188 The Pi.ant^World. 



□ M 



in great masses; Protoeoecoideae are numerous, but Desmids 



rare. 



4. The phytoplankton of the Central European alpine lakes 

 is different according as the lakes are low-lying large lakes or 

 low-alpine, the latter having a poor phytoplankton made up of 

 Diatoms and Flagellates. 



5. The phytoplankton of the Mediterranean lakes, of the 

 North American lakes, of the Asiatic lakes, and of the tropical 

 lakes are still insufficiently known. 



The life conditions and adaptation forms of the plankton of 

 each geographical area are discussed at length, and the data 

 summarized as follows: The fresh water plankton is character- 

 ized by its well-marked cosmopolitanism; the fresh water asso- 

 ciations (communities), in contrast to all other associations on 

 land and water, everywhere contain the same types and nearly 

 everywhere the same species. The fresh water plankton is 

 amongst the oldest associations of the earth. This fact may be 

 explained by its very great power of adaptation to outward con- 

 ditions. 



The different plankton associations of the Danish fresh waters 

 are arranged in nine categories; the phytoplankton includes: 



1. The true lake plankton, which differs according to depth 

 of the lakes. In deep lakes the Diatoms produce enormous 

 maxima from March to November, although in the warmest 

 period Ceratium hirundinella is the main form. Chlorophyceae 

 are very rare; Myxophyceae few (only Oscillatoria and Anabaena 

 flos aquae are at times dominant). The shallow lakes are chiefly 

 characterized by the water-bloom, which is due to the dominance 

 of various Myxophyceae; the Diatoms are of minor importance, 

 while the Chlorophyceae may be common. 



2. The plankton of coast lakes is little investigated, but 

 it seems to be very poor; it contains much detritus. 



3. The plankton of dune lakes contains very few Myxo- 

 phyceae and Diatoms, but Flagellates in considerable quantities, 

 also many small Chloiophyceae. 



4. The plankton of heath lakes is very like number three, 

 but Dinobryon seems to be dominant. 



5. The pond plankton is very rich in species and varies 

 greatly from pond to pond. In contrast to the lake plankton 



