FUNGI OR MUSHROOMS.

185

epigone, and forms a sporangium or capsule; the epigone either remaining as a sort of sheath round the base of the sporangium, which is called the vaginule, or its upper part is carried upward as a sort of hood or styloid calyptra.

The sporangia vary much in different genera.

The spores have usually two coats, like pollen-cells, and the outer coat also frequently presents markings of different kinds; but in Marchantia the spores have but one coat. They mostly germinate without any intermediate prothallium, although some produce a kind of prothallium in the form of a confervoid mass, or protonema, like a Moss.

SECTION 2.

REPRODUCTIVE ORGANS OF THALLOPHYTES.

The Thallophytes may be divided into four large natural groups or orders, called respectively, Fungi, Lichens, Characeæ, and Algæ, in each of which again several subordinate divisions have been made.

1. Fungi or Mushrooms.-To give a detailed description of the various organs of reproduction occurring in the different sub-divisions of this order would be beyond the scope of this volume.

A few examples of the different methods by which reproduction takes place must suffice. Fungi are divided into the following groups, viz.: (i) Phycomycetes, (ii) Hypodermiæ, (iii) Basidiomycetes, and (iv) Ascomycetes.

(i) Phycomycetes. Cystopus candidus, a fungus which is commonly found growing upon cruciferous plants, is an example of this group. On examining a plant infested by Cystopus, it will be seen that the greatly elongated onecelled mycelium of the fungus is interwoven, as it were, among its cells, and draws nourishment from it by means of little rounded projections, known as haustoria. After vegetating for some time in this manner, erect branches grow out from the surface of the epidermis, from which conidia—that is, reproductive cells which are produced

asexually-are formed by a process of budding. From these conidia, when moistened with dew, rain, etc., zoospores are formed, and these settling down upon a similar plant will, under favorable circumstances, again develop the Cystopus mycelium.

But Cystopus can also produce zoospores by means of a sexual process, which takes place in the interior of its host.

(ii) Hypodermiæ.-Puccinia graminis, which we will take as the type of this group, is remarkable not only in showing a distinct alternation of generations; but also in the fact that each generation is developed upon a different host; and thus affording a good example of what has been called heterocism, or changing from host to host during different stages of development.

Thus, in the spring, the fungus may be seen in one phase of its existence growing on the Barberry, while in the summer, upon certain Grasses, fungous growths may be seen which have been developed from spores formed while the Puccinia was inhabiting the Barberry, and which in fact constitute the second generation.

If a section be made through one of the yellowish swellings seen on the leaf of a Barberry plant which is affected by the fungus, the whole tissue of the leaf at the spot in question will be found to be permeated by the mycelium of the Puccinia, while two kinds of fructification may be noticed, one on either side of the leaf. On the upper surface are somewhat rounded spaces, termed spermogonia, full of very delicate hair-like bodies, and from the floor of the cavity very small spore-like structures, the spermatia, are formed. On the under suface are the much larger acidium fruits or acidia. These consist of closelypacked vertical hyphæ, from which, by a process of continuous budding, a great number of conidia-like spores are detached. It is by the germination of these spores and their growth on Grasses that what are known as the uredofruits are produced.

THE COMMON MUSHROOM.

187

(iii) Basidiomycetes.-As an example of this group we will briefly describe what is known of the life history of the common Mushroom (Agaricus campestris). That which is ordinarly known as the Mushroom is in reality the receptacle, fructification, or spore-producing structure, growing from a mycelium (Fig. 227, my), which is vegetating underneath the surface of the ground, or other substance upon which the fungus may be growing. The receptacle, in the case which we are considering, consists of two parts, viz., the cap or pileus, p, and the stalk or stipe, st. The former may be regarded as the essential part of

vol

FIG. 227.-Vertical section of

the common Mushroom (Agaricus campestris). my. Mycelium. vol. Remains of volva. st. Stipe, an. Annulus. h. Hymenium with its gills or lamella, la. p. The pileus.

the receptacle, the spores being produced on its under surface, while the stalk simply serves the purpose of raising the pileus some distance

above the ground.

In the earlier stages of development the young receptacle consists of small, solid, somewhat pear-shaped bodies (Fig. 228), made up of a dense mass of hyphal tissue continuous with that of the mycelium, m. As growth proceeds in these bodies, an annular air cavity is formed near the upper part, the roof of which forms the under side of the pileus, and, growing rapidly in a transverse direction, ultimately becomes

covered by a number of closely set vertical folds placed

188

STRUCTURE OF CAP.

in a radiating direction from the center to the margin : these are the lamellæ or gills, and collectively constitute the hymenium (Fig. 227, la), upon which the spores are produced in a manner to be presently described. The growth of the cap gradually causes the floor of the cavity, known as the veil or indusium, to give way from the margin, so that it comes at last to hang from the stalk in the form of a fringe or annulus (Fig. 227, an).

In some species of Agaricus, as the present, the whole plant is entirely inclosed at first in a kind of veil or covering, called the volva, which ultimately becomes ruptured, and free from the tissue forming the membrane on the upper surface of the pileus; but its remains may be seen at the base of the stalk (Fig. 227, vol).

If a transverse section of one of the lamellæ of a ma

more

and

or less

rounded to

ward the pe

riphery, where

FIG. 229.-Transverse section of a lamella of the mature hyme- they form what nium of Agaricus campestris. t. Trama. sh. Sub-hymeni- is known al layer. g. Paraphyses. ', s", "". Basidia in different

as

stages of development, showing formation of spores. " the sub-hymeniBasidium after the spores have fallen off. (After Sachs.)

al layer, sh.

Placed upon and derived from this layer are the densely crowded club-shaped cells known respectively as the ba

THE ERGOT FUNGUS.

189

sidia, s', s'', s''', s'""', or paraphyses, q, according as they produce spores or remain sterile.

From each basidium, in this species, two spores are produced, the process of their development being as follows: On the free rounded surface of the basidial cell there first appear two little processes (Fig. 229, s′), which quickly become swollen at their extremities, s". The swelling in each instance increases, and finally a protoplasmic cell is produced, s'"', which ultimately becomes separated from the little stalk, s'''', and forms a spore. The spores, thus formed, when placed under favorable circumstances, are capable of producing the mycelium, or dense network of hyphæ, from which again the fructification or receptacle is developed. Judging from analogy, we should have expected the fructification to be the result of a sexual process taking place in the mycelium, thus giving rise to an alternation of generations, but from the latest researches on the subject this does not seem to be the case.

(iv) Ascomycetes.-As an example of this division of Fungi, Claviceps purpurea, or the Ergot Fungus, may be selected. If we trace the development of this Fungus upon the ovary of the affected Grass (Rye being the one more commonly selected), we find that it first produces what is known as the sphacelia. On examining a section of an ovary in this condition, it is seen to be almost completely surrounded by a dense mass of hyphal tissue, which also penetrates more or less into its interior, and gradually, in fact almost entirely, takes the place of the proper structure of the ovary—this being more particularly the case toward the base of the organ.

From the free ends of the outer hyphæ great numbers of conidia are produced by budding, which appear to have the power of again producing sphacelia in other Grasses. Finally, the hyphal tissue becomes much more dense, this taking place gradually from the base to the apex, until the Sclerotium or Ergot, which is ultimately a