g6 BOTANICAL GAZETTE [februaxy 



those relating to the chemical and physical quality of the soil. I call 



02T 



more 



groups 



Historical factors are those which in the 



course of time exert an influence on the topography and thus indirectly 



on the plant covering. 



Another series of factors analogous to the physical factors influen- 

 cing plant life are the biotic. They are either zoobiotic or phytobiotic. 

 Of the former especially the influence of man has to be recognized in 

 connection with the study of vegetation on coastal sands. The 

 phytobiotic factors are those caused by the plants themselves, and the 

 mutual relationship of sand plants will be discussed in another paper. 



Atmospheric factors 



1m 



factors influencing plant life, or to give one of these factors precedence 

 in rank before another, because this depends in different cases on 

 different conditions. It may be said, however, that the whole group 

 of atmospheric factors is the most important, especially because of their 

 influence on the transpiration of the plants. Sand vegetation 

 particularly affected by: (i) the large amount of heat absorbed b) 

 the sandy ground and reflected from its surface; (2) the intensity « 

 illumination, both direct on the open, unprotected formation, and 

 reflected from the white sand ; (3) the exposure to winds, which con- 

 stantly change the atmosphere. 



Temperature. — If we compare in a general way the temperature 

 conditions in a few of the localities under consideration, we find tha 

 there is no significant difference in results as regards development 



formation 



On 



the dunes along the Gulf of Finland, where the vegetation is in a dor- 



months 



during winter, when the grains are covered with a thin surface of tf e - 

 and are smooth. The friction is less and they are able to move skwJ 

 forward. The herbaceous vegetation is absent at this time, and t e 

 deciduous shrubs are without leaves. There is consequently nothing 

 to arrest the movement of the sand. 



Q 



semitroDical climate 







