35^ 



VASCULAR CRYPTOGAMS. 



of the veins ; or the sorus may run for a considerable distance by the side of the 

 veins. Sometimes the fertile veins run close to the margin of the leaf, in other cases 

 close to the mid-rib of the lamina. 



The Developmeftt of the sporangium ^ is accurately known only in the Poly- 

 podiacese ; it arises there from a papillose outgrowth of one of the epidermal cells 

 from which the sorus originates. Rees has shown that before the formation of 

 the sporangium the epidermal cell concerned has been already divided cross-wise ; 



the papilla is cut off by a septum, another 

 septum arising, after further elongation, in 

 the mother-cell of the sporangium thus 

 formed; the lower cell forms the pedicel, 

 the upper cell the capsule of the spor- 

 angium. The pedicel is usually trans- 

 formed, by intercalary transverse divisions 

 and longitudinal walls, into three rows of 

 cells ; the nearly hemispherical mother-cell 

 of the capsule is next transformed, by 

 four successive oblique divisions, into four 

 plano-convex parietal cells and a tetra- 

 hedral inner cell ; in the former further 

 divisions follow perpendicular to the sur- 

 face, while the inner cell again forms 

 four tabular segments which are parallel 

 to the outer parietal cells. These inner 

 parietal cells also divide perpendicularly to 

 the surface of the capsule, the wall of 

 which thus consists of two (or, according 

 to Rees, of three) layers. The cells of the 

 outer parietal layer from which the annulus 

 is to be formed are further divided by 

 parallel walls perpendicular to the surface 

 of the sporangium and to the median line 

 of the annulus, until the prescribed num- 

 ber of cells of the annulus is reached ; 

 these cells then project above the surface 

 of the capsule. While the tetrahedral 

 central cell is now producing by successive 

 bipartitions the mother-cells of the spores, the cells of the inner parietal layers are 

 absorbed, and the cavity of the sporangium is considerably enlarged by this means 

 and by the superficial growth of the outer parietal layer ; so that the mass of 

 mother-cells (according to Rees there are always twelve), floats entirely free in 

 the fluid that fills the sporangium (Fig. 266). For the further peculiarities ex- 



FlG. 267.— Development of the sporangium of Asple- 

 niinn Trichonianes ; the order of succession according to 

 the letters a-i; in i the annulus r is shown ; the other 

 figures are seen in optical longitudinal section, EUid the an- 

 nulus is perpendicular to the paper (X550). 



* When the first sporangia are ripening, all stages of development of the younger ones may 

 be found in the same sorus side by side. 



