CONCLUSION 293 



among the Desmids, the external form of the individual 

 cells is also very complex. The sexual process is well 

 marked, but of the simplest kind, consisting in the union 

 of the contents of similar vegetative cells, with, at most, 

 only the slightest indication of any difference between 

 the sexes. The Conjugate, so far as we know, do not 

 lead on to any of the higher groups of plants. 



In quite a different direction green unicellular organ- 

 isms appear to have given rise to the remarkable group 

 represented among our types by VaucJieria. Here the cell 

 has become niultinucleate, and grows out into a large and in 

 some cases complex thallus, but without dividing, so that we 

 find a highly organised plant with non-cellular structure. 

 VaucJieria itself, though its thallus is simple compared 

 with that of the true Siphonese, has attained a very high 

 level as regards reproduction, for its sexual organs are as 

 sharply differentiated as in the higher multicellular plants. 



Differences of sex appear quite independently in 

 many diverse lines of affinity, among which that repre- 

 sented by Vaucheria is one of the most distinct. 



VaucJieria is also of great importance because it leads 

 directly to the Fungi. There can be no doubt of the 

 close relationship of such a Fungus as Pythium to such 

 an Alga as VaucJieria ; in fact Pythium might fairly be 

 described as a member of the Yaucheriaceae which has 

 lost its chlorophyll. The most important difference is the 

 disappearance of the spermatozoids, which are no longer 

 differentiated in the Fungus, the male protoplasm being 

 carried to the ovum by the fertilising tube. This change 

 has been compared to the change from fertilisation by 

 spermatozoids to fertilisation by a pollen-tube, in passing 

 from Cryptogams to Phanerogams. In both cases the 

 disappearance of motile male cells is correlated with 

 the loss of aquatic environment. 1 The gradual extinction 



1 See, however, p. 303. 



