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 problems 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 very 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 
