SECTION X.

RUDIMENTS OF THE GENERAL MORPHOLOGY AND PHYSIOLOGY OF THE EUMYCETES.

CHAPTER XXXIX.

GENERAL MORPHOLOGY.

§ 217.-Articulation of the Thallus.

ALREADY in the first volume (§ 22) the algae and the fungi were arranged in a single group, that of the Thallophytes, in contradistinction to all other plants, the latter being classed in the group of Cormophytes. The distinguishing characteristics of these two groups were stated to be the absence in the former, and the presence in the latter, of an articulation of the body of the individual organism into leaf and stem. At the same time, it was mentioned that the corporeal form assumed by the Thallophytes, and differing fundamentally from the cormus of the Cormophytes, has received the general name of thallus. That intermediate forms between these two types should exist, and that the thallus of the highest Thallophytes should approximate to the cormus of the lowest Cormophytes, is perfectly natural and in accordance with the general laws of phylogenetic evolution.

Although the fungi, the only class of Thallophytes with which we are now concerned, do not exhibit division into leaf and stem, their thallus is not entirely destitute of all articulation. True, in one of the two chief divisions of the fungus family, the Schizomycetes, the articulation of the thallus is practically undiscernible, the individual organisms taking the form of globular or oval cells, or straight or bent rods of variable length. Should any extensive development of the thallus occur, this may almost invariably be regarded as either a malformation preceding death-e.g. the branching of bacteroids (§ 195) and Bacterium aceti (§ 211)—or as an assemblage of several individual organisms giving rise to a deceptive appearance of articulation, as in the case of Cladothrix (§ 197). Again, in many species of bacteria, the colonies known as zooglea seem to exhibit a more or less well-developed articulation; but these cannot be con

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sidered as a thallus, since they represent assemblages of many uniform cells, and not separate individual organisms.

The case is, however, different in the second division of the fungus family (§ 23), namely, the Eumycetes or higher fungi. These differ from the Schizomycetes in universally possessing the faculty of forming true branchings. This characteristic resides in the immediate and uninterrupted connection between the plasma of the branch (or the oldest cell of same) and that of the main stem from which the branch proceeds. The divergence of form and the luxuriance of this branching vary in the different orders of fungi, and, in general, increase the higher we

Shows the unicellular mycelium sprung from the spore; together with three organs of fructification, a, b, c, in different stages of development, raising themselves from the mycelium. Magn. about 10. (After Kny.)

get in the system and the nearer we approach the Cormophytes. In the lowest members, on the other hand, this tendency is often greatly simplified and restricted, thus approximating to the Schizomycetes.

A more careful examination of the thallus of the higher Eumycetes, even with the unassisted eye, will reveal something more than the existence of a more or less copious branching. It will soon be found possible to dissect the thallus into two parts (Fig. 91), which though intimately connected serve entirely different purposes: one, known as the mycelium, having charge of the nutrition and maintenance of the individual plant; whilst, on

the other, or organ of fructification, devolves the task of reproduction, and therefore the maintenance of the species. This latter organ develops special cells or spores, which are mostly globular or oval, and from each of which under favourable circumstances, a new individual of the same species can be produced.

The

The mycelium may therefore be defined as the portion of the thallus spreading in or upon the nutrient medium and extracting nutriment therefrom. It proceeds from a spore. As soon as this latter comes under the influence of circumstances favourable to its germination, it absorbs water and other nutrient materials from the surroundings, swells up more or less, and usually puts forth one or more tubular buds (Fig. 92). These continue to develop in two directions, increasing in length, and forming lateral branches which in turn continue to act in like manner name hypha is given to each of these branchings, and the whole group of hypha that have resulted from a single spore and serve to nourish the individual plant in question, is called the mycelium. The spore may also germinate by the process of gemmation described in § 219. In some fungi the spores can only germinate in the one manner, whilst in others the germination is restricted to the other type. The mycelium, in order to fulfil its task, must continue to penetrate towards more remote portions of the nutrient medium; and accordingly, the hyphæ must progressively increase in length. Now this growth is confined to the apex of the hyphæ, i.e. the part farthest from the centre of development. On the other

FIG. 92. -Mucor mucedo. Germinating spore, which has already put

forth two buds. Magn. 300. (After Brefeld.)

hand, the parts nearest that centre quickly cease to extend and branch; consequently, the Eumycetes exhibit acrogenous growth. This behaviour constitutes another feature of difference between Eumycetes and Schizomycetes, since, in the latter, the growth of the cells is not restricted to one end only, but proceeds by the extension of the whole body.

