292 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



in a great number of rushes, bulrushes, and reed-like grasses. If when the dew 

 falls heaviest one roams through a thicket of willows, or across a moor, one may see 

 plenty of drops hanging from the under side of the leaves, but they do not actuallj' 

 wet this surface, and on the slightest movement of the leaves they roll off and fall 

 down. It is, indeed, in consequence of this that one is more likely to get thoroughly 

 wet by walking through meadows and dwarf willows than by an excursion through 

 country overgrown with ordinary herbs. The two white stripes, so well known on 

 the under side of fir leaves, are also formed by a waxy coat, which prevents the 

 stomata below from being wetted. In species of juniper {e.g. J. comTnunis, nana, 

 Sabina) the two white stripes occur on the upper side of the leaf, and it is interest- 

 ing to see how the distribution of the stomata again corresponds ; for junipers 

 belong to that group of plants whose under leaf-surface is free from stomata, these 

 being present only on the wax-coated region of the upper side of the leaves. 

 Many grasses, to which we shall refer later for other reasons (e.g. Festuca j^unctoria), 

 only possess stomata on the upper side of the leaf, and again only where the strips 

 of wax are situated. Generally speaking, wax is a protection from moisture, and is 

 most frequently formed when the stomata make their appearance on the upper side 

 of the leaf. The leaves of peas, nasturtiums, woodbine, poppies, fumitory, many 

 pinks, cabbages, woad, and many other cruciferous plants, which have stomata on 

 the upper surface, also produce a covering of wax there. Water poured on the 

 upper surface of a cabbage-leaf rolls off in the form of drops, exactly as it runs off 

 a duck's back, without wetting the surface. In the fronds of ferns (e.g. Polypodiuni 

 glaucophyllwm and sijorodocarpum), on the upright leaves of Irises {Iris ger- 

 manica, purtiila, pallida), as well as on the vertical leaves and leaf -like branches of 

 many Australian acacias and myrtles, and lastly in the erect whiplike, leafless or 

 scantily-leaved papilionaceous plants {Retama, SpartiuTn), the stomata are pro- 

 tected from the wet by a coat of wax. 



The formation of hairs furnishes another barrier to the entrance of water into 

 the stomata. We shall come back again to these structures, which serve so many 

 different purposes in the plant economy, but here only those hairy and felted 

 coverings whose task is to hinder the wetting of the stomata will be considered. 

 Examples of these are furnished by many Malvaceae which grow in marshes and 

 ditches {e.g. Althoea oficinalis), and also by some mulleins {e.g. Verhascum Thapsus, 

 phlomoides), whose leaves are provided with stomata on both surfaces, and with 

 hairy coverings which it is impossible to moisten. In the damp meadows of the 

 valleys of the Lower Alps grows Gentaurea Pseudophrygea, whose large leaves, 

 hairy on both sides, are very rough and much wrinkled. The stomata are only 

 situated in the hollows between the ridges. When rain falls, or the leaf becomes 

 bedewed, the water remains in the form of drops on the hairs of the elevated por- 

 tions, and the cells in the hollows are not wetted. In many alpine plants, for 

 example the Hairy Hawkweed {Hieracium villosum), after a fall of rain or dew 

 the long projecting hairs of the leaves are thickly beset with drops of water, 

 none of which can reach the stomata on the epidermis beneath. 



