34 U >CATI( >\ I IF ST( IMATA 



shade <>r in the presence of an abundant soil-moisture develop 

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

 there is no necessity of conserving the water supply. \-<<r the 

 same reason some aquatics have lost altogether the power ol 

 closing their stomata. < ta the other hand plants that are ex] 

 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. _m. s), or in furrows, as in certain gn 

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

 depressions remove the stomata from the dry wind- and prevenl 



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

 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 thud 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 hanks 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 

 r 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. Ever) one 

 is familiar with the waxy appearance or bloom of man) 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 man) grass haves and of the jewel weed when 



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



the light, ^ccordinglj the water does nol really touch the leaf 

 and you notice thai it is quite <lr\ when it is removed from th< 

 ire for a few drops which readily run <<\i. Protection 

 againsl wetting is also obtained b) the development of coatings 

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

 where th- nstantly exposed to heav) fogs, dews, and rains 



