FORMS OF J'EGETATION. 309 



Through this competition between species one kind may be 

 forced to migrate to some different region in order to main- 

 tain itself. The capacit)' of a plant to adapt itself to a differ- 

 ent environment determines the possibilit)' of its occupying 

 a new region, for here it must come into competition with 

 other sorts, and can only maintain itself if it is capable of so 

 modifying its form and structure as to adapt them to the new 

 conditions, and that as well as or better than the occupants 

 it finds in possession. In the beginning it was probably by 

 competition between species that water plants were gradually 

 adapted to an amphibious life, and then to a terrestrial life, 

 all the while advancing in complexity ; later some green 

 plants adapted themselves to a parasitic or saprophytic life; 

 plants of moist regions gradually nioved out and occupied 

 even the deserts ; plants loving the shade adapted themselves 

 to the direct light of the sun; and so on, until all parts of 

 the earth's surface and even considerable depths of the ocean 

 have been occupied. 



418. Environment. — In order to understand the variety 

 of factors which are acting upon any particular plant, it will 

 be instructive to consider the conditions which surround the 

 ordinary land plant. A portion of such a plant is imbedded 

 in the soil, and the remainder rises into the air. The sub- 

 terranean part is profoundly influenced by the size and form 

 of the soil piarticles, as well as by their chemical composition. 

 It is exposed to contact with water varying in amount, some- 

 times from day to day and always from time to time during 

 the year, holding man}' substances in solution in varying 

 amounts and kinds at different periods. It is subject, also, 

 to variations of temperature from day to day and from season 

 to season. 



The aerial part of such a plant is exposed to greater or 

 less variations of temperature from hour to hour, from day 

 to night, from day to day, and from season to season. It is 



