21 



leaf nutrition stimulated by warmth but root-absorption 

 also increases with a rise in temperature, so that all nutri- 

 tive processes are favoured by warmer conditions of soil 

 or climate. 



A secondary but important function of leaves was re- 

 ferred to in a preceding lecture, namely, the importa^nce 

 of getting- rid of a certain amount of w^ater vapour mto 

 the atmosphere. This process of transpiration aids the 

 upward passage of sap and also causes the concentration 

 of the necessary mineral salts, which are only contained 

 in dilute solution in the soil. 



Let us now consider how the structure of leaves is 

 adapted to perform its important functions. For both of 

 them it is essential that the air should be in close contact 

 with the cells of the leaf. It is essential to prevent too 

 great a loss of water from the leaves and also for pro- 

 tective purposes it is necessary that the surface of the 

 leaf should be covered with a more or less resistant layer. 

 Consequently the passage of air to the interior of the leaf 

 is effected through microscopic pores called stomata, which 

 are usually found in greatest number on the under sur- 

 face of leaves. These pores have the power of opening 

 and closing so that the aperture can be regulated to the 

 needs of the plant. They open with the approach of day- 

 light and thus enable the carbonic acid to enter the leaves 

 when the conditions are favourable for leaf nutrition. At 

 the same time, of course, water vapour escapes outwards 

 through the pores. The inner parts of the leaf are so 

 arranged that the cells containing most of the green colour- 

 ing matter are near the upper surface of the leaf where 

 they will receive most light . Throughout the interior of 

 the leaf there are vvide spaces through which the air can 

 circulate readily between the cells. The veins w^hich bring 

 up the supply of water and which also have to collect 

 and conduct away through their bast portion the organic 

 material formed by the leaf, become very finely branched 

 so as to be in contact with all parts of the leaf. 



If w^e consider that the larger the leaf surface the 

 larger the amount of light absorbed and hence the greater 

 the nutrition, we might wonder why all plants have not 

 larger leaves. The limitation in the size of leaves is 

 largel}/ due to mechanical considerations. A slender stem 

 cannot bear leaves beyond a certain size. Aloreover, 

 though a large leaf would increase the process of carbon 



