FACTORS OF ENVIRONMENT. 479 



934. In the Sahara Desert the winds come mainly from the cool Medi- 

 terranean at the north, so that there is little or no precipitation. 



935. Temperature. Besides the physical effects of heat, heat 

 is to be considered as a climatic factor. The amount of heat 

 during the period of growth and reproduction has a very im- 

 portant bearing on the limits of plant distribution. Plants vary 

 in their total heat requirement, so they are drawn into broad 

 climatic zones, from polar lands to the equator and also more or 

 less parallel with mountain chains. Ocean currents affect the 

 climate, and also deflect these life zones, as they are called (see 

 Chapter XLVIII). "Glacial" epochs also profoundly influenced 

 the vegetation of the earth, since the southward extension of the cold 

 wave was so great as to destroy plant life over large regions, and 

 cause a general southward shifting of the life zones which moved 

 northward again as the glaciers retreated (see Chapter XLVIII). 



936. Physiography. The physiography of the earth's sur- 

 face exerts a powerful influence upon vegetation. Mountain 

 chains, oceans, rivers, etc., present barriers to plant migration. 

 They also produce lines of tension or stress between them and 

 adjacent territory. Rivers also act as conductive agencies. On 

 mountain sides lines of stress are also produced because of dif- 

 ference in altitude, and consequently difference in temperature, 

 so that a zonal arrangement of vegetation results. Thus the 

 life zones which are in general transcontinental are deflected far 

 to the south or north because of variations in temperature accom- 

 panying variations in altitude, or influenced by the different ocean 

 currents. In hilly countries, as well as in mountains, exposure 

 to light, the sun's heat, etc., affects vegetation, while the undu- 

 lating surface is one of the factors in determining the water 

 content of the soil. 



HI. Biotic Factors. 



937. Factors for pollination and distribution. Some of the 

 biotic factors have been discussed in other places. The agency of 

 insects in pollination, in the case of entomophilous flowers in Chap- 

 ter XLIII, has a great influence in increasing the fertility and 



