186 THE ALG^E 



attracted by light and collect at the illuminated side of a vessel, 

 forming a green cloud in the water. Because of these habits, 

 and their rapid darting to and fro in the water, they are often 

 called swarm spores. 



The zoospores of Ulothrix are developed most numerously at 

 night and are set free from the parent filaments chiefly during 

 the morning hours. Sometimes the entire protoplast slips out as 

 a single large zoospore, but more often 2, 4, or 8 zoospores are 

 formed in each parent cell (Fig. 180, B}. They are roundish or 

 pear-shaped (Fig. 180, C), with four cilia at the pointed end, and 

 each contains a red pigment spot, chromatophore, and nucleus. 

 The zoospores thus resemble the organisms called flagellates 

 (Sec. 204), and like them swim freely around in the water by 

 the lashing movements of their cilia. But the zoospores have 

 a relatively short free-swimming period, for after perhaps an hour 

 or more they attach themselves by the ciliated ends to various 

 objects and grow into new Ulothrix filaments (Fig. 180, D). 



At times a much greater number of zoospores may be de- 

 veloped in the parent cells,- -perhaps 32 or 64, or even more 

 than a hundred (Fig. 180, E). These have generally only two 

 cilia and are much smaller than the four-ciliate zoospores, but 

 otherwise have the same structure. They swim very actively 

 for a short time, and then come together in pairs in the water 

 and fuse with one another (Fig. 180, F). This cell union is a 

 sexual process (see Sec. 200), and the small two-ciliate zoospores 

 are therefore sexual elements and are called gametes. The result 

 of this fusion is a sexually formed cell called a zygospore 

 (meaning a yoked spore), because the two gametes are similar, 

 like the halves of a yoke, and not different in form, as the sperm 

 and egg (see Fig. 178, Volvox). This simple type of sexual 

 reproduction is termed isogam.y, because the gametes have the 

 same form, or morphology. The zygospore, of course, corresponds 

 to the fertilized egg, or oospore, characteristic of higher plants. 



The zygospore of Ulothrix swims about for a short time with 

 its four cilia, and may only be distinguished from the large 



