TEMPERA.TURE. 121 



heat and cold, and they may therefore be conveyed from one country 

 to another with safety. 



On the eastern continent there are, it is thought, about 160 natural 

 groups or families of plants, the tropical parts of which contain types 

 of the whole, and beyond which they become gradually extinct; 

 scarcely one half appearing in lat. 48 '. In lat. 65^ there are only 40, 

 and but 17 near the polar regions. In the tropics the woody species, 

 or the trees and shrubs, are equal to the herbaceous, annual, biennial 

 and perennial; but this proportion decreases from thence to the poles; 

 yet the perennial increase over the annual and biennial, so that near 

 the limits of vegetation they are as 20 to 1. The same elevations in 

 corresponding latitudes, do not however equally favor the vegetation 

 of the same plants, as circumstances often modify the degree of heat. 

 The greater the depth of valleys the greater is the cold on the sum- 

 mits of neighboring mountains ; hence plants thrive on some which 

 will not thrive on others of the same altitude, and this is likewise true 

 in respect to valleys. 



Temperature. The gelatinous, gummy and mucilaginous parts of plants 

 may be converted into sugar. Thus apple jelly treated with a vegeta- 

 ble acid dissolved in water, yields a sugar like that of the grape. So 

 also with the gum of the pea, placed with oxalic acid in a temperature 

 of 125 '. The gum from starch, mixed with the juice of green grapes, 

 renders it saccharine, and tartaric acid, aided by heat, produces the 

 same effect : this is the cause why most fruits become sweet when 

 cooked. The production of sugar and of many flavors, oils, etc., in 

 plants with little or no oxygen is attributable to high temperature and 

 bright sun light, while acidity is the result of opposite circumstances. 

 The effect of an excessive high temperature upon unisexual plants, is 

 to cause the production of male flowers, while that of a low tempera- 

 ture is to produce female flowers. Thus the same fruit-stalks may be 

 made to produce male or female flowers, in conformity with these cir- 

 cumstances. Plants are incapable of decomposing carbonic acid or 

 water at a low temperature and without light, and consequently of 

 assimilating their food ; they are therefore deprived of their green 

 color, flavor, sweetness, nutriment, fruit and flowers. A preternatural 

 elevation of heat and evaporation of moisture faster than the roots 

 can supply, hastens the secretions, so that the plants cannot elaborate 

 them or furnish parts to receive them, and the old leaves burn or dry 

 up and the young ones perish. But this heat and dryness is favor- 

 able for certain secretions and the former, with moisture, produces 

 leaves and branches, but these only : hence wheat, etc., are often, 

 raised in some climates as grass for fodder only, they producing no seed. 



A freezing temperature produces different effects, according to the 

 specific nature of the plants, some being destroyed by it, while others 

 endure great cold. The general effects are that the fluids within the 

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