LOW TEMPERATURE. 109 



temperature, there is a proportionate abstraction of moisture, 

 the loss of fluid, by perspiration and evaporation, goes on faster 

 than the roots can make it good, or the tissue transmit it; old 

 leaves " burn " and dry up ; and young leaves perish as fast as 

 they are formed. 



Such being the result of preternaturally high temperature 

 in dryness and in moisture, it is 5asy to conceive that, although 

 such extremes cannot but be prejudicial, yet that they may be 

 approached for particular purposes with advantage. A high 

 temperature, and dryness, will be favourable to the formation 

 of secretions of whatever kind, while a high temperature, with 

 moisture, will lead to the production of leaves and branches 

 only. 



According to Humboldt^ this iappens 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 plaat. 



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 diminishing it are not less 

 disadvantageous. If the temperature to which a growing plant 

 is exposed is not lowered 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 the earth and air, but 

 it cannot assimilate it ; its tissue grows, but is not solidified by 

 the incorporation of assimilated matter; aqueous particles 

 accumulate in the interior, a general yellowness ensues, partly 

 from the want of a sufficient power of decomposing carbonic 

 acid, and partly from inability to decompose the water collected 

 in the interior. The consequence is a want of the means of 

 forming the usual secretions; flavour, sweetness, nutritive 



