NATURE OF PLANTS 39 



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. 

 Plant hairs do not always serve this purpose. Desert plants 

 and those living in dry localities are frequently characterized by 

 hairy coatings. Such plants are frequently termed xerophyjies 

 in contradistinction to aquatics or hydrophytes and to plants 

 living in moderately moist soils, which are called mesophytes. 

 The hairy coatings of xerophytes give them their familiar silvery 

 or hoary appearance as seen in the sage-brush, dusty miller, and 

 mullein. The hairs are usually empty tubes or cells which re- 

 flect a large portion of the intense light and heat and consequently 

 materially lessen the loss of water, just as straw sprinkled upon 

 the ground keeps the earth below moist and cool. In many in- 

 stances plant hairs develop mucilage or resinous secretions which 

 protect as with a varnish growing regions as buds or young shoots 

 and leaves of birches, alders, poplars, and peach. In this con- 

 nection it should be noted that dense hairy coatings reduce the 

 loss of water only when the plant is exposed to air currents 

 not in still air. Cutin, wax and resinous deposits retard trans- 

 piration under all conditions. Oil and poisonous substances are 



