WITH ISTOTES ON OTHER SPECIES. 65 



mTRODUCTORY. 



The greater simplicity or apparent primitiveness of structure which is usually 

 associated, among the simplest plants, with life in the water, together with the fact 

 that various theoretical considerations point to the water as the probable habitat of 

 the earliest forms of life, lends to the study of the aquatic Thallophytes the highest 

 interest. Leaving out of consideration the natural and fairly circumscribed group 

 of the 8cliizophyta, the great body of these forms is made up of plants containing 

 chlorophyll, belonging to the algal stock, and presenting various lines of relationship 

 and descent. From this stock must undoubtedly have been derived the great 

 collateral chlorophyll-less groups of fungi. Most of the latter have been so modified 

 to meet the conditions of aerial life, that the absence of chlorophyll has become 

 of minor importance as a distinguishing character. A few, however, which cling to 

 the ancestral habitat have preserved so many of the essential algal features that 

 it becomes difficult to separate them from the Algue except on the basis of 

 their acquired saprophytic or parasitic habit and consequent loss of chlorophyll. 



All of the characteristically aquatic groups of fungi belong to the most 

 primitive or alga-like division — the Pliycomycetes — and they are all probably 

 to be regarded as primitively aquatic. Several interesting cases of transition 

 from aquatic to aerial life are, however, presented by species immediately related 

 to some aquatic ones and referrible to natural groups which may, in general, 

 be called aquatic. The aquatic Phycomycetes may be grouped under five heads, 

 as follows : 



1. ChytridiaceoB. 4. Saprolegniacece. 



2. Ancylistacem. 5. Pythium. 



3. Monoblepharidacece. 



The GhytridlaceoB constitute a heterogeneous group of considerable size and 

 of much variety of structure. Its members are regarded by some writers as the 

 most primitive of the fungi, and by others as degenerate forms derived from 

 the higher Phycomycetes. They possess a very rudimentary mycelium or none 

 at all, and commonly produce both sporangia, with zoospores, and resting spores. 

 An evident conjugation precedes the formation of resting spores in a very 

 few cases; but, for the most part, the group presents no traces of sexuality. 

 A considerable number of the species are parasitic upon terrestrial Phanerogams, 

 usually such as grow in wet places ; but the aquatic species constitute the larger part 

 of the group and are also chiefiy parasites. Their hosts include Pi'otozoa, Ayigail- 

 lulcB and Rotifera ; Saprolegniacece, Algce of every group, various spores and pollen 



