Plant Defences 105 



also hindered. Place a silver leaf in water and notice how the 

 silvery sheen is brightened. The thick coat of hairs retains a 

 layer of air which the rain cannot replace. It is because of 

 this that vapour can pass off without interruption. A bamboo 

 leaf under water glistens on the lower surface quite as brilliantly, 

 but the upper surface comes out wet. The stomata which are 

 most numerous on the under surface are sunken, and protected 

 by projecting rods of wax. On the upper surface wax is want- 

 ing but the few stomata are protected in another manner. 



FIG. 88. Vegetation in the Karroo where there are long droughts. The 

 soil is alkaline. 



Peculiar colourless cells of the epidermis lie in rows midway 

 between the veins. When there is sufficient moisture these pro- 

 ject above the surface. As moisture is withdrawn, they col- 

 lapse without injury, allowing the leaves to roll ; the stomata 

 lying on either side of these cells are thus covered. 



A bamboo leaf which has withered for several days will re- 

 vive if a stream of water is allowed to flow over the upper sur- 

 face. While the wax rods make it impossible to wet the under 

 surface, thin walled cells on the upper surface actually absorb 

 moisture. 



The sugar bush has another cunningly devised method of 



