Freshwater Algce 11 



Class 4. Heterokontce (or the Yellow-green Algae), contain- 

 ing a large proportion of a yellow pigment known 

 as xanthophyll. The stored product of assimila- 

 tion is 'a fatty substance. Freshwater. 



Class 5. Bacillariece (or the Diatoms), containing a brown 

 colouring-matter diatomin, which much resembles 

 the phycophnsin of the brown Algae. Universal 

 both in fresh and salt water. 



Class 6. Myxophyceie (or the Blue-green Algae), containing 

 a blue colouring-matter known as phycocyanin. 

 The stored product of assimilation is most probably 

 glycogen. Mostly freshwater. 



By far the greater part of the vegetation of the sea consists of 

 marine Algae, and with few exceptions these marine forms are of 

 quite a different nature from the freshwater ones. It is only with 

 freshwater Algae that this volume is concerned. 



Certain Algae are known in a fossil state. These are mostly 

 Diatoms, the siliceous valves of which are eminently suited for 

 fossilization, and a few others in which the thallus was encrusted 

 with carbonate of lime. The majority of other Alga? are of much 

 too fragile and delicate a nature to become fossilized, and most of 

 the records of such fossil Algae are of very doubtful value. 



Freshwater Algae exhibit a variety of types of structure. 

 Some of them are unicellular, each plant consisting of a single 

 protoplasmic unit or energid (i.e. a mass of protoplasm containing 

 a single nucleus) surrounded and enclosed by a definite cell-wall 

 (e.g. Desmidiacese, Bacillarieae, and many Protococcoideae) ; others 

 are unseptate or ccenocytic plants composed of an aggregate of 

 protoplasmic units enclosed within a common cell-wall (e.g. Sipho- 

 nese, Hydrodictyon); others are incompletely septate plants, each 

 segment containing a number of protoplasmic units within a cell- 

 wall, the septation of the plant going on independently of the 

 divisions of the nuclei (e.g. Cladophorales) ; others are multi- 

 cellular or completely septate plants, each segment containing one 

 protoplasmic unit (e.g. Zygnemaceae, Chaetophorales, etc.). 



Many of the unicellular forms are solitary cells, but others 

 occur as colonies, in which the individual cells are more or less 

 loosely held together in a common mucilaginous envelope, which 

 is either secreted by the protoplasm of the cells or is derived from 



