THE GEEEN ALG^ (CHLOEOPHYCE^) 197 



of branching enables it to expose much chlorophyll to the 

 light. Its rapid and luxuriant growth is evidence that it is 

 a successful plant. 



182. Cladophora : reproduction. It is probable that Cladoph- 

 ora reproduces itseK vegetatively by continued growth of 

 broken parts. Like Spirogyra, it does not have any regular 

 method of bringing about vegetative reproduction. At times, 

 however, in some of the segments, the contents divide into 

 large numbers of small zoospores. These escape from the old 

 wall, swim about for a time, and then become attached and grow 

 into new Cladophora plants. This method of spore formation 

 is much like that of the relatives of Pleurococcus, except that 

 zoospores of Cladophora are formed in much greater numbers, 

 by a special cell rather than by the whole plant, and under con- 

 ditions suited to abundant and wide distribution. In Fig. 159 

 is shown the method of reproduction of Ulothrix, an unbranched 

 plant which in its reproduction is quite similar to Cladophora. 

 Plants such as Cladophora and Ulothrix have still another 

 method of reproducing themselves. At times the cells, in- 

 stead of forming zoospores, form bodies which are like the 

 zoospores in form and movement, but smaller. It seems that 

 one of these alone cannot reproduce the plant, or, if it should 

 germinate, it produces a plant that does not live. It is not 

 a spore, since it cannot directly reproduce the plant which 

 formed it. Two of these bodies may unite and form a cell 

 which is capable of reproduction. These zoospore-like bodies 

 are gametes and unite to form a zygospore, which then pro- 

 duces a new plant. In Cladophora and other plants ( Ulothrix, 

 Drapamaldia, Fig. 160) that reproduce themselves as it does 

 there are interesting suggestions as to the origin of sexual re- 

 production among plants, namely, the origin of gametes from 

 small and apparently weakened zoospores. By the union of 

 these simple zobspore-like gametes there is formed the simplest 

 kind of sex spore, the zygospore. Spirogyra forms its zygospore 

 in somewhat the same way as does Cladophora, but the process is 

 more complex and no relation of gametes to zoospores is shown. 



