LEAF STRUCTURE 



11 



sive. Every cell is said to have part of its surface next to an 

 air space and the total exposed surface of all the cells inside 

 the leaf is from five to thirty times as great as the total of 

 the upper and lower surfaces of the leaf. Examination of 

 Figure 15 will give an idea of these surfaces. More surface 

 per unit of volume has been found in the palisade than in the 

 spongy portion by Turrell, who has contributed much to our 

 knowledge of the internal structure of leaves. This surface 

 is very important in the absorption of carbon dioxide from 

 the atmosphere by the cells. The carbon dioxide diffuses 

 from the atmosphere through the stomata into the inter- 

 cellular spaces, from which it is absorbed by the water on the 

 walls of the cells and diffuses into the cells. When the 

 atmospheric content of carbon dioxide is considered, its rate 

 of entry into the cell can be explained only by the fact that 

 the stomata are very numerous and the area of cell walls 

 exposed to the air spaces in the leaf is so much greater than 

 the surface of the leaf. 



There are a number of modifications of leaves which are 

 of interest. Each flower is a group of leaves so different that 

 we give them their special names. The leaves that are sub- 

 merged in water are usually without an epidermis or sto- 

 mata; obviously there is little need for protection against 

 the injury or loss of water. Water plants die quickly when 

 placed in the air. Some plants adapted to dry conditions, 

 such as some of those selected for rock gardens, store quan- 

 tities of water in their leaves, the central portion of which 

 contains large cells to hold water, but few or no chloroplasts. 

 The insectivorous plants have leaves which catch insects and 

 secrete an enzyme which digests them. It is believed to be 

 a valuable source of nitrogen for them. The pitcher plant 

 has a hollow leaf containing water in which insects drown, 

 after which they are digested. The Venus' fly trap catches 

 insects between two flaps of the leaf which come together 

 when they are touched on their trigger hairs by the entering 



