THE PTERIDOPHYTA : FILICALES, THE FERNS 491 



The archesporial cell then divides to produce a transitory layer of cells on 

 the outside, termed the tapetum, and a small number of central cells, usually 

 about twelve in number, called the spore mother cells. It is from these 

 mother cells that the spores are developed. Each spore mother cell divides 

 into four spores. During the divisions of each spore mother cell there is a 

 meiosis, in which the chromosome number is halved. We see, therefore, 

 that in each mature sporangium there are forty-eight spores, and each spore 

 contains the monoploid number of chromosomes. 



Fig. 480. — Dryopteris filix-mos. Rudiments 

 of sporangia showing their origin from 

 single epidermal cells. 



The sporangium itself is a small, stalked capsule containing the spores 

 which have dark, rugged walls. The capsule is not spherical, but is shaped 

 like a biconvex lens. The wall of the sporangium is composed of a single 

 layer of cells, which are flat and thin-walled, except round the edge, where 

 a row of specially thickened cells, called the annulus, partially surrounds the 

 capsule. Where it stops short on one side, there is a group of narrow thm- 

 walled cells, the stomium, at which point the ripe sporangium breaks open. 



The bursting of the sporangium is caused by a ver>^ efltective mechanism. 

 As will be seen from Fig. 482, the cells of the annulus are peculiarly thickened ; 

 the inner tangential and radial walls are thick, while the outer tangential 

 wall is thin. It is due to the behaviour of the cells of the annulus in relation 

 to water that the bursting of the sporangium is achieved. In a young state 

 water glands are developed on the stalks of the sporangia, which secrete 

 moisture and maintain a damp atmosphere under the indusium. When the 

 sporangia are mature the indusium shrivels and the glands cease to secrete. 

 The sporangia thus become exposed to dry air and loss of water from the 

 cells, due to evaporation, causes the thin outer walls of the annulus to contract. 

 Meanwhile the water which is held in cohesion between the thickened radial 

 walls tends to draw them together. Under the strain so produced the cells 



