OF TEMPERATURE. 83 



them can be formed ; the old leaves " burn" and dry 

 up ; and young leaves perish, in like manner, as fast 

 as they are formed. 



109. Such being the result of preternaturally high 

 temperature in dryness and in moisture, it is easy to 

 conceive that, although such extremes cannot but be 

 prejudicial, yet that they may be approached for parti- 

 cular purposes with advantage. A high temperature 

 and dryness will be favourable to the formation of se- 

 cretions of whatever kind ; while a high temperature, 

 with moisture, will lead to the production of leaves 

 and branches only.* 



110. An unnaturally low temperature is productive 

 of evils of another kind. A certain amount of heat 

 is necessary to each particular species, to enable it to 

 grow, at all: the immediate effect of heat being to 

 rouse the vital forces, and to bring them into action. 

 If the amount of heat to which a plant is exposed be 

 sufficient to effect this purpose, the functions of the 

 plant are natural and healthy ; the consequences of 

 exceeding it have been explained, those of diminish- 

 ing it are not less disadvantageous. If the tempera- 

 ture to which a growing plant is exposed is not low- 

 ered so much as to destroy it, but just reduced to that 

 point within which it will continue to live, the plant 

 is brought, by the absence of a sufficient exciting 

 cause, into a state not unlike that already described as 

 resulting from over-excitement. It absorbs food from 



* According to Humboldt, this happens to the Wheat grown about 

 Xalapa in Mexico, which will not mount into ear, but produces an 

 abundance of grass, on which account it is cultivated as a fodder plant 



