182 



THE ALG^E 



unfavorable seasons and are sometimes developed so numerously 

 as to cover the bottom of pools and rock hollows with a red 

 deposit. The phenomenon called " red snow " is due to deposits 

 of the resting cells of Sphcerella nivalis on fields of snow and ice. 

 Some species of Sphcerella (Fig. 176, C) develop a much greater 

 number of daughter elements, 32 to 64, which are much smaller 



than those just described, but 

 have the same structure. These 

 smaller cells swim about freely 

 for a short time, and then come 

 together in pairs and completely 

 fuse with one another, beginning 

 at the ciliated ends (Fig. 176, D). 

 A cell fusion of this character is a 

 sexual act (Sec. 200) and the cells 

 which unite are gametes. The sex- 

 ually formed fusion cell or zyg- 

 ospore of Sphcerdla soon settles 

 down on some surface and, losing- 

 its four cilia, remains quiet for 

 several days or weeks, finally de- 

 veloping within itself several 

 motile cells of the usual type. 



Volvox (Fig. 178, A) is a colo- 

 nial form consisting of many hun- 

 dreds of cells (sometimes more 

 than twenty thousand) imbedded 

 in a gelatinous substance in the 

 form of a sphere, with the pairs of cilia pointing outwards. 

 These remarkable organisms, as large as pin heads, roll slowly 

 through the water of quiet pools and ponds, sometimes gather- 

 ing in great numbers in open sunlit portions, free from water 

 weeds and algal growths. Daughter colonies (Fig. 178, A, d) are 

 formed from certain cells which after a period of growth develop 

 a large number of motile cells like the parent. These small cells 



EIG. 177. Chlamydomonas 

 Braunii 



Chlamydomonas is not uncommon 

 in the same sort of situations as 

 Sphserella. It may be distinguished 

 from the latter by the absence of 

 a thick gelatinous envelope around 

 the cells. Some of the forms show 

 important advances over Sphse- 

 rella in their sexual processes, for 

 the gametes may be of two sizes, 

 large female and small male cells, 

 as shown above, z, the zygospore. 

 After Goroschankin 



