CCENOBIE^. 99 



within the parent colony for some time, and eventually 

 escape. These daughter-colonies may often be seen within 

 the parent colony under the form of dark green balls. 

 Towards the autumn the asexual mode of reproduction is 

 replaced by a sexual method; certain cells recognized by 

 their large size and dark green colour appear — these are 

 oogonia; the large end projects into the interior of the 

 sphere, and the contents become transformed into an 

 oosphere. Other cells, the antheridia, produce antherozoids, 

 which become free within the sphere and eventually blend 

 with the oospheres, which after fertilization become covered 

 with a thick cell-wall covered with prominences, and change 

 to a brick-red colour. When the oospores are mature, the 

 mother-colony breaks up, and they sink to the bottom of 

 the water as resting-spores. 



In the genus Hydrodidyon, popularly known as " water- 

 net," thecoenobium presents the appearance of an inflated net, 

 the wall being formed of slender cylindrical cells combined 

 to form numerous irregular polygons. Asexual reproduction 

 takes place by the protoplasm of one of the cells breaking 

 up into numerous zoospores, each furnished with four cilia ; 

 after moving about in the mother-cell or zoosporangium for 

 some time, the zoospores arrange themselves in such a way 

 that by increasing in size they form a net which is liberated 

 from the mother-cell, and by continued growth within the 

 course of a few weeks reaches to the size of the parent net. 

 In the above mode of reproduction the zoospores do not 

 conjugate. Other cells of the net give origin to numer- 

 ous minute zoogametes, furnished with only two cilia, which 

 leave the parent-cell and conjugate, the resulting zygospore 

 remaining for some time as a resting-spore. It has been 

 clearly demonstrated, that by varying the nutrient solution in 



