FACTORS OF ENVIRONMENT. 467 



905. Hydrodynamic forces. There should be added also the 

 hydrodynamic forces as a factor in influencing vegetation. The 

 beating of the waves on rocks, or the force of the water at water- 

 falls, or in rapids, or flowing water, encourages a characteristic 

 water flora, and water also assists in the distribution of seeds or 

 plant parts. Of the greatest importance is the action of water 

 in eroding the surface of the earth, resulting in the so-called lev- 

 elling process; also the effect of floods and the cutting of the 

 channels for streams. The mechanical effect of precipitation is 

 sometimes injurious, especially in the case of hail, and in heavy 

 snows which cling to branches and weight them down. 



906. Light. Light as an ecological factor influences plants 

 in several ways: ist. Influence of light in photosynthesis enables 

 the plant to obtain the necessary carbohydrates. 2d. The influ- 

 ence of light in the direction of growth of stems and in the 

 position assumed by leaves brings the plant organs in a position 

 favorable for photosynthesis. 3d. In very many plants the 

 leaves remain rudimentary unless they are in the light, and 

 light in most cases is necessary for the formation of the green 

 chlorophyll. Since some plants are able to outgrow other plants 

 in their adjustment to the light relation, many of these, instead 

 of dying out for want of brilliant illumination, have accommo- 

 dated themselves to various degrees of intensity of diffused light, 

 as in the case of shade plants and deep-water plants. Many 

 <hade plants are injured when subjected to direct sunlight, so 

 that light as well as water is harmful to certain plants under cer- 

 tain conditions, and its modifying influence is therefore greater 

 in plant forms than it otherwise would be. An abundance of 

 light is usually necessary for development of flowers and fruit. 

 Light also accelerates transpiration, and is thus of direct benefit 

 to the plant when supplied with an optimum of soil moisture, 

 and injuries sometimes result to plants when root absorption is 

 active and transpiration is slow. Leaves exposed to direct sun- 

 light have a thicker epidermis than shade leaves, and in the 

 latter the palisade layer of cells is less developed and the leaves 

 are thinner and softer. 



