Il6 FRESH-WATER BIOLOGY 



in the determination of form. It was formerly thought that such 

 a polymorphism was characteristic for the majority of the higher 

 algae, but though frequent it is by no means universal. 



Certain of the genera of the unicellular algae must be closely 

 related to certain genera of the filamentous forms, such as Stichococcus 

 and Hormidium, Botrydiopsis and Conferva. The structure of 

 the cell, the color, size, and shape of the chromatophores, the repro- 

 duction, the chemica substances formed by the cells of the differ- 

 ent genera, are in each case identical, and practically the only 

 difference is that in the one case the cells are cylindrical and united 

 into a filament, while in the other case they may be somewhat 

 spherical and solitary. 



The resemblance is so great between the Chloromonadaceae, 

 Conferva, Botrydiopsis, and other forms in reference to the light 

 color, the small chromatophores, the nature of the zoospores, and 

 several other points, that many modern writers classify them 

 together under the head of Heterokontae, in spite of the fact that 

 some are unicellular, some flagellate, and some filamentous forms. 

 Though this resemblance is fully recognized by the writer, in this 

 brief outline of the fresh-water algae the older classification of 

 Wille will be retained. 



The adult algal cell is a typical plant cell, bounded by a mem- 

 brane, usually of cellulose, but in the Diatomaceae of a siliceous 

 nature. Just within the membrane is a layer of protoplasm which 

 encloses one or more vacuoles and in which are imbedded one or 

 more chromatophores occupying either a parietal or a central 

 position. The nucleus usually lies near the center. In by far the 

 larger number of species there is a single nucleus in a cell, but in 

 the Cladophoraceae and the non-septate Siphonales there are many 

 nuclei. The non-septate algae are called coenocytes. 



The chromatophores of the algae are large in proportion to the 

 size of the cell, and may be disc-shaped, plate-like, star-shaped, 

 or spiral. They may be regular or irregular, perforated, netted, 

 or entire. Nowhere else in the plant kingdom do we find such a 

 variety of shapes and structures among chromatophores as among 

 the algae. Within the chromatophores of many species is a body 

 denser in structure and albuminous in character, the pyrenoid. 



