INFLUENCE OF THE AIR AND MOISTUKE. 475 



growth, and though two localities may have the same mean annual tem- 

 perature, it does not follow that their maximum of cold for the winter, 

 and their maximum of heat for the summer, should coincide. It was 

 such considerations that led to the construction of isotheral lines, or those 

 of equal summer heat, and isochimenal lines, or those of equal winter 

 cold. 



Into the causes which bring about this difference of heat distribution 

 it is not necessary for us here to inquire minutely. They Causes of the 



are very various. The prevalent winds at different seasons S " 



of the year, ocean currents, the geological structure of a coun- heat. 

 try, even what might be termed its optical qualities, that is, its power of 

 absorbing the rays of the sun (for instance, the great Desert of Sahara 

 disturbs the temperature of all Europe), and upon like principles must 

 act the removal of extensive forests, and their substitution by equivalent 

 surfaces of cultivated, differently colored, and differently absorbing lands, 

 elevation above the sea level, for the higher the country the lower its 

 temperature : these, and a multitude of other such conditions, impress 

 an effect upon the distribution of heat. The mean annual temperature 

 represents these and all other such influences, and includes all the varia- 

 tions, diurnal and nocturnal, monthly and seasonal, for the year. 



The organic functions of a plant demand particular temperatures at 

 particular times. There is, doubtless,^ special degree best suited to the 

 period of germination, another to the period of aerial growth, another to 

 the period of fertilization, and another to that of ripening the seeds ; and 

 these degrees differ in the case of different plants. Where the require- 

 ments become so complicated, it would be erroneous to expect that the 

 mean annual temperature should satisfy them all. 



Connected in part with temperature, and in part with elevation above 

 the sea, are the variations in the density of the air. These influence of va- 

 control, to a certain extent, the aerial supply to plants, the riations in the 



. i . . , T density of the 



quantity presented to their leaves diminishing as the density air moisture, 

 becomes less. etc ' 



The same observation may be made respecting moisture, which, as is 

 very well known, constitutes one of the most influential conditions in de- 

 termining the growth of plants, and this in a double way, either as va- 

 por contained in the air or as rain. The effect of rain in this respect is 

 twofold : it diminishes the quantity of atmospheric carbonic acid by 

 exerting over it a solvent power, carrying it into the ground, and thereby 

 reducing, by sometimes as much as one half, the supply on which the 

 leaves are depending ; it also brings in larger quantities to the interior 

 of the plant the saline constituents of the soil which are requisite for tis- 

 sue development. 



To variations in the temperature, the density of the air, and its moist- 



