LIGHT HEAT AIE. 121 



The effects of light and heat in favouring vegetation being 

 nearly always found in union, causes that which is really due 

 to light often to be attributed to heat, as in the growth of 

 tropical plants ; where both are combined, there will be the 

 greatest powers of vegetative growth. The absence of light 

 and air often causes the upward growth of a plant to seek 

 them ; a curious instance of this, altering the habits of a 

 tree, is mentioned by Dr. Livingstone, who says : 



"As we traverse a succession of lawns and open forests, 

 it is interesting to observe something like instinct developed 

 even in trees. One, which when cut emits a, milky juice, 

 and if met with on the open lawns grows as an ordinary 

 umbrageous tree, and shows no disposition to be a climber, 

 when growing in a forest still takes the same form, then 

 sends out a climbing branch which twines round another 

 tree until it rises 30 or 40 feet, or to the level of the other 

 trees, and there it spreads out a second crown, where it can 

 enjoy a fair share of the sun's rays. In parts of the forest 

 still more dense than this, it assumes the form of a climber 

 only, and at once avails itself of the assistance of a tall neigh- 

 bour, by winding vigorously round it, without attempting to 

 form a lower head. It does not succeed so well as parasites 

 proper, but when forced to struggle for space, it may be 

 mistaken for one which is invariably a climber." 



The absence of heat with plenty of light is shown in the 

 vegetation of mountainous districts, where there is great 

 size, but the absence of leafy expansion, as in the pine tribe 

 (fig. 5). Where both light and heat are deficient, nothing 

 but fungi, mosses, and lichens of the most stunted nature 

 will grow. Experiments in hothouses prove that although 

 tropical plants be supplied with their proper amount of heat, 

 yet without the very greatest supply of light that can be 

 obtained, they do not come to any degree of perfection. 

 The influence of light is chiefly upon the leaves, that of heat 

 upon the earth and root, which becomes more fully deve- 

 loped. When a seed is placed in the earth, moisture is 

 absorbed by it from the earth, and a development of the 

 cells of the cellular tissue of which the seed is composed 

 takes place, and from the peculiar nature of these cells they 



