THE GREEN ALG^ 95 



cheria obviously exhibits very marked and fundamental analogies 

 with the Phycomycetes, and it seems probable that Vaucheria 

 shows us a remnant of those forms which were transitional 

 between the Siphoneae and the Phycomycetes. The reproduc- 

 tive organs of Vaucheria in themselves exemplify a number of 

 possible stages in the transition between the two groups, as in 

 the antheridium of this genus we still have liberation of the indi- 

 vidual motile elements as spermatozoids, in the zoosporangium we 

 have coalescence of the motile elements to form the synzoospore 

 (or we might call it ccenozoospore), whilst in the oogonium we 

 have a motionless coenogamete, becoming uninucleate at maturity 

 (cf. also Lotsy 46). The fact that some species of Vaucheria 

 favour a terrestrial habitat may perhaps have some connection 

 with the evolution of its oogonium and peculiar type of zoospore. 



The process of zoospore-formation in the curious genus 

 Derbesia (a member of the Siphoneae), which has recently been 

 investigated by Davis (Davis 20a), shows many points of interest 

 in this connection. The young zoosporangium, as in Vaucheria^ 

 contains numerous nuclei, a large number of which, however, 

 subsequently degenerate. Cleavage of the protoplasm takes 

 place around the remaining nuclei, and in this way a number 

 of uninucleate zoospores are formed. Each zoospore is provided 

 with a circle of cilia at its front end. In the zoosporangium 

 of Derbesia one therefore finds the same tendency towards 

 abortion of nuclei as in the oogonium of Vaucheria or of the 

 Phycomycetes, but this process of abortion does not go to the 

 extreme of leaving only a single functional nucleus within the 

 sporangium. In its vegetative characters Derbesia shows con- 

 siderable resemblance to Vaucheria ; the results of Davis's 

 investigation of the former afford additional evidence of the 

 existence of a group intermediate between Siphonales and 

 Phycomycetes. 



If we decide to regard Vaucheria as a form leading over 

 to the Phycomycetes, we have still to consider its relation to 

 the siphoneous series. Although the spermatozoids show an 

 indication of Heterokontan characters (cf. below, p. 104), the 

 units of the zoospore are typically Isokontan {i.e. with two 

 equal cilia) and agree with the type of motile element prevalent 

 among the Siphoneae. The chloroplasts of Vaucheria are, on 

 the whole, more like those of the Siphoneae than those of the 

 Heterokontae ; I am not aware that the yellowish tinge of 



