112 PLANT SOCIOLOGY 



structure and arrangement of plant communities. Even the slightest 

 differences in water supply are clearly shown in the accompanying 

 vegetation. 



Moisture, as a climatic habitat factor, is determined by the amount, 

 duration, and seasonal distribution of the precipitation as rain or snow 

 and by the humidity of the air. 



Soil moisture will be considered in connection with edaphic factors. 



A. Precipitation 



The water economy of the plant aims at an equality between income 

 and outgo of water, between absorption and transpiration. The water 

 requirement is generally met by rain and dew, precipitating the water 

 vapor of the air in liquid form. Hoarfrost and hail, though only 

 occasional, are harmful to vegetation. It is quite otherwise with snow. 

 The effect of snow upon the forms of vegetation in the cold and cool- 

 temperate regions has been rightly appreciated only within the last few 

 decades. 



Rainfall. — Next to the distribution of heat the annual distribu- 

 tion of rain is the most important factor for the general character and 

 periodicity of vegetation. The longer and more intense the dry season 

 the more obvious is the tendency of vegetation to complete its chief 

 development during the rainy season. In Europe the difference 

 between the sharply periodic wet and dry climate of the Mediterranean 

 region and the rainy climate of the Atlantic region with its evenly 

 distributed precipitation is conspicuously expressed in the vegetation. 

 In the Atlantic region luxuriant evergreen meadows, P^ricaceae heaths, 

 and broom thickets (Ulex, Sarothamnus, Genista) are continuously 

 photosynthetic. In the Mediterranean region, with its spring and 

 autumn vegetative periods separated by dry summer and winter, we 

 have the dull colors of the broad sclerophylls and therophytes (Fig. 

 62). Eastward and southward the rain periods of the autumn and 

 spring seasons converge into an autumn-winter rain period (southern 

 Italy, northern Africa). 



As we approach the great desert plateaus of Africa and Asia, the 

 summer drought becomes more intense and the winter rains more 

 sparse. Here the beginning and the end of the yearly cycle of vegeta- 

 tion are bound up with the duration of the winter rains. In the desert, 

 at the limits of life, a few halophilous chamaephytes and emphemeral 

 therophytes wait for years in a drought rigor only to shoot forth in 

 an hour into new life with the vivifying rain. 



Contrasts in Precipitation. — The sharpest known boundaries of 

 vegetation are formed by high mountain chains lying across the course 



