CH. X] REPRODUCTION. 131 



the sporangium of the fern is a trichome, i.e. a hair-like 

 structure ; the multicellular hairs which occur on the 

 surface of many plants being each similarly developed from 

 a single epidermic cell. 



The chief stages in the development of the sporangium 

 are shown in fig. 58. The epidermic cell divides into a 

 stalk and a head, which are the parent-cells of the stalk 

 and head of the future sporangium. The stalk cell 



a 



FIG. 58. 



DEVELOPMENT or THE FERN SPORANGIUM. 



a. The out-growth from an epidermic cell is divided into two cells ; a 

 rounded apical cell giving rise to the head of the sporangium, and a 

 basal cell from which the stalk is developed. 

 6. Two out of the four oblique walls, by which the archespore is marked 



out, are shown. 



c. A third oblique wall has appeared, and the wall of the sporangium 

 is seen to be developing. 



divides by numerous cross-walls and comes to consist of 

 several stages or storeys one over the other, each layer 

 consisting of four cells. The fate of the head-cell is more 

 complex : it will be enough to say that by four cell-walls 

 (of which two are shown in fig. 58, 6) a large triangular 

 cell, the archespore, is marked out in the middle of what 

 was once the headlike half of the original epidermic cell. 

 From part of the archespore what are known as the 



92 



