THE GREEN ALG^ 97 



the zoospore of VaucJieria and the gemmae of DicJiotomosiphon. 

 In the structure and character of the sexual organs there is 

 almost complete agreement between the two genera, but their 

 position in Dichotomosiphon is peculiar, since they are here 

 seated at the ends of the final forking branchlets of the filament 

 and not laterally as is the case in the majority of species of 

 Vaucheria ; but even in this respect we find in certain species 

 of the latter genus (e.g. V. piloboloides) some analogy. The 

 previous considerations seem sufficient to establish a fairly close 

 relationship between Vaucheria and the Siphoneae ; at the same 

 time we have learnt to regard the former as a transitional type 

 to the Phycomycetes. 



The Ulotrichales, to which we may now turn our attention, 

 do not require a lengthy treatment. We have already seen how 

 this series can be derived from the Protococcaceous stock by 

 way of a form like Chlorosphcera^ which combines their two 

 chief characteristics — vegetative division and zoospore-forma- 

 tion. Another view derives them from the " filamentous " Tetra- 

 sporaceae (Blackman 5) but, as already pointed out above, the 

 forms included under this name are probably largely stages of 

 Ulotrichales, and, although not altogether indisputable, such an 

 ancestry is not easy to support in the light of our present 

 knowledge. The actual line of evolution followed by the Ulo- 

 trichales is shrouded in darkness. They show the most elaborate 

 development of the septate filamentous habit among the green 

 Algae, especially if (as above advocated) we include the Clado- 

 phoraceae in this series. They are the most likely group from 

 which to derive the higher plants (Archegoniatae), but how 

 this derivation was effected it is at present impossible to say, 

 although a form like Draparnaldia (Berthold 4), with its upright 

 and almost purely supporting main axis bearing richly branched 

 lateral assimilating threads, shows the morphological possibilities 

 of the simple filamentous habit. There are various other 

 evolutionary tendencies among the Ulotrichales that may be 

 briefly indicated. One of the most important is the tendency 

 to depart from the upright habit and to replace it by an epi- 

 phytic thallus (Huber 35) of richly branched filaments emanating 

 from a central point (well seen in the common genus Aphano- 

 chaie and its allies) ; in the highest forms {Coleochcete^ etc.) this 

 culminates in the formation of a compact disc-like plate, which 

 in some cases may even become an endophyte. It is probable 



7 



