PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 165 



of temperature occur, the same plants will not be found in 

 both. It is, perhaps, most important to note the tempera- 

 ture during certain critical periods in the life of plants, 

 such as the flowering period of seed-plants. 



Although the temperature problem may be compara- 

 tively uniform over any given area, the effect of it may be 

 noted m the succession of plants through the growing sea- 

 son. In our temperate regions the spring plants and summer 

 plants and autumn plants differ decidedly from one another. 

 It is evident that the spring plants can endure greater 

 cold than the summer plants, and the succession of flowers 

 will indicate somewhat these relations of temperature. 



It should be remarked, also, that not only is the tem- 

 perature of the air to be noted, but also that of the soil. 

 These two temperatures may differ by several degrees, and 

 the soil temperature especially affects root activity, and 

 hence is a very important factor to discover. 



At this point it is possible to call attention to the effect 

 of the combination of ecological factors. For instance, in 

 reference to the occurrence of plants in any association, the 

 water factor and the heat factor cannot be considered each 

 by itself, but must be taken in combination. For example, 

 if in a given area there is a combination of maximum heat 

 and minimum water, the result will be a desert, and only 

 certain specially adapted plants can exist. It is evident 

 that the great heat increases the transpiration, and trans- 

 piration when the supply of water is very meager is pecu- 

 liarly dangerous. Plants which exist in such conditions, 

 therefore, must be specially adapted for controlling tran- 

 spiration. On the other hand, if in any area the combina- 

 tion is maximum heat and maximum water, the result will 

 be the most luxuriant vegetation on the earth, such as 

 grows in the rainy tropics. It is evident that the possible 

 combinations of the water and heat factors may be very 

 numerous, and that it is such combinations that chiefly 

 affect plant associations. 



