216 THE ALG^E 



margins of ponds and lakes. Fig. 202 presents a diagram of 

 such distribution on two isolated rocks. It will be seen that 

 there is a clear zone of a red alga (PolysipJionia] just below 

 low-water mark, another zone (Ceramium) at or a little above 

 this mark, while the Irish moss grows at some depth. The 

 sea lettuces and rockweed are well above low-water mark, as 

 is also Nemalion, which is exceptional in its habits for a red 

 alga. On the northerly New England coast and beyond there 

 are usually two distinct zones on the rocks, one well above 

 low-water mark, composed chiefly of rockweeds, and the other 

 near this point, but below, and made up mostly of Irish moss 

 with other red algse, including the dulse. 



243. Nemalion. Nemalion illustrates excellently the structure 

 of the sexual organs and the sexually formed fructification of 

 the red algse, called the cystocarp (meaning a fruit cavity). The 

 plant body is a rather soft, cord-like, branching structure, com- 

 posed of an immense number of filaments held together by a 

 stiff gelatinous substance around the cells. There is a central 

 axis of delicate threads, while the outer regions consist of short 

 filaments pointing outward. The cells of the outer filaments 

 contain each a single chromatophore, and the vegetative activi- 

 ties (photosynthesis) as well as the reproductive take place in 

 this region of the plant. 



The male organs, or antheridia, consist of clusters of small cells 

 at the surface of the plant (Fig. 203, A], each of which develops 

 a single sperm, spherical in form and without cilia, and con- 

 sequently nonmotile. The female organ (Fig. 203, B] is devel- 

 oped at the end of a short branch and consists of a cell which 

 bears a long, hair-like extension called the trichogyne (meaning 

 female hair), which is the receptive organ for the sperms. The 

 sperms are applied to the trichogynes largely by the contact of 

 male plants with the female as they are washed about by the 

 movements of the water. When a sperm fuses with the tri- 

 chogyne its nucleus (male) passes down into the swollen base of 

 the female cell and unites there with a female nucleus. The 



