Introduction 5 



vegetable organisms occurring in the plankton form the food of 

 most of the smaller aquatic animals, and so, indirectly, form the 

 basis of the food-material of lacustrine and river fishes. Little is 

 known concerning the food-value of the freshwater plankton, but 

 statistics of this nature have been very carefully compiled with 

 regard to the Baltic Sea. Brandt 1 states that the chemical com- 

 position of the plankton of this sea in autumn and winter is 

 intermediate between that of " rich pasturage " and green lupines. 

 The proportion of fat is greater than in land products used as 

 fodder, but in spring the great abundance of Diatoms causes such 

 a great increase in the amount of ash as to preclude direct com- 

 parison with land plants. Many of the Algas found in the plankton 

 are more or less characteristic, some of them being largely and 

 others entirely surface organisms. The majority of them belong 

 to the Palmellaceae, Protococcaceae, Volvocaceae, Desrnidiaceae and 

 Bacillariese. Most of the Desmids of the plankton possess very 

 long spines or processes which terminate in spines, and in those 

 species which normally possess long spines the latter are of greater 

 length when the plants occur in the plankton than when found in 

 other situations. Some of the Protococcacese and Diatoms have 

 also acquired long spines. The assumption of this spined con- 

 dition is to be correlated with their free-floating existence and 

 their consequent need for greater protection against those animals 

 of the plankton which feed on Algae 2 . 



Algae exist under very varied conditions of temperature. In 

 temperate and arctic climates many of them can survive prolonged 

 freezing even when in the ordinary vegetative condition. It is 

 quite possible to melt out from the ice numerous healthy Alga3 

 which have suffered in no way from their exposure to such a low 

 temperature. In the arctic and antarctic regions, in the Alps and 

 in the Andes, there is a gjio^-flora. consisting principally of Algae 

 which pass their entire existence on the snow and ice. This 

 collection of Algae, which is known as the ' Cryoplankton,' consists 

 of a few forms which are more or less universal in such situations 3 . 



1 Brandt in Wissensch. Meeresimtersuchungen, N. Folge, Bd iii, Heft 2, 1898 ; 

 consult also American Naturalist, xxxii, Dec. 1898. 



2 West & G. S. West, 'Scott. Freshw. Plankton,' Journ. Linn. Soc. Bot. xxxv, 

 Nov. 1903, p. 554. 



3 The most interesting of these Algae are Sphcerella nivalis Sommerfeldt (the Eed 

 Snow plant) and a Desmid, Ancylonema Nordenskioldii, first discovered byBerggren 

 in the snows of Greenland, and afterwards by Lagerheim in the Andes and by Chodat 

 on Mont Blanc. 



