RHIZOCARPEAZ 31 



floating megaspores. The roundish megaspore, which does not nearly 

 fill up the sporange, is completely covered by a very thick warty layer 

 of hardened frothy mucilage, its epispore, which projects far above the 

 apex, and separates, in its upper part, into either three or nine large 

 pear-shaped masses of the same substance, terminating in a tuft of fine 

 threads. These bodies constitute a floating apparatus for the megaspore, 

 the fine threads floating on the surface of the water, and suspending 

 beneath them the float-corpuscles, either three in number or more 

 numerous, containing abundance of air-cavities, and the megaspores, to 

 which the microspores are attached by their glochidiate processes. 



The two genera of Salviniaceae, Salvinia and Azolla, each include 

 but a small number of species, all annual plants, widely distributed over 

 the globe, especially in its warmer regions. Those of Azolla form green 

 or red floating patches of considerable size, with the habit of a Junger- 

 mannia. No economical application is known of either genus. 



ORDER 2. MARSILEACE^E. 



The female prothallium attains here a much smaller degree of 

 development than in the Salviniaceae. It arises within the apical papilla 

 of the megaspore, the protoplasm of which breaks up into several cells, 

 which remain for a time unclothed with cellulose, and only subsequently 

 constitute a tissue containing a small quantity of chlorophyll. Even 

 after this the prothallium still remains for some time completely enclosed 

 within the apical papilla of the megaspore, being covered by the epi- 

 dermal layers of the apex of the spore itself, and shut off from the 

 spore-cavity within and below by a diaphragm which is attached to the 

 internal coat Of the spore. By the further growth of the prothallium 

 the epidermal layers of the apical papilla are broken through, and the 

 dorsal ridge of the prothallium projects into the funnel formed by the 

 absence at this spot of the thick outer layers of the epispore. The 

 diaphragm subsequently arches convexly, and the prothallium is pushed 

 further outwards, but still lies as a hemispherical mass in the funnel- 

 shaped opening. 



In those species which have hitherto been examined each prothallium 

 produces only a single archegone. Even before the prothallium breaks 

 through the megaspore, the large central cell may be recognised in it, 

 covered only by four cells arranged in a cross, which form at the same 

 time the apex of the prothallium. From these are developed the more 

 or less projecting neck and the stigmatic cells of the archegone. As in 

 Salviniaceae, a neck-canal-cell is separated from the central cell, which 

 pushes up between the neck-cells, as well as a smaller ventral canal-cell ; 



