1 62 Practical Plant Biology. 



forms a little rounded cushion, the placenta, and to this the stalks 

 of the sporangia and of the indusium are fixed. The structure 

 of the sporangia is peculiar and most interesting, as revealing a 

 beautiful natural mechanism for the distribution of the spores. 

 Each sporangium is attached by its edge to its stalk. A single 

 row of cells forms a ridge round its edge. This row is composed 

 of two unequal parts. The longer part, the annulus, which 

 occupies three-quarters of the circumference starting from the at- 

 tachment of the stalk, is composed of about fifteen similar cells. 

 Each is approximately cubic in form. Its inner wall and those 



FIG. 40. Aspidium filix-mas, sorus on leaf, transverse section, x 45. 

 a, annulus ; d, indusium ; /, stomium or lip-cells. (After Kny.) 



separating it from its neighbours in the row are much thickened 

 and are brown in colour. The material of these walls is very rigid 

 and elastic and forms a U-shaped band partly enclosing each cell, 

 while that of the remaining outer and side walls is soft and pliant 

 like pure cellulose. The cells of the annulus soon lose their proto- 

 plasmic contents but remain filled with water. The remainder of 

 the marginal row, the stomium, is composed of uniformly thin- 

 walled cells, more flattened in form than those of the annulus. 

 Their thin walls are easily torn asunder. The convex sides of the 

 sporangium are formed of a single layer of flat cells, also with thin 

 walls. The cavity enclosed by the annulus and sides is filled with 



