CH. X] THE GREEN ALG^ 419 



against the wall in regular order, grow together at points of 

 contact, and then elongate between those points. When set 

 free by decay of the wall, the cells of the nets, and therefore 

 the nets themselves, increase immensely in size. These plants 

 reproduce sexually by conjugation of gametes formed in great 

 numbers by special cells. The zygospores rest for a time, 

 then produce small nets, from the cells of which, large ones 

 later develop. 



The motile colonial forms, of which there are several, 

 reach their perfection in the beautiful Volvox (Fig. 288). 

 In this remarkable plant, the cells, each retaining its two 

 cilia, form a hollow 

 gelatinous sphere, com- 

 posed often of thou- 

 sands of individuals, 

 and as large as a pin 

 head. In this condi- 

 tion it can be seen roll- 

 ing slowly through the 

 water in clear ponds, 

 or occasionally in tanks 

 in greenhouses. The 

 microscope shows that 

 the individuals are con- 

 nected by protoplas- 

 mic threads extending FlG . 2 88. Volvox Giobator ; x 125. 



through the gelatinous The smaller irregular cells are the vegeta- 



rp, . tive individuals, each with two cilia; the 



matnx. IniS arrange- i arger rou nd or pear-shaped ceUs are egg cells 



ment is not found lower in various stages of development ; arid three 



, other large cells contain sperm cells in their 



among plants, but pre- ra diate, parallel, and free stages, the latter 



Vails in the higher kinds J ust P rior to their eg c a Pe into the cavity of the 



t , . , colony. (After a wall chart by Dodel-Port.) 



(page 40), on which 



account the Volvox might be considered a multicellular in- 

 dividual, rather than a colony. At first the cells are all 

 alike, but later some enlarge and enter the cavity, where 

 they form new colonies which are ultimately set free by 



