172 
PLANT STUDIES 
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 eifect 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 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 society, 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 tran- 
spiration when the supply of water is very meager is pe- 
culiarly 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 com- 
binations of the water and heat factors may be very numer- 
ous, and that it is the combination which chiefly affects 
plant societies. 
