LIGHT AND HEAT. 119 



from the absence, not of heat, but of light ; hence alpine plants flour- 

 ish in the rays of the sun, far at the north. They flourish even on a 

 base of ice, as seen in a valley of the Borneo mountains filled with 

 ice Some earth having been brought down by avalanches upon it, 

 food is thus grown for flocks in that region. When secluded from 

 light, plants become lax, and send out long, thin and whitish shoots, 

 and in fact become bleached from the effect of oxygen in forming an 

 acid with their carbon, which oxygen they obtain from water but can- 

 not give off. Darkness favors their length by softening their parts 

 while light, by maturing nourishment, consolidates the parts. A due 

 adjustment of their materials, therefore, depends on the balanced alter- 

 nations of day and night. Plants spring up in the arctic region whilst 

 the sun is constantly above the horizon ; and the incessant light acting 

 on them perfects them before they have time to acquire much weight. 

 Direct rays of the sun, we know, contribute more to the flowing of the 

 sap of the sugar-maple than heat a frost at night succeeded by a 

 sunny day being best for the flow of this j uice. 



When the temperature is 62' 6', nature revives during spring in 

 the north, the mean temperature of which is from 42 to 46. The 

 heat most congenial to tropical plants can be imitated and they will 

 flourish in the long sunny days of summer, but in winter, though a 

 high temperature be maintained, they will languish or die. Thus 

 plants are located according to the light they require. Much light is 

 required to stimulate to action all plants of a succulent nature, with 

 watery leaves, few pores, and containing very resinous and oily juices, 

 or which have a great extent of surface. Others, such as mosses, 

 ferns, and some evergreens, flourish best beneath the shade. But 

 we have elsewhere alluded to the effects of light on plants and fruit. 



Heat is obviously the most powerful agent in the life and growth of 

 plants. All vegetation ceases in winter if not aided by artificial heat. 

 As they are chiefly nourished by water, or its parts, after decomposition, 

 vegetation is necessarily arrested by a temperature below the freezing 

 point, as it is in deserts where the heat is so great as to dry up the 

 moisture of the earth. Trees, however, having long roots resist the 

 influence of both heat and cold, the soil at some depth being both 

 warmer in winter and more moist in summer. The tree partakes of 

 this warmth in winter through the influence and communication of 

 the roots, as it partakes of the moisture obtained from sub-soils and 

 communicates it to the plant in like manner. The greater the size of 

 the stem or branch, therefore, or the greater the number of layers be- 

 tween its surface and the pith, the better do plants resist the effects 

 of cold and heat. The pith being the softest and most moist part, is 

 most susceptible of these effects, and a tree or shrub, as it grows 

 older, is less liable thereto. Thus the plant, pride of India is destroyed 

 by a little cold when young, but when older it sustains four times the 



