_a._ CARPOGONIA OF GELATINOUS LICHENS. Stahl’s[548] work on various Collemaceae followed on the same lines as that of Fuisting. The first species selected by him for examination, _Collema_ (_Leptogium_) _microphyllum_’ is a gelatinous lichen which grows on old trunks of poplars and willows. It has a small olive-green thallus which, in autumn, is crowded with apothecia; the spermogones or pycnidia appear as minute reddish points on the edge of the thallus. Within the thallus, and midway between the upper and lower surface, there arises, as a branch from a vegetative hypha, a many-septate filament coiled in spiral form at the base, with the free end growing upwards and projecting a short distance above the surface and occasionally forked (Fig. 91). The tip-cell is slightly swollen and covered with a mucilaginous coat continuous with the mucilage of the thallus. The whole structure, characterized by the larger size and by the richer contents of its cells, was regarded by Stahl as a carpogonium, the coiled base representing the ascogonium, the upright hypha functioning as the receptive organ or trichogyne, comparable to that of the Florideae. The spermatia, which mature at this early stage of carpogonial development, are expelled from a neighbouring spermogonium on the addition of moisture and easily reach the protruding trichogyne. They adhere to the mucilaginous wall of the end-cell, and, in two or three instances, Stahl found that copulation had taken place. As the affixed spermatium was empty, he concluded that the contents had passed over into the trichogyne, and that the nucleus had travelled down to the ascogonium. Certain degenerative changes that followed seemed to confirm the view that there had been fertilization: the cells of the trichogyne had lost their turgidity and at the same time the cross-walls had swollen considerably and stood out like knots in the hypha (Fig. 92). The ascogonial cells had also increased not only in size but in number by intercalary division, so that the spiral arrangement became obscured. Ascogenous hyphae arose from the ascogonial cells, and asci cut off by a basal septum were finally formed from these hyphae. Lateral branches from below the septum also formed asci. [Illustration: Fig. 92. _Collema microphyllum_ Ach. Carpogonium and trichogyne after copulation × 500 (after Stahl).] Stahl’s observations were repeated and extended by Borzi[549] on another of the Collemaceae, _Collema nigrescens_. In that plant the foliaceous thallus is of thin texture and has a distinct cellular cortex. The carpogonia were found at varying depths near to the cortical region; the ascogonium, of two and a half to four spirals, consisted of ten to fifteen cells with very thin walls, the trichogyne of five to ten cells, the terminal cell projecting above the thallus. Borzi also found spermatia fused with the tip-cell.