228 OUTLINES OF PLANT LIFE. 



species better suited to the conditions of that region. 

 Through this competition between species one kind may be 

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

 tain itself. The capacity of a plant to adapt itself to diifer- 

 ent surroundings determines the possibility 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, at least as well as the occupants it' finds 

 in possession. In the beginning it was probably by competi- 

 tion between species that water plants were gradually forced 

 to adapt themselves to an amphibious life, and then to a ter- 

 restrial life, all the while advancing in complexity ; later some 

 green plants adapted themselves to a parasitic or saprophytic 

 life ; plants of moist regions gradually moved out and occu- 

 pied even the deserts ; plants loving the shade adapted them- 

 selves 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. 



317. 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 or- 

 dinary land plant. A portion of such a plant is embedded in 

 the soil, and the remainder rises into the air. The subterra- 

 nean part is profoundly influenced by the size and form of the 

 soil particles, 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 many 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 



