422 GENERAL MORPHOLOGY OF PLANTS 



600. 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 

 important in photosynthesis, in determining direction of growth as 

 well as in determining the formation of suitable leaves in most 

 plants, and has an influence on the structure of the leaf accord- 

 ing 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. (= 43 F.), or in the 

 tropics at 10 to 12 C., but a much higher temperature is usually 

 necessary for reproduction. Some arctic algae, however, fruit at 

 i.8C. The upper limit of temperature favorable for plants in 

 general is 45 to 50 C., while the optimum is below this. Very 

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

 algae grow in hot springs where the temperature reaches 80 to 

 90 C. Som'e 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 extracting water from the protoplasm (see para- 

 graph 83). Wind may serve a useful purpose in pollination and 

 in aeration, but severe winds injure plants by causing too rapid 

 transpiration, by felling trees, by breaking plant parts, by deform- 

 ing 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 im- 

 portant for many trees and shrubs. It also prevents alternate 

 freezing and thawing of the ground, which " heaves " some plants 

 from the soil. Leaves and other plant-remains mulch the soil 

 and check evaporation of water. The influence of the chemical 



