Occurrence and distribution 139 



quite a different character from the ordinary zoogonidia of the gametophytic 

 thallus. The zoospore on germination produces a small asexual plant, and 

 there may be several generations of such asexual plants before a full-grown 

 sexual plant is formed. This phase in the life-history of Coleochsete has 

 generally been regarded in recent years as a rudimentary sporophyte gene- 

 ration, for the reasons that it is both post-sexual and asexual, and occupies 

 the same relative place in the life-cycle of the Alga as the sporophyte of the 

 Archegoniatas. In a recent publication by Allen ('05), who followed out the 

 nuclear changes on the germination of the zygote, it is stated that the first 

 division is a reduction division. Some confirmation of this statement is 

 certainly desirable since it means that the so-called sporophyte of Coleoch&te 

 is haploid and therefore part of the gametophyte generation. This develop- 

 ment of Coleochxte is by analogy a simple sporophyte, and it seems probable 

 that the precisely similar post-sexual development of (Edoyonium is homologous 

 with it ; but in view of Allen's statements, especially if confirmed, it is scarcely 

 possible to regard these post-sexual phenomena as phylogenetic fore-runners 

 of the sporophyte of the Archegoniatse. 



The term " sporophyte " has in recent years acquired a definite interpre- 

 tation as the non-sexual phase which occurs between sexual fusion and 

 chromosome-reduction, and if it be true that in the Chlorophycere chromosome- 

 reduction follows immediately upon sexual fusion, the sporophyte generation 

 simply does not exist in these Algse. 



Since in the Chlorophyceas the actual plant is the gametophyte, and the 

 production of motile asexual reproductive cells does not involve any cytological 

 change, they are referred to throughout this volume as ' zoogonidia.' 



OCCURRENCE AND DISTRIBUTION. The Chlorophycese exist in the most 

 varied habitats, occurring in every conceivable damp or wet situation to which 

 light can penetrate. Some are epiphytes (Coleoch&te, Aphanoch&te, Trente- 

 pohlia, etc.), others endophytes (Endoderma, Acroch&te, Blastophysa, etc.), 

 and a few are partial parasites (Phyllosiphon spp., Cepladeuros spp., Coccomyxa 

 Opliiurte). The great majority of the Green Algse are exclusively confined to 

 fresh water. The marine forms mostly belong to the relatively small groups 

 of the Siphonales, Siphonocladiales, and Ulvales, all three of which also include 

 a few freshwater representatives. 



The Chlorophyceous vegetation of northern seas differs very much from 

 that of tropical seas. In the more northern colder oceans the Green Alga? 

 are of little importance except for the Ulvacese and some of the Cladophoracese, 

 and such forms as occur are mostly confined to the uppermost part of the sub- 

 littoral region. In the tropics, on the other hand, not only are various members 

 of Ulvacese and Cladophoracese abundant, but the three families Codiacese, 

 Valoniacese and Caulerpacese, which are almost absent from northern seas, are 



