122 FRESH-WATER BIOLOGY 



gonium, and Bulbochaete are found. Chara and Nitella are found in 

 huge beds at the bottom of lakes at a depth of from one to many feet. 



Of the unicellular forms also, different species occur under different 

 conditions. An especially favorable position for this group is 

 among the leaves and on the surface of the higher aquatic plants. 

 Indeed unless higher algae or phanerogams exist in certain locali- 

 ties but few of the minute forms are ever found. There seem to be 

 certain preferences on the part of different species of unicellular 

 algae as to the forms of the higher plants with which they associate. 

 This may be simply that the shape, texture, and arrangement of 

 parts of certain of these plants afford a better shelter and protec- 

 tion for the single cells than do others, but it is more probable that 

 the plant itself exerts some chemical influence which is attractive 

 or repulsive to these forms. For instance, enormous numbers of 

 different species may be found growing among Chara, while in 

 cormection with Ceratophyllum, the leaves of which are very finely 

 cut, but few species occur. 



The endophytic forms, such as Endosphaera and Scotinosphaera, 

 live principally in the tissues of Potamogeton, Lemna, and other 

 water plants, though they may also be found outside of the tissues. 

 The discoid forms, such as Coleochaete and Ulvella, are found on the 

 surface of the broader-leaved types of submerged plants, especially 

 on Potamogeton; and the unicellular blue-green forms occur abun- 

 dantly among the Charas, though they are also numerous in most 

 stagnant water. 



In the plankton are always foimd many species that exist in the 

 littoral region, but there are also many forms which are distinctively 

 plankton types. These are characterized by a great surface in 

 proportion to the mass of the cells, thus rendering them more 

 buoyant. This is provided for in several ways: by the presence 

 of long gelatinous or cellulose spines, as in Chodatella and Rich- 

 teriella; by the union of cells into ribbons or bands, as in Fragila- 

 ria; and by the production of a homogeneous gelatinous matrix in 

 which the cells are imbedded, as in Kirchneriella and Sphaerocystis. 



In studying the Hfe history of the algae, cultivation is absolutely 

 essential in order that development may be traced from step to 

 step without confusing the different phases of the form in ques- 



