THE THALLOPHYTA 



283 



point already well up in the scale of algal evolution. The higher 

 members of the class display an unquestionable alternation of 

 generations. Two typical genera will illustrate the complexities 

 of reproduction in this group. 



Nemalion (Fig. 160) is a common form with a branching, 

 filamentous habit and represents the simplest of the red 

 algae. The numerous small antheridia profluce non-motile, 



Fig. 161. — Polysiphonia fihrillosa. A, Portion of a tetrasporic plant showing 

 three groups of tetraspores. These spores occur in tetrads or groups of four. 

 X 90. B, portion of a male plant. Above, an immature antheridium; below, a 

 mature one, producing large numbers of sperms on its surface. X 215. C, 

 portion of a female plant showing a ripe cystocarp, its wall enclosing a group 

 of carpospores. X 45. 



binucleate sperms. The female sexual organ, which is here 

 called the procarp, consists of two cells — a basal carpogonium, 

 containing the single female cell and homologous with the oogo- 

 nium of other algae; and a terminal cell, the trichogtjne, which is 

 drawn out into a hairlike tip. The sperm is carried bj^ water 

 currents to the trichogyne, down which its contents passes into 

 the carpogonium. The fertilized carpogonium does not develop 

 a new plant directly, but produces a group of short filaments on 

 the end of each of which is borne a non-motile carpospore. This 

 whole structure of spores, filaments, and carpogonium is known as 

 the cystocarp. A carpospore germinates directly into a new 

 plant. 



Polysiphonia (Fig. 161), another common member of tli(> class, 

 displays a much more complex sexual history and is tyj^ical of 



