502 ECOLOGICAL CLASSIFICATION OF PLANTS 



cal zones are distinguished by their kinds of plants as well as 

 by their difference in temperature. When the temperature is 

 extremely low, as in the polar ice regions, or extremely hot, as 

 in some deserts, very few or no plants at all are able to live. 

 Even on the same area, as in a woods or a field, if the plants are 

 not disturbed, one can observe the effect of the heat factor in 

 the succession of plants through the growing season, the spring 

 plants being very different from the summer and autumn 

 plants. To secure crop plants adapted to the temperatures of 

 the different agricultural regions is also one of the pro])lems of 

 Agriculture. 



Light. — Not all plants in an association can receive the same 

 amount of light, and some plants are so adjusted that they can live 

 in the shade. They are known as shade plants, and the Ferns, 

 common in the woods, are examples of such plants. But even 

 in an association of herbaceous plants, as in a field of weeds, 

 many small plants grow among and in the shade of the taller 

 ones. Some plants, like the Pumpkins and Melons which grow 

 well along with Corn, have a ver}^ large leaf surface which may 

 compensate for the lack of light. Plants climb other plants or 

 walls, grow tall erect stems, and adjust themselves to neigh- 

 boring plants in various other ways in order to obtain sufficient 

 light. 



Soil. — The soil in regard to its chemical and physical prop- 

 erties determines largely the kinds of plants that can grow in a 

 given region. Thus the plants on a sandy beach or sand dune 

 differ from those on a clay or loam soil. The chemical elements 

 of a soil and its power to retain water both have a determining 

 effect upon the growth of plants. Some plants, like Alfalfa and 

 some of the Clovers, are more sensitive than the grains to acids 

 in the soil. Some weeds, like the Sheep Sorrel, grow best in an 

 acid soil. Some plants require more potash, nitrates, or some 

 other element than other plants. Even water plants are some- 

 what dependent upon the soil, for the minerals in a pond or 

 lake are carried in from the soil. One of the chief problems of 

 Agriculture consists in putting the soil in a suitable condition 

 for plants and in choosing plants adapted to the different types 

 of soil. 



Wind. — The wind tends to dry out plants by increasing their 

 transpiration, while at the same time it is an important agent 



