EFFECT OF TEMPERATURE. 107 



that of freezing, yet no plant is able to grow unless the 

 temperature is above 32°, for physical reasons that require no 

 explanation. When temperature rises, the air contained in the 

 minute cells of plants expands, the fluids become thinner, the 

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

 insensible perspiration is commenced, the effect of which is to 

 augment the absorbing powers of the roots, and thus to set 

 the machinery of vegetation in action. The degree of tem- 

 perature 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 different climates. For 

 example, the common weeds called Chickweed, GroundseU, 

 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 temperature 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. 



When, in the case of a given plant, the temperature is 

 permanently maintained at a much, higher degree than the 

 species requires, it is over-excited. If the atmosphere is pre- 

 served in a proportional state of humidity, the tissue grows 

 faster than the vital forces of the plant are capable of solidifying 

 it, its excitability is gradually expended, the whole of its 

 organization becomes enfeebled, the vital functions are 

 deranged, and a state of general debility is brought on. 



According to Mr. Knight, tlie effect of an excessively high tempera- 

 ture is to cause, in imisexual plants, the production of male flowers 

 only, while a very low temperature produces the contrary result. A 

 Water Melon plant was grown in a house, the heat of which was some- 

 times raised to 110° during the middle of warm and bright days, and 

 which generally varied, in such days, from 90° to 105°, declining 

 dtiring the evening to about 80°, and to 70° in the night ; the air was 

 kept dajmp by copious sprinkling with water, of nearly the temperature 

 of the external air, and little ventilation was allowed. The plant, 

 under these circumstances, grew with great health and luxuriance, and 

 afforded a most abundant blossom; but aU its flowers were male. 



