FILICINEM. 



453 



latter is cup-shaped and therefore open at the apex, whereas the wall of the former 

 completely encloses the sorus, as does the indusium of Cyathea. The sporocarp 

 of the Salviniaceae is then a sorus, the term being used in the sense in which it is 

 applied to the fructification of Ferns, but here the indusium is much more fully 

 developed, consisting of two layers of cells, the walls of which, toward the upper 

 part of the capsule, become lignified in Azolla. 



Each sporocarp contains either microsporangia or macrosporangia, but both 

 kinds of sporocarp occur in the same plant and even upon the same leaf, so 



Fig. ^Tg.—Salvtm'a nutans -, A transverse section of a stem bearing a whorl of leaves, I aerial leaves, iu submerged leaf with 

 several teeth, ysporocarps on it (natural size) ; B longitudinal section through three fertile teeth of a submerged leaf, a a sporo- 

 carp with macrosporangia, i i two sporocarps with microsporangia ; C transverse section of a sporocarp with microsporangia 

 ■mi; D transverse section of an aerial leaf, hu hairs of the under side, ho hairs of the upper side, ep epidermis, I air-cavities, the 

 dark ones show the vertical walls of the tissue in the background (B—D X lo) ; E cells of a lamella of tissue in the leaf; F one of 

 the cells after contraction of the contents in glycerine. 



that the plant is monoecious. The microsporangia are contained in Azolla^ as 

 in Salvinia, in large numbers within the sporocarp; in Azolla only a single 

 macrosporangium is formed in a sporocarp, whereas in Salvinia several macro- 

 sporangia are formed. All the spores derived from the mother-cells (i6) in a 

 microsporangium reach maturity, but only one of the (4x16) spores of a macro- 

 sporangium ever becomes ripe, so that in Azolla, where there is but one macro- 

 sporangium in a sporocarp, a single macrospore is enclosed by the wall of the 



