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 
Siphonez 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 Volvocinee and Confervacee. 
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 
odgonium, where it is fertilized by the minute biciliate 
spermatozoids. 
The Siphonee exhibit great variety also in the non- 
sexual reproduction. Most of them produce zodspores, 
which are usually provided with two cilia, but, in the 
case of Vaucheria, are apparently multiciliate, owing 
to the fact that the individual biciliate zodspores are 
discharged in a mass and never separate. Besides 
the zodspores, 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. 
