Page:EB1911 - Volume 19.djvu/123

 similar material occurs along branching and anastomosing tracts through the protoplasm of the sporangium, giving rise to the capillitium. The greater part of the lime granules pass out of the protoplasm and are deposited in the capillitium, which in the ripe sporangia of Badhamia is white and brittle with the contained lime (cf. fig. 8). In this genus some granules are found also in the sporangium-wall. Strasburger concludes that the sporangium-wall of Trichia is a modification of cellulose (29).

It has been stated (16), but the observation requires confirmation, that a fusion of the nuclei in pairs occurs early in the development of the sporangium.

At a later stage, after the capillitium is formed, the nuclei undergo a mitotic division which affects all the nuclei of a sporangium simultaneously. This was first described by Strasburger (29). While it is in progress the protoplasm of the sporangium divides, into successively smaller masses, until each daughter nucleus is the centre of a single mass of protoplasm. These nucleated masses are the young

spores. A spore-wall is soon secreted and the sporangium has now resolved itself into a mass of spores, traversed by the strands of the capillitium and enclosed in a sporangium-wall, connected with the substratum by a stalk. As ripening proceeds, the wall becomes membranous and readily ruptures, and the dry spores may be carried abroad on the currents of air or washed out by rain.

We may now review some of the main differences in structure presented by the sporangia. They may be stalked or sessile (fig. 13). If the former, the stalk is usually, as in Badhamia utricularis, the continuation of the sporangium-walls (figs. 11 and 12), but in Stemonitis and its allies (figs. 17 and 18) it is an axial structure. A central columella may project into the interior of the sporangium, either in stalked (fig. 15) or sessile (fig. 13) forms.

The sporangium-wall may be most delicate and evanescent (fig. 17), or consist of a superficial network of threads (fig. 18), which in Dictydium (fig. 19) present a beautifully regular arrangement.

In Chondrioderma (fig. 13) the wall is double, the inner layer being membranous, the outer thickly encrusted with lime granules. In Craterium the upper part of the sporangium-wall is lid-like and falls away, leaving the spores in an open cup (fig. 14).