58 EVOLUTION OF PLANTS 



a thallus of the type found in Vaucheria may have 

 arisen. 



The largest members of the order, which reaches its 

 highest development in the tropical seas, generally have 

 the large thallus made up of closely interwoven, much- 

 branched filaments, which, however, seldom show any 

 divisions except those by which the reproductive organs 

 are cut off. In regard to the latter, these large marine 

 Siphoneae are less highly developed than some of the 

 otherwise much simpler fresh-water genera. 



Within the series we find much the same progression 

 in the development of the reproductive parts that has 

 been described in the Volvocinese and Confervaceae. 

 Most of them show both non-sexual and sexual reproduc- 

 tion, the latter being of a low type in the greater number 

 of them, with little or no difference between the male 

 and female cells. The genus Vaucheria, however 

 (Fig. 12), shows perfectly differentiated sexual cells, 

 the larger passive egg-cell being retained within the 

 oogonium, where it is fertilized by the minute biciliate 

 spermatozoids. 



The Siphonese exhibit great variety also in the non- 

 sexual reproduction. Most of them produce zoospores, 

 which are usually provided with two cilia, but, in the 

 case of Vaucheria, are apparently multiciliate, owing 

 to the fact that the individual biciliate zoospores are 

 discharged in a mass and never separate. Besides 

 the zoospores, there are various forms of non-motile 

 spores, and the plants often increase in number by the 

 separation of a portion of the thallus. Indeed in Cau- 

 lerpa (Fig. 11) this is the only known method of repro- 

 duction. 



