Page:EB1911 - Volume 16.djvu/600

Rh gonidia. In L. granatina the primary alga is Pleurococcus, the secondary, Gleococapsa.

Cephalodia.—In about 100 species of lichens peculiar growths are developed in the interior of the thallus which cause a slight projection of the upper or lower surface. These structures are known as cephalodia and they usually occupy a definite position in the thallus. They are distinguished by possessing as gonidia algae foreign to the ordinary part of the thallus. The foreign algae are always members of the Cyanophyceae and on the same individual and even in the same cephalodium more than one type of gonidium may be found. The function of these peculiar structures is unknown. Zukal has suggested that they may play the part of water-absorbing organs.

The exact relation of gonidia and hyphae has been investigated especially by Bornet and also by Hedlund, and very considerable differences have been shown to exist in different genera. In Physma, Arnoldia, Phylliscum and other genera the gonidia are killed sooner or later by special hyphal branches, haustoria, which pierce the membrane of the algal cell, penetrate the protoplasm and absorb the contents (fig. 11, C). In other cases, e.g. Synalissa, Micarea, the haustoria pierce the membrane, but do not penetrate the protoplasm (fig. 11, D). In many other cases, especially those algae possessing Pleurococcus as their gonidia, there are no penetrating hyphae, but merely special short hyphal branches which are in close contact with the membrane of the algal cell (fig. 3).

There are three methods of reproduction of the lichen: by fragmentation, by soredia, by the formation of fungal spores. In the first process, portions of thallus containing gonidia may be accidentally separated and so may start new plants. The second method is only a special process of fragmentation. The soredia are found in a large number of lichens, and consist of a single gonidium or groups of gonidia, surrounded by a sheath and hyphae. They arise usually in the gonidial layer of the thallus by division of the gonidia and the development around them of the hyphal investment; their increase in number leads to the rupture of the enclosing cortical layer and the soredia escape from the thallus as a powdery mass (fig. 12). Since they are provided with both fungal and algal elements, they are able to develop directly, under suitable conditions, into a new thallus. The soredia are the most successful method of reproduction in lichens, for not only are some forms nearly always without spore-formation and in others the spores largely abortive, but in all cases the spore represents only the fungal component of the thallus, and its success in the development of a new lichen-thallus depends on the chance meeting, at the time of germination, with the appropriate algal component.

Conidia.—Contrary to the behaviour of the non-lichen forming Ascomycetes the lichen-fungi show very few cases of ordinary conidial formation. Bornet describes free conidia in Arnoldia minitula, and Placodium decipiens and Conidia-formation has been described by Neubner in the Caliciae.

Spermatia.—In the majority of genera of lichens small flask-shaped structures are found embedded in the thallus (fig. 13). These were investigated by Tulasne in 1853, who gave them the name spermogonia The lower, ventral portion of the spermogonium is lined by delicate hyphae, the sterigmata, which give origin to minute colourless cells, the spermatia. The sterigmata are either simple (fig. 13, C) or septate—the so-called arthrosterigmata (fig. 13, B). The spermogonia open by a small pore at the apex, towards which the sterigmata converge and through which the spermatia escape (fig. 13). There are two views as to the nature of the spermatia. In one view they are mere asexual conidia, and the term pycnoconidia is accordingly applied since they are borne in structures like the non-sexual pycnidia of other fungi. In the other view the spermatia are the male sexual cells and thus are rightly named; it should, however, be pointed out that this was not the view of Tulasne, though we owe to him the designation which carries with it the sexual significance. The question is one very difficult to settle owing to the fact that the majority of spermatia appear to be functionless. In favour of the conidial view is the fact that in the case of Collema and a few other forms the spermatia have been made to germinate in artificial cultures, and in the case of Calicium parietinum Möfler succeeded in producing a spermogonia bearing thallus from a spermatium. For the germination of the spermatia in nature there is only the observation of Hedlund, that in Catillaria denigrata and C. prasena a thallus may be derived from the spermatia under natural conditions. In relation to the view that the spermatia are sexual cells, or at least were primitively so, it must be pointed out that although the actual fusion of the spermatial nucleus with a female nucleus has not been observed, yet in a few cases the spermatia have been seen to fuse with a projecting portion (trichogyne) of the ascogonium, as in Collema and Physcia, and there is very strong circumstantial evidence that fertilization takes place (see later in section on development of ascocarp). The resemblance of the spermatia and spermogonia to those of Uredineae should be pointed out, where also there is considerable evidence for their original sexual nature, though they appear in that group to be functionless in all cases. The observations of Möller, &c., on the germination cannot be assumed to negative the sexual hypothesis for the sexual cells of Ulothrix and Ectocarpus, for example