34 PB ACTIO AL GUIDE TO GABDEN PLANTS 



can absorb the necessary food or throw off surplus gases through the 

 walls of the cells instead of through stomata. Those floating on the 

 surface have stomata above, but none or very few beneath. 



Functions of the Leaf. — The most important work of the leaf is to 

 construct, manufacture, ' elaborate ' or make digestible food for the plant 

 out of the raw materials in the sap, and to give off through its pores 

 surplus gases and watery vapour. 



It is only imder the hght of the sun that the living protoplasm in 

 the cells of leaves is capable of changing mineral matters and gases into 

 plant food. From the air carbonic acid gas is taken in through the 

 pores in the leaf and is absorbed through the cell walls by the proto- 

 plasm in the cells. The carbon is retained and the oxygen is given off. 

 This process of manufacturing food is called assimilation, and may 

 be likened to what is known as digestion with human beings and 

 animals. By its means starch, sugar, oils and various other substances 

 found in plants are obtained, and constitute the food of man and 

 animals. The absorption and liberation of gases by means of the pores 

 is known as respiration, owing to its being somewhat akin to the 

 breathing of animals, the great difference, however, being that plants 

 breathe out oxygen during the day instead of carbonic acid gas, and 

 thus keep the air in a purified state. 



Transpiration. — Almost every part of a living plant is continually 

 giving off vapour from its tissues, although the quantity varies with 

 atmospheric conditions. When in active growth the roots often absorb 

 more water from the soil than is actually needed, and the surplus 

 is given off into the air by means of the stomata in the leaves and 

 minute pores in the stems. This continual discharge of watery vapour 

 is known as transpiration, and according as the cells become emptied 

 by evaporation they absorb fresh supplies from contiguous cells by 

 means of the process already referred to as Osmosis, see p. 23. 



"Whether large or small quantities are given off depends a good deal 

 upon the wetness or dryness of the atmosphere, and upon heat and cold 

 — that is, practically on the state of the weather. It is not merely a 

 mechanical process of evaporation, as the amount given off is regulated by 

 the plant itself. When too much vapour is being given off, the stomata 

 begin to close, as if realising that the loss of great quantities of water 

 means injury to the plant as a whole. So long as the supply of water 

 from the roots exceeds that given off by the leaves, the latter remain 

 plump and fresh, as the cells composing them are turgid or full of 

 watery sap. But as soon as the leaves throw off more water than is 

 supplied by the roots, the cells of the leaves gradually become emptied 



