314 PLANT STKUCTDEES 



make almost identical demands upon their surroundings. 

 LTence it is usual for a plant association to be made up of a 

 large number of unrelated plants. 



There are numerous factors which combine to deter- 

 mine associations, and it is known as yet only in a vague way 

 how they operate. 



178. Ecological factors. — Water. — This is a very impor- 

 tant factor in the organization of associations, which are 

 usually local assemblages. Taking plants altogether, the 

 amount of water to which they are exposed varies from 

 complete submergence to perpetual drought, but within 

 this range plants vary widely as to the amount of water 

 necessary for living. 



Heat. — In considering the general distribution of plants 

 over the surface of the earth, great zones of plants are out- 

 lined by zones of temperature ; but in the organization of 

 local associations in any given area the temperature condi- 

 tions are nearly uniform. Usually plants work only at 

 temperatures between 32° and 122° Fahr., but for each 

 plant there is its own range of temperature, sometimes 

 extensive, sometimes restricted. Even in plant associations, 

 however, the effect of the heat factor may be noted in the 

 succession of plants through the working season, spring 

 plants being very different from summer and autumn 

 plants. 



Soil. — The great importance of this factor is evident, 

 even in water plants, for the soil of the drainage area deter- 

 mines the materials carried by the water. Soil is to be 

 considered both as to its chemical composition and its 

 physical properties, the latter chiefly in reference to its 

 disposition toward water. Soils vary greatly in the power 

 of receiving and retaining water, sand having a high recep- 

 tive and low retentive power, and clay just the reverse, 

 and these factors have large effect upon vegetation. 



Light. — All green plants can not receive the same amount 

 of light. Hence some of them have learned to live with a 



