PLANTS, FUNGI AND INSECTS 11 



which the epidermal cells are not cuticularised, water passes off 

 from the whole leaf -surf ace, but this ceases when a cuticle is 

 formed, and the rate at which transpiration occurs is then 

 controlled by the stomata, which have the power of opening 

 and closing as circumstances demand. 



(4) Elaboration of plant-food (Photosynthesis). The mineral 

 salts absorbed in solution by the root-hairs are of no use in feeding 

 the plant until they have undergone various chemical changes, 

 and have been built up into more complex substances. This is 

 effected in the green parts of the plant, and can go on only in the 

 presence of light. Carbon dioxide is taken in from the air, and, 

 through the agency of chlorophyll, the carbon is combined with 

 water into carbohydrates. The oxygen from the carbon dioxide 

 is liberated, and passed back into the air through the stomata. 

 Further chemical changes take place in which the nitrates are 

 built up into more complex nitrogenous substances. The whole 

 process is very complicated, buti ultimately, sugars, starch and 

 nitrogenous foods are formed, and are passed on to the other 

 parts of the plant. This photosynthesis is carried out chiefly 

 by the leaves, and light is essential for the process, so that when- 

 ever a plant is much shaded, its development is arrested; on 

 this account the leaves of a tree are arranged so as to form a 

 kind of mosaic, whereby the fullest use is made of the light 

 available. 



During the daytime respiration and assimilation go on side by 

 side, so that the plant is absorbing oxygen and giving out carbon 

 dioxide at the same time as it is absorbing carbon dioxide and 

 giving out oxygen, but usually the output of oxygen is greater 

 than that of carbon dioxide. At night, in the absence of light, 

 assimilation ceases, and the plant only absorbs oxygen and 

 exhales carbon dioxide. 



(5) Translocation of plant-food. It has already been seen 

 that the raw food material dissolved in water is carried up to the 

 leaves through the cavities of the wood vessels. The prepared 

 food is transferred from the leaves through the phloem or bast, 

 and is utilised in building up the tissues. Starch is insoluble, 

 so that, before it can be moved from the leaves, it is converted 

 into sugar, and then, if an excess be formed, it may either be 

 stored up in some part of the plant as sugar, e. g. beetroot, 

 or else re-converted into starch, and stored up in that form, as 

 in the potato. 



