394 



VASCULAR CRYPTOGAMS. 



compared to the sorus of a Fern. In INIarsilea the processes are still more complicated. 

 In this genus the sporocarp has somewhat the form of a bean, the stalk running 

 up one of its edges (Fig. 292). The interior of the sporocarp may be compared 

 somewhat to two book-cases placed parallel to one another, and separated by a 

 wall, or to a chest in which bags are placed horizontally and side by side in two 

 vertical rows : each of the smaller transverse compartments contains a sorus, the 

 placenta of which extends on its external face from the dorsal to the ventral edge 



Fig. 294. — Salvinia natins ; A transverse section of a stem bearing a wiiorl of leaves, / aerial leaves, 7v submerged leaf with 

 several teeth,/" sporocarps on it (natural size) ; B longitudinal section through tliree fertile teeth of a submerged leaf, a a sporocarp 

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

 fni ; 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. 



of the sporocarp, and projects inwards in the form of a ridge ; along the centre the 

 placenta bears a row of macrosporangia, and on either side of this rows of micro- 

 sporangia. (On the part played by these compartments in germination, vide infra 

 Fig. 300.) Corresponding to each placenta a fibro-vascular bundle runs on the 

 inner side of the envelope of the sporocarp, springing from the primary bundle 

 which runs along the dorsal edge and branches towards the ventral edge. 



Our knowledge of the history of development of the sporocarps is still very 

 imperfect. With respect to that of Salvinia it is known that an annular zone becomes 