Examination of the mycelia of a large number of species of Eumycetes, under a power of about 100 diameters, soon leads to a separation of these specimens into two groups: the one comprising species whose mycelium, however large and extensively branched, consists of only one single cell; whilst the other group contains the species wherein the mycelial hyphæ are subdivided into cylindrical parts of variable length by transverse walls (septa) perpendicular to the longitudinal axis. This fundamental and highly important difference constitutes the chief basis for the separation of the Eumycetes into two main subdivisions: Eumycetes with a unicellular mycelium on the one hand, and Eumycetes with a septated mycelium on the other.

In mycelial structure and several other particulars, the members of the first subdivision bear a remarkable resemblance to certain unicellular algæ, for which reason they have received the name Phycomycetes, or algic fungi. On the other hand, the Eumycetes of the second group, with septated mycelia, bear the groupname Mycomycetes. In comparison to the others these latter stand on a higher plane of development, and are almost exclusively aerial; whereas the majority of the Phycomycetes still incline to subaqueous existence. The following scheme will easily fix the foregoing classification in the reader's memory:

§ 218.-The Typical Mycelium.

We will now consider the development of the mycelium of a Mycomyces from its spore, Fig. 93 helping to make this clear. Soon after the tubular buds have sprouted from the spore, a septum forms between the spore and each of the buds. The tube then increases in length, and develops in its interior a septum which divides it into two cells, the one nearer the spore (or centre of growth) being termed the inner cell, whilst the outer one is called the terminal or crown cell. Now, whereas the inner cell ceases to increase in length the crown cell continues to grow longitudinally, and in turn develops a septum, whereby an inner cell (of the second order) is again formed; and this operation is repeated at convenience. Meanwhile the inner cells are not inactive, since, although they do not increase in length, they throw out lateral projections, which develop into branches separated from the inner cell by septa. These branches grow longitudinally, and separate into a crown cell and a secondary inner cell by developing another septum, an operation repeated by the crown cells as often as external circumstances will admit. This faculty of the inner cells of the first order is also shared by those subsequently formed, so that we have lateral branches of the third, fourth, and other orders. The whole of these hyphæ or mycelial threads together constitute the mycelium. The serial order in which the inner cells begin to throw off lateral branches is, as a rule, in accordance with their age, the oldest starting first. Consequently, the development of the mycelium proceeds laterally from the spore (the "basis") towards the periphery or apex the mycelial thread, such a mode of growth being termed basi

of

fugal or acropetalous. Again, the lateral position of the branches in question is, as a rule, very uniform; those from the inner cells of odd-numbered orders all branching from one (e.g. the left) side, and all those springing from inner cells of evennumbered orders appearing on the opposite (e.g. right) side of the respective inner cells. When the branching from any given

FIG. 93.-Mycelial development of a Mycomyces (the ordinary bread mould,
Penicillium glaucum).

A, the ripe spore. B and C, the same, with respectively one and three tubular offshoots. In D each of these has become separated from the spore by a septum, s. In E each tube has become divided by the formation of a second septum, s', into a terminal cell (e) and an inner cell (b), whilst branching has commenced in two places. F shows each of the three tubes developed into a main branch (I. to III.), which in turn has thrown out lateral branches of the first to the third order (1, 2, 3). Magn. 400. (After Zopf.)

cell is confined to a single lateral offshoot the system is termed monopodial, the main and branch cell together being called a monopodium.

The progress of mycelial development in a Phycomyces naturally differs from that just described, inasmuch as no septa are