridium) grows up in close contact with the archicarp, and like it is shut off by a part.i.tion from its filament. It is more slender than the archicarp, but otherwise differs little from it. No actual communication can be shown to be present between the two cells, and it is therefore still doubtful whether fertilization really takes place. Shortly after these organs are full-grown, several short branches grow up about them, and soon completely envelop them (_D_, _E_). These branches soon grow together, and cross-walls are formed in them, so that the young spore fruit appears surrounded by a single layer of cells, sufficiently transparent, however, to allow a view of the interior.

The antheridium undergoes no further change, but the archicarp soon divides into two cells,--a small basal one and a larger upper cell.

There next grow from the inner surface of the covering cells, short filaments, that almost completely fill the s.p.a.ce between the archicarp and the wall. An optical section of such a stage (Fig. 39, _F_) shows a double wall and the two cells of the archicarp. The spore fruit now enlarges rapidly, and the outer cells become first yellow and then dark brown, the walls becoming thicker and harder as they change color. Sometimes special filaments or appendages grow out from their outer surfaces, and these are also dark-colored.

Shortly before the fruit is ripe, the upper cell of the archicarp, which has increased many times in size, shows a division of its contents into eight parts, each of which develops a wall and becomes an oval spore. By crus.h.i.+ng the ripe spore fruit, these spores still enclosed in the mother cell (ascus) may be forced out (Fig. 39, _H_). These spores do not germinate at once, but remain dormant until the next year.

[Ill.u.s.tration: FIG. 41.--Forms of mildews (_Erysiphe_). _A_, _Microsphaera_, a spore fruit, 150. _B_, cl.u.s.ter of spore sacs of the same, 150. _C_, a single appendage, 300. _D_, end of an appendage of _Uncinula_, 300. _E_, appendage of _Phyllactinia_, 150.]

Frequently other structures, resembling somewhat the spore fruits, are found a.s.sociated with them (Fig. 39, _I_, _K_), and were for a long time supposed to be a special form of reproductive organ; but they are now known to belong to another fungus (_Cicinn.o.bulus_), parasitic upon the mildew. They usually appear at the base of the chains of conidia, causing the basal cell to enlarge to many times its original size, and finally kill the young conidia, which shrivel up. A careful examination reveals the presence of very fine filaments within those of the mildew, which may be traced up to the base of the conidial branch, where the receptacle of the parasite is forming. The spores contained in these receptacles are very small (Fig. 39, _K_), and when ripe exude in long, worm-shaped ma.s.ses, if the receptacle is placed in water.

The mildews may be divided into two genera: _Podosphaera_, with a single ascus in the spore fruit; and _Erysiphe_, with two or more. In the latter the archicarp branches, each branch bearing a spore sac (Fig. 41, _B_).

The appendages growing out from the wall of the spore fruit are often very beautiful in form, and the two genera given above are often subdivided according to the form of these appendages.

A common mould closely allied to the mildews is found on various articles of food when allowed to remain damp, and is also very common on botanical specimens that have been poorly dried, and hence is often called "herbarium mould" (_Eurotium herbariorum_).

[Ill.u.s.tration: FIG. 42.--_A_, spore bearing filament of the herbarium mould (_Eurotium_), 150. _B_, _C_, another species showing the way in which the spores are borne--optical section-- 150. _D_, spore fruit of the herbarium mould, 150. _E_, spore sac. _F_, spores, 300. _G_, spore-bearing filament of the common blue mould (_Penicillium_), 300. _sp._ the spores.]

The conidia are of a greenish color, and produced on the ends of upright branches which are enlarged at the end, and from which grow out little prominences, which give rise to the conidia in the same way as we have seen in the mildews (Fig. 42, _A_).

Spore fruits much like those of the mildews are formed later, and are visible to the naked eye as little yellow grains (Fig. 42, _D_).

These contain numerous very small spore sacs (_E_), each with eight spores.

There are numerous common species of _Eurotium_, differing in color and size, some being yellow or black, and larger than the ordinary green form.

Another form, common everywhere on mouldy food of all kinds, as well as in other situations, is the blue mould (_Penicillium_). This, in general appearance, resembles almost exactly the herbarium mould, but is immediately distinguishable by a microscopic examination (Fig. 42, _G_).

In studying all of these forms, they may be mounted, as directed for the black moulds, in dilute glycerine; but must be handled with great care, as the spores become shaken off with the slightest jar.

Of the larger _Ascomycetes_, the cup fungi (_Discomycetes_) may be taken as types. The spore fruit in these forms is often of considerable size, and, as their name indicates, is open, having the form of a flat disc or cup. A brief description of a common one will suffice to give an idea of their structure and development.