RHIZOCARPE^ 25 



oups. The megasporanges are often considerably larger than the 

 icrosporanges. 



In the early stages of their development no difference is exhibited 

 ;tween the megasporanges and microsporanges. In both cases the 

 lorange originates in a papilla placed on the placenta, which divides 

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

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

 (Grange, and dividing ultimately into a large central tetrahedral 

 rhespore, surrounded by a layer which almost immediately breaks up 

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

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

 )ecial spore-cells arranged tetrahedrally. A difference is now mani- 

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

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

 microspore, while the tapetal cells become disorganised, and changed 

 to the frothy mucilage which subsequently hardens and encloses the 

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

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

 timately fiUing up the cavity of the sporange. The remaining sixty- 

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

 sliquesce into a frothy mucilage which envelops the ripe megaspore, 

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

 " the prothallium. In AzoUa the mucilage of the microsporanges forms 

 le peculiar massulcs which will be described later. 



A more detailed description requires the division of the Rhizocarpes 

 ito the two orders Salviniacece (Salvinia and Azolla) and Marsileacece 

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

 ilated to one another. 



Order i, — Salviniacece. 



The female prothallium of Salvinia is formed within the epical papilla 

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

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

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

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

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

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

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

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

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

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



