130 



FERN. 



[CH. X 



differing morphologically from the indusium of Pteris. 

 Fig. 57 shows that the sporangia arise from a cushion 

 (the placenta), which is simply a swollen vein. 



FIG. 57. 



TRANSVERSE SECTION THROUGH A SORUS OF FTERIS : 



P, the placenta, bearing hairs and sporangia, two of which contain 



spores : R, the annulus or ring of the large empty sporangium. 



The sporangium consists of a hollow head mounted on 

 a delicate stalk. Within the cavity of the head are 

 contained numerous minute brown cells, which are the 

 spores. The spore-wall is differentiated into two layers, 

 an inner cellulose membrane and an external cuticularised 

 layer, resembling in fact the outer wall of an epidermic 

 cell. To understand the development of the spores it is 

 necessary to have a general idea of the development of 

 the sporangium. 



Each sporangium is the product of a series of cell 

 divisions occurring in a single epidermic cell. This mode 

 of development gives a certain morphological value to a 

 sporangium, which is technically expressed by saying that 



