THE ALGAE 



167 



becomes established between them and also with the pericentral cell. This 

 general cell-union finally results in the formation of a large irregular central 

 cell. This cell therefore contains the two diploid nuclei and the disorganizing 

 monoploid nuclei of the component auxiliary cells. 



The diploid nuclei now divide repeatedly and lobes (gonimolobes) 

 are formed on the surface of the central cell. Into each of these a diploid 

 nucleus passes and divides, a cross wall being formed to separate a stalk cell 

 from a terminal carpospore. 



In each species while this development has been going on, an envelope 

 has grown up from the peripheral siphons of the axis, which completely 



Carpospore 



Cystocarp wall 



Fig. 157. — Polysiphonia sp. Mature cystocarp attached to 

 branch showing contained carpospores. 



encloses the whole of the female organ and later the carpospores. This 

 structure is swollen and urn-shaped, and it, together with the reproductive 

 organs within, is termed the cystocarp (Fig. 157). 



Asexual Reproduction 



On liberation, the carpospores give rise to plants exactly like the parents, 

 but on which asexual spores arise from the pericentral cells of the young 

 branches (Fig. 158). Only one pericentral cell in each tier is fertile. It 

 divides longitudinally by a tangential wall, and the inner half then divides 

 horizontally to produce a lower stalk cell and an upper cell w^hich becomes 

 the tetrasporangium. Its protoplast divides tetrahedrally into four tetra- 

 spores, and it is here that meiosis occurs, so that the spores and the sexual 

 plants which they subsequently produce are monoploid (Fig. 159). 



