148 PLANT PHYSIOLOGY AND ECOLOGY 



by the presence of either of these. It often occurs, however, on 

 the lower side of a leaf that is cutinized above. Excretions of 

 wax or of salts render the epidermis highly impermeable, and 

 correspondingly reduce the loss of water through the epidermal 

 cells. The thickening of the outer wall of the epidermis to form 

 a cuticle is the most perfect of all devices for decreasing permea- 

 bility, and thus reducing transpiration. In many desert plants 

 the greatly thickened cuticle completely prevents all transpiration 

 except that which occurs through the stomata. In these the 

 cuticle is also regularly developed in such a way as to protect 

 the guard-cells. Some species have an epidermis that comprises 

 two or more layers of cells. While this is an effective protection 

 against water loss, it is not frequent. 



A coating of hairs decreases transpiration by screening the 

 epidermis so that the amount of light and heat is diminished, 

 and the access and movement of dry air impeded. A few scattered 

 hairs are of little or no value for this purpose, but a uniform com- 

 pact layer is of the greatest service, since it protects the stomatal 

 openings as well as the epidermal cells. Hairs are of the most 

 various sizes and forms, but all hairy coverings serve the same 

 purpose, even when they are primarily for water storage, as in a 

 few plants. The fact that hairs protect the stomata as well as 

 the epidermal cells explains the occurrence of a hairy covering 

 on the lower surface, even when it is absent from the more exposed 

 upper side. In some cases, hairs are developed only to serve as 

 screens to the stomata. 



169. Modifications of the stomata. Since the great bulk of the 

 water lost under ordinary conditions passes through the stomata, 

 the changes of the latter are of the utmost importance in reducing 

 transpiration. Their modifications for this purpose are many, 

 but practically all of them are concerned with number or position. 

 Species growing in dry places have fewer stomata to the same 

 leaf area than those in moist habitats. The number on both sur- 

 faces decreases as the danger of excessive water loss increases. 

 The decrease is usually more rapid upon the upper surface, which 

 finally loses its stomata entirely. Stomata are usually more 

 numerous on the less exposed or lower surface of the leaf. Excep- 

 tions occur in many shade plants where the exposure of the two 

 surfaces is equal, and in aquatic plants, in which water loss is 

 beneficial instead of harmful. The change in the number of 



