POLYSIPHONIA 



219 



Besides the sexual plants (male and female) there is an asexual 

 condition in Polysiplwnia called the tetrasporic plant. Tetra- 

 sporic plants are individuals which develop asexual spores, called 

 tetraspores because they are formed in groups of four, termed 

 tetrads, in mother cells (Fig. 204,7?, F). The tetraspore mother 

 cells arise from the central siphon near the ends of the branches. 



FIG. 204. Polysiphonia violacea 



A, tip of filament showing two antheridia, a; /, cross section of a portion of an 

 antheridium illustrating the development of the sperms at the ends of the very 

 numerous short branches ; C, a procarp with the projecting trichogyne t, from 

 the female cell (carpogonium), which is hidden by the surrounding sterile cells; 

 I), mature cystocarp with the urn-shaped envelope inclosing the cluster of 

 carpospores, a single spore shown at the right; E, a short branch from a tetra- 

 sporic plant; F, two groups of tetraspores from a branch similar to E ; note 

 the peculiar arrangement of the tetraspores in a group of four, or tetrad 



Some recent investigations clearly indicate that the tetrasporic 

 plants come from carpospores, and that the tetraspores develop 

 into sexual plants. So there is an alternation of sexual and 

 tetrasporic plants in the life history of Polysiphonia. 



246. Summary of the red algae. It is quite certain that the 

 red algre have had their origin from a very much higher level 



