OP TEMPERATURE. 81 



the excitability of the tissue is aroused, and, at the 

 same time, insensible perspiration is commenced, the 

 effect of which is to bring into play the absorbing 

 powers of the roots, and thus to set the machinery of 

 vegetation in action. The degree of temperature 

 required to produce this effect is extremely variable 

 in different species of even the same climate, and is, 

 of course, much more variable between plants of dif- 

 ferent climates. For example, the common weeds 

 called Chick-weed, Groundsel, and Poa annua, 

 evidently grow readily at a temperature very near 

 that of 32° ; while the Nettles, Mallows, and other 

 weeds around them, remain torpid. In like manner, 

 while our native trees are suited to bear the low tem- 

 perature of an English summer, and, in most cases, 

 suffer if they are removed into a country much 

 warmer, such plants as the Mango, the Coffee, &c., 

 inhabitants of tropical countries, soon perish, even in 

 our warmest weather, if exposed to the open air. 



108. When, in the case of a given plant, the tem- 

 perature is permanently maintained at a much higher 

 degree than the species requires, it is over-excited. 

 If the atmosphere is preserved in a proportional state 

 of humidity, the tissue grows faster than the vital 

 forces of the plants are capable of solidifying it, by 

 the decomposition of carbonic acid, and by other 

 means ; its excitability is gradually expended, the 

 whole of its organisation becomes enfeebled, the vital 

 functions are deranged, and a state of general debi- 

 lity is brought on.* Such plants are soft and watery, 



* According to Mr. Knight, the effect of an excessively high tern 



4* 



