CELL-NUCLEUSFERTILISATION 237 



encased by the sporangial wall. The central 

 cells increase, and whenever their nuclei 

 divide, the full, unreduced number of chromo- 

 somes can be seen, just as it may be ob- 

 served in any other dividing nuclei of the fern 

 plant. 



But a time arrives when the central cells 

 within the sporangium become free from 

 each other. Each one proceeds to grow, and 

 it finally divides twice, to give rise to four 

 spores. It is during these two divisions that the 

 reduction in the. number of chromosomes takes 

 place in the manner already described, and 

 hence the nucleus of each spore only contains 

 half the number of these nuclear structures. 

 When the spores are ripe the sporangium 

 bursts and the spores are scattered. If they 

 happen to alight on a suitable spot, they 

 germinate, but they do not bring forth a plant 

 like a fern (Fig. 27). A filamentous body is 

 formed which gradually develops into a heart- 

 shaped green expansion known as a prothallus. 

 It is very delicate, and is easily dried up, and 

 consequently is only suited to live where 

 conditions of moisture prevail. 



The prothallus sometimes multiplies vegeta- 

 tively, by the dying off of part of the plant, 

 while the living fragments grow into new 

 prothalli. Sexual organs, called antheridia 

 and archegonia, are developed on its under 

 side. In the former a number of sperms are 

 produced, while each archegonium, when 

 mature, contains a single egg. We need not 



