34 LOCATION OF STOMATA 



shade or in the presence of an abundant soil-moisture develop 

 the stomata on a level with the leaf surface (Fig. 24), because 

 there is no necessity of conserving the water supply. For the 

 same reason some aquatics have lost altogether the power of 

 closing their stomata. On the other hand plants that are exposed 

 to arid conditions or drying winds develop the stomata well 

 below the surface of the leaf, as in the cactus and in the needles 

 of conifers (Fig. 23, s), or in furrows, as in certain grasses, 

 or at the bottom of minute pores, as in the oleander. These 

 depressions remove the stomata from the dry winds and prevent 

 the direct contact of the moist air in the leaf with the dry atmo j 

 sphere. The pores in the oleander contain hairs which would 

 check transpiration just as a plug of cotton in the neck of a 

 flask would lessen the evaporation of the fluid in the flask. The 

 chief purpose of this arrangement, however, is to prevent the 

 stopping of the stomata with water. This plant grows naturally 

 along the banks of streams where it is subject almost nightly to 

 heavy dews. When the stomata become filled with dew it re- 

 quires several hours of sunshine to drive the moisture from these 

 capillary openings. Consequently during this time there could 

 be no interchange of gases and the vital functions of the leaves 

 would be largely stopped. Many devices appear that prevent 

 water entering the stomata. Attention has been called to the 

 development of stomata on the under surface of leaves and to 

 waxy coatings as protective features of this nature. Every one 

 is familiar with the waxy appearance or bloom of many fruits, 

 as the plum; or of leaves, as the cabbage; and of stems, as the 

 raspberry. Water is unable to spread over the wax and enter 

 the stomata because a thin layer of air clings to it. The silvery 

 appearance of many grass leaves and of the jewel-weed when 

 immersed in water is due to this thin film of air which reflects 

 the light. Accordingly the water does not really touch the leaf 

 and you notice that it is quite dry when it is removed from the 

 water save for a few drops which readily run off. Protection 

 against wetting is also obtained by the development of coatings 

 of hairs. This is noticeable in many Alpine and polar plants 

 where they are constantly exposed to heavy fogs, dews, and rains. 



