﻿334 BOTANICAL GAZETTE [may 



less highly specialized than the first and the Saprolegniales lower 

 than the Peronosporales with respect to sexual processes. Bat 

 oogenesis in these two groups shows such marked differences 

 in their evolutionary tendencies that the question of the relative 

 level of each process has very little import. 



The Saprolegniales are more difficult to understand in rela- 

 tion to a coenogamete ancestry than the Peronosporales, because 

 the many eggs without periplasm suggest at once the stage in 

 heterogamy illustrated by Sphaeroplea. However, the processes 

 of oogenesis are probably very different in the two types. The 

 ^gg origins of Saprolegnia hav^e a great many potential gamete 

 nuclei, and that stage indicates strongly the coenogamete ances- 

 . try. By numerical reduction ot the gamete nuclei the o,^^ of 

 Saprolegnia has proceeded to a point where it has almost ceased 

 to be a coenogamete, that condition only being presented in the 

 bi- and trinucleate eggs. 



It will be difficult for many to give up the idea that Vaucheria 

 is not a suitable starting point for^the line of higher Phycomy- 

 cetes. The chief objection is the incompatibility of the processes 

 of oogenesis where relationship demands agreement even in the 

 details of cytology. We have only the accounts of Oltmanns 

 (1895), Behrens (1890), and Klebahn (1892, p. 237), which are not 

 in complete agreement on some important points, and perhaps 

 further stwdy may reveal conditions that are only suspected. 

 In considerations of this sort it is important to know the relation 

 that Vaucheria bears to the algae as a whole. Although gen- 

 erally classed among the Siphonales, Vaucheria has little in 

 common with that group excepting the coenocytic thallus. It 

 stands alone as the only heterogamous form (oosporic) in a very 

 large assemblage characteristically isogamous. Generally pre- 

 sented as a type of the Siphonales, Vaucheria is not really 

 representative of that group, which is much better illustrated by 

 such forms as Codium, Bryopsis, or Penicillus. The affinities of 

 Cladophora with the Siphonales are now better understood, and 

 we see that this form except for the septate thallus — whose cells 

 are, however, multinucleate — has all the characters of the 

 Siphonales. It is this region of the algae (Cladophora, Codium, 



