172 



PLANT STUDIES 



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

 hotli. 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-jolants. 



Altliough the temperature proljlem may be compara- 

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

 noted in the succession of plants through the growing sea- 

 son. In our temperate regions the spring plants and summer 

 plants and autumn jilants ditt'er 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 tlie 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 fa('tors. For instance, in 

 reference to the occurrence of plants in any society, the 

 water factor and the heat factor cannot he considered each 

 by itself, Init must be taken in combination. For examiflo, 

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

 and minim uni water, the residt will be a desert, and only 

 certain specially adapted plants cari exist. It is evident 

 that the great heat increases the transpiration, and tran- 

 spiration when tlie supply of watei' is very meager is pe- 

 culiarly dangerous. Plants which exist in sucli conditions, 

 therefore, must be s]>ecially adapted for controlling tran- 

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

 tion is maximum lieat aiul nuiximum 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 com- 

 binations of the water and heat factors may be verv numer- 

 ous, and tliat it is the combination wlii(;h chiefly a.ft'ects 

 plant societies. 



