RHIZOCARPE.^ 25 



groups. The megasporanges are often considerably larger than the 

 microsporanges. 



In the early stages of their development no difference is exhibited 

 between the megasporanges and microsporanges. In both cases the 

 sporange originates in a papilla placed on the placenta^ which divides 

 first into an upper and a lower cell, the latter developing, by repeated 

 transverse septation, into the pedicel, the former into the body of the 

 sporange, and dividing ultimately into a large central tetrahedral 

 archespore, surrounded by a layer which almost immediately breaks up 

 into two layers of tapetal cells or mantle-layers. The archespore further 

 divides into sixteen spore-mother-cells, and each of these into four 

 special spore-cells arranged tetrahedrally. A difference is now mani- 

 fested according as the sporange is to develop into a mega- or a micro- 

 sporange. In the latter case each of the sixty-four cells develops into 

 a microspore, while the tapetal cells become disorganised, and changed 

 into the frothy mucilage which subsequently hardens and encloses the 

 spores. In the former case only one of the sixty-four cells develops 

 into a megaspore, growing rapidly at the expense of the others, and 

 ultimately filling up the cavity of the sporange. The remaining sixty- 

 three spore-cells, as well as the tapetal cells, become disorganised, and 

 deliquesce into a frothy mucilage which envelops the ripe megaspore, 

 ultimately hardening into the epispore, which splits to allow the emergence 

 of the prothallium. In Azolla the mucilage of the microsporanges forms 

 the peculiar massulcz which will be described later. 



A more detailed description requires the division of the Rhizocarpe^ 

 into the two orders Salvhiiacece (Salvinia and Azolla) and Marsileacete 

 (Marsilea and Pilularia), which are, perhaps, not in reality very nearly 

 related to one another. 



Order i. — Salvixiace.e. 



The female prothallium of Salvinia is formed within the apical papilla 

 of the megaspore. The protoplasm in this papilla appears to separate from 

 that of the rest of the spore, and then breaks up by free-cell-formation 

 into several portions, which remain for a time unclothed with cellulose ; 

 subsequently they secrete cell-walls, and form a tissue, which breaks 

 through the cell-wall of the papilla, and forces its way through the epi- 

 spore, which splits into a three-lobed body. The prothallium, when it 

 first emerges from the epispore, has a somewhat triangular form, with an 

 elevated ridge along its median line, and two wing-like appendages, 

 subsequently forced apart by the growth of the embryo, which hangs 

 down on each side of the spore. It contains a considerable amount of 



