38 



LOCATION OF STOMATA 



with the sun's rays. This arrangement of the leaf permits a 

 direct illumination of the leaf only in the morning and afternoon 

 when the intensity of the light is feebler. 



The position and character of the stomata in relation to exter- 

 nal conditions show many interesting relations. Plants living in 

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



Fig. 24. Cross-section of a leaf of the inch plant, Tradescantia, showing 

 the delicate character of the cells and the raised stoma of this shade plant. 



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 de- 

 pressions remove the stomata from the dry winds and prevent 

 the direct contact of the moist air in the leaf with the dry atmos- 

 phere. 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- 



