288 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Different plants are variously adapted as to the minimum, optimum, 

 and maximum temperatures for their life as a whole as well as for 

 its component physiological functions, even though these actual 

 temperatures may change with variations in other conditions and 

 with the state of the plant (as well as, of course, differently with 

 different plants). 



Winter is normally a resting period when activity is at a minimum 

 in temperate regions, though many plants are active at much lower 

 temperatures in the polar lands and waters — some even below o^ C. 

 On the other hand, temperatures above the freezing point may 

 already be lethal to tropical plants ; so may temperatures above 

 45° C. if evaporation does not cool and save them. However, there 

 can be few places that are naturally too hot or too cold for any 

 plants. Important indirectly are clouds and other influences reduc- 

 ing the amount of direct insolation, and relative humidity which 

 greatly affects the loss of water by evaporation. The soil also has 

 a marked local effect, dry and dark types warming up much more 

 quickly than heavy waterlogged ones. 



Temperatures vary markedly at different levels as well as at 

 different times, and meteorological means {i.e. averages) are there- 

 fore of little value to the ecologist. An annual mean well below 

 freezing point is found in some continental regions where forests 

 abound, and although monthly means, and especially monthly mean 

 maxima and minima, are of more value to the plant scientist than 

 annual means, a thermograph tracing showing the continuous change 

 on the spot is most desirable for ecological purposes. To give any- 

 thing like a complete picture of the temperature-climate as it strikes 

 the plants, tracings should be obtained synchronously at each 

 different level or layer of vegetation and root-infested soil. In 

 intricate work involving, for example, the surfaces or the internal 

 tissues of leaves, thermocouples are employed instead of ther- 

 mometers, while for determining the approximate temperatures of 

 hard surfaces, etc., thin shavings of paraffins of different known 

 melting points are convenient. Fig. So indicates (A) the annual 

 mean temperature, and (B) the mean temperature of the warmest 

 month of the year, in different parts of the world. 



(3) Precipitation. The amount of rain, especially, falling in an 

 area during the year constitutes a factor of outstanding importance, 

 as it often mainly determines the availability of water for growth 

 and other vital processes. To this availability the local vegetation 

 largely corresponds ; and although the year's total is apt to be the 



