102 THE VEGETATIVE ORGANS 



of the sun's rays in bringing about and carrying on many 

 important changes in the substances entering the cells, 

 also of rearranging their component atoms and building 

 up new compounds from them. During these chemical 

 changes much heat is evolved, as we have already seen in 

 the experiment with germinating peas ; but the leafy shoots 

 of plants are always cool, and commonly cooler than the 

 surrounding air. How is this ? With all the chemical 

 changes going on in plant-tissues, why does the temperature 

 of the plant not rise much above that of the air, as it does 

 in our own bodies ? Some heat may be lost by radiation, 

 but for a fuller answer we must go back to our experiments 

 on transpiration and try to realize the enormous amount 

 of heat required to convert the water of the cell-sap 

 into vapour, and the large amount of vapour given off 

 by an average leafy shoot. It is estimated that over 

 90 per cent, of the heat absorbed by a plant is dissipated 

 in this way. No wonder, then, that the foliage of a plant 

 feels cool to the touch. 



But our experiments with water-cultures suggest another 

 interesting point in this connexion. Is the soil-water 

 (or its artificial representative, a water-culture solution) 

 a dense, or a weak, food-solution ? Is it necessary for the 

 solution to be a weak one ? and, if so, why ? If a plant needs 

 to take up an enormous amount of water in order to obtain 

 a sufficiency of solid food, what is the consequence ? The 

 necessities of osmosis, of conduction and transmission, 

 require a weak food-solution. This involves the absorption 

 of an excess of water above that needed for the building 

 up of tissue-materials. Hence we see the value of a thin, 

 flat leaf whose exposed surface is very large compared 

 with the amount of its tissue. Again, the spongy tissue 

 of a leaf, with all its cells hung out, as it were, in drying- 

 chambers, has an interesting meaning. These chambers, 

 communicating by way of the stomata with the air outside, 



