CONCLUSION 293 



members of the group, namely, in some Desmids, the ex- 

 ternal 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 Algae 

 appear to have given rise to the remarkable group 

 represented among our types by Vaucheria. Here 

 the cell has become multinucleate, 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. Vaucheria itself, though its thallus 

 is simple compared with that of the true Siphoneee, 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. 



Vaucheria 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 Vaucheria ; in fact Pythium might fairly be 

 described as a member of the Vaucheriacese 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 



