Page:EB1911 - Volume 11.djvu/356

Rh host on which the uredospores (if present) and the teleutospores are developed. A few examples are appended:

Some of the Uredineae also exhibit the peculiarity of the development of biologic forms within a single morphological species, sometimes termed specialization of parasitism; this will be dealt with later under the section Physiology.

Cytology of Uredineae.—The study of the nuclear behaviour of the cells of the Uredineae has thrown great light on the question of sexuality. This group like the rest of the Basidiales exhibits an association of nuclei at some point in its life-history, but unlike the case of the Basidiomycetes the point of association in the Uredineae is very well defined in all those forms which possess aecidiospores. We find thus that in the eu and opsis forms the association of nuclei takes place at the base of the aecidium which produces the aecidiospores. There we find an association of nuclei either by the fusion of two similar cells as described by Christmann or by the migration of the nucleus of a vegetative cell into a special cell of the aecidium. After this association the nuclei continue in the conjugate condition so that the aecidiospores, the uredospore-bearing mycelium, the uredospores and the young teleutospores all contain two paired nuclei in their cells (fig. 17). Before the teleutospore reaches maturity the nuclei fuse, and the uninucleate condition then continues again until aecidium formation. In the hemi, brachy, micro and lepto forms, which possess no aecidium, we find that the association takes place at various points in the ordinary mycelium but always before the formation of the uredospores in the hemi and brachy forms, and before the formation of teleutospores in micro and lepto form. Whether the association of nuclei in the ordinary mycelium takes place by the migration of a nucleus from one cell to another or whether two daughter nuclei become conjugate in one cell, is not yet clear. The most reasonable interpretation of the spermatia is that they are abortive male cells. They have never been found to cause infection, and they have not the characters of conidia; the large size of their nuclei, the reduction of their cytoplasm and the absence of reserve material and their thin cell wall all point to their being male gametes. Although in the forms without aecidia the two generations are not sharply marked off from one another, we may look up the generation with single nuclei in the cells as the gametophyte and that with conjugate nuclei as the sporophyte. The subjoined diagram will indicate the relationship of the forms.

Basidiomycetes.—This group is characterized by its greatly reduced life-history as compared with that of the eu forms among the Uredineae. All the forms have the same life-history as the lepto forms of that group, so that there is no longer any trace of sexual organs. There is also a further reduction in that the basidium is not derived

from a teleutospore but is borne directly on the mycelium. Formerly, before the relationship of promycelium and basidium were understood, the Uredineae were considered as quite independent of the Basidiomycetes. Later, however, these Uredineae were placed as a mere subdivision of the Basidiomycetes. Although the Uredineae clearly lead on to the Basidiomycetes, yet owing to their retaining in many cases definite traces of sexual organs they are clearly a more primitive group. Their marked parasitic habit also separates them off, so that they are best included with the Basidiomycetes in a larger cohort which may be called Basidiales. Most of Basidiomycetes are characterized by the large sporophore on which the basidia with its basidiospores are borne.

It must be clearly borne in mind that though the Basidiomycetes show no traces of differentiated sexual organs yet, like the micro and lepto forms of the Uredineae, they still show (in the association of nuclei and later fusion of nuclei in the basidium), a reduced fertilization which denotes their derivation, through the Uredineae, from more typically sexual forms. No one has yet made out in any form the exact way in which the association of nuclei takes place in the group. The mycelium is always found to contain conjugate nuclei before the formation of basidia, but the point at which the conjugate condition arises seems very variable. Miss Nichols finds that it occurs very soon after the germination of the spore in Coprinus, but no fusion of cells or migration of nuclei was to be observed.

Protobasidiomycetes.—This, by far the smaller division of Basidiomycetes, includes those forms which have a septate basidium. There are three families—Auriculariaceae, Pilacreaceae and Tremellinaceae. The first named contains a small number of forms with the basidium divided like the promycelium of the Uredineae. They are characterized by their gelatinous consistence and large size of their sporophore. Hirneola (Auricularia) Auricula-Judae is the well-known Jew’s Ear, so named from the resemblance of the sporophore to a human ear.

The Pilacreaceae are a family found by Brefeld to contain the genus Pilacre. P. Petersii has a transversely divided basidium as in Auriculariaceae, but the basidia are surrounded with a peridium-like sheath. The Tremellinaceae are characterized by the possession of basidia which are divided by two vertical walls at right angles to one another. From each of the four segments in the case of Tremella a long outgrowth arises which reaches to the surface of the hymenium