76 



PLANT SOCIOLOGY 



The differences in dominance from one year to another are primarily 

 traceable in these communities to differences in precipitation. During 

 the extraordinarily dry year 1921 in Europe (288-mm. rainfall) there 

 occurred a great outburst of Protococcales and a reduction of Desmidi- 

 aceae. In the following year (804-mm. rainfall) the relations were 

 reversed and the desmids greatly predominated. 



For the development of plankton communities the temperature of 

 the water, its salt content, its acidity, and its nutritive content are of 

 decisive importance. Upon this last factor Thienemann (1926) 

 distinguishes three great types of continental waters : dystrophic, with 

 Sphagnum and Desmidiaceae, water rich in humus with acid reaction ; 



1919 1920 1921 1922 



Fig. 43. — Annual fluctuations in total plankton in a pond near Fontainebleau. 

 line, Desmidiaceae; dotted line, Protococcales. {After Denis.) 



Solid 



eutrophic, poor in humus, often rich in calcium, with a broad zone of 

 rooted plants, including Phragmites, Potamogeton, Cyanophyceae, 

 and many other forms; and oligotrophic, distinguished quantitatively 

 from the eutrophic, possessing only a narrow zone of rooted forms 

 because of the rapidly deepening waters (cf. Strom, 1928). 



Phytoedaphon. — The quantitative investigation of the edaphon and 

 aeroplankton is still in its infancy, and one must be content to get by 

 means of the culture methods of the bacteriologists an approximate idea 

 of the great wealth of species of the air and soil floras. The number of 

 organisms per gram of soil as reported by many investigators must 

 be taken with much reserve. By culture under determined artificial 

 conditions certain organisms will naturally be favored and will develop 

 rapidly. At the same time other species which perhaps were originally 

 very abundantly present may fall behind or be wholly suppressed by 

 the luxuriant growth of competitors. 



