34 FRESH- WATER ALG^. 



Algse are able themselves to produce carbonaceous food- 

 materials out of carbonic acid by assimilation ; they are not 

 therefore usually either parasites or saprophytes, but can maintain 

 an independent life. Their dependence on assimilation requires 

 that they should inhabit localities where there is free access of 

 light ; while Fungi are not absolutely dependent on light for their 

 supply of food. 



The Fresh-Water Algae ought, from a strictly scientific point 

 of view, to be considered in conjunction with the marine Algae, as 

 they both belong to the same group. The only excuse for consider- 

 ing them apart is that of convenience to those microscopists who 

 desire some acquaintance with the organisms to be found in the 

 numerous ponds and canals of their neighbourhood. 



The classification of Algae, like that of Fungi, is still in a 

 transition state, and for the same reason, viz., that we are as yet 

 unacquainted with the full details of the life-history in many 

 species. 



For practical purposes, the classification adopted by Cooke in 

 his recently completed work on the Fresh-Water Algae answers 

 very well. He mentions that all Algae are associated under five 

 classes, viz. : — 



I. — CHLOROPHYLLOPHYCEiE, with the Cell contcnts mostly of a 

 chlorophyll-green. 



2. — PHYCOCHROMOPHYCEiE, with the cell contents mostly of a 

 bluish-green. 



3. — Melanophyce^, with the cell contents olive, brownish, 

 or blackish. 



4. — Rhodophyce^, with the cell contents rosy, purple, 

 crimson, or violet. 



5. — DiATOMOPHYCE/E, with an incombustible siliceous skeleton. 



The third class are all marine, and the majority of the fourth, 

 so that, exclusive of diatoms, which are a special study, the 

 Fresh- Water Algae are mainly included in the first two classes. 



The classification proposed by German botanists and accepted 

 by some in this country is made according to the method of 

 reproduction, and is similar to that of the Fungi, viz. : — 



I. — Protophyta, in which the multiplication of individuals is 

 effected by fission of the vegetable cells, 



