162 PLANTS MODIFIED BY THEIR SURROUNDINGS 



amount of water in the soil and air surrounding them. Such are 

 most of our forest and fruit trees, and most of our garden vege- 

 tables. Conditions of moderate moisture are called mesophytic; 

 the plants living thus are known as mesophytes. 



It may easily be seen that plants which are mesophytes at one 

 time may under some conditions of weather be forced to undergo 

 xerophytic or hydrophytic conditions. An oak tree may receive 

 no water through the roots during the winter because the surface 



A mesophytic condition. A valley in central New York. 



of the ground is frozen, thus preventing water from finding its way 

 below the surface ; on the other hand, during excessive rains in 

 the spring it might exist for a time under almost hydrophytic 

 conditions. But many trees are annually killed in districts where 

 lumbering is going on through the damming of streams and forma- 

 tion of artificial ponds, which increase the water supply of the trees 

 near by and soon kill them. 



Other Factors. It is a matter of common knowledge that plants 

 in different regions of the earth differ greatly, from one another in 

 shape, size, and general appearance. If we study the causes for 



