FACTORS INFLUENCING VEGETATION TYPES. 465 



which means a study of plants in their home or a study of the 

 household relations of plants. These factors are of three sorts: 

 ist, physical factors; 2d, climatic factors; 3d, biotic factors. 



901. Physical factors. Some of these factors are water, light, 

 heat, wind, chemical or physical condition of the soil, etc. Water 

 is a very important factor for all plants. Even those growing on 

 land contain a large percentage of water, which we have seen is 

 rapidly lost by transpiration, and unless water is available for 

 root absorption the plant soon suffers, and aquatic plants are 

 injured very quickly by drying when taken from the water. 

 Excess of soil water is injurious to some plants. Light is impor- 

 tant in photosynthesis, in determining direction of growth as 

 well as in determining the formation of suitable leaves in most 

 plants, and has an influence in the structure of the leaf according 

 as the light may be strong, weak, etc. Heat has great influence 

 on plant growth and on the distribution of plants. The growth 

 period for most vegetation begins at 6 C. (=43F.), or in the 

 tropics at io-i2C., but a much higher temperature is usually 

 necessary for reproduction. Some arctic algae, however, fruit 

 at i.8C. The upper limit favorable for plants in general is 

 45-5o C., while the optimum temperature is below this. Very 

 high temperatures are injurious, and fatal to most plants, but 

 some algae grow in hot springs where the temperature reaches 

 8o-90 C. Some desert plants are able to endure a temperature 

 of 70 C., while some flowering plants of other regions are killed 

 at 45 C. Some plants are specifically susceptible to cold, but 

 most plants which are injured by freezing suffer because the 

 freezing is a drying process of the protoplasm (see p. 374). Wind 

 may serve useful purposes in pollination and in aeration, but 

 severe winds injure plants by causing too rapid transpiration, 

 by felling trees, by breaking plant parts, by deforming trees and 

 shrubs, and by mechanical injuries from "sand-blast." Ground 

 covers protect plants in several ways. Snow during the winter 

 checks radiation of heat from the ground so that it does not 

 freeze to so great a depth, and this is very important for many 

 trees and shrubs. It also prevents alternate freezing and thaw- 



