MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 291 



this circumstance could be explained thus, that the upper side is usually turned 

 towards the rain, and that the stomata are on this account collected together on 

 the under side, which is sheltered from it. This explanation, however, which at 

 first sight seems so plausible, does not quite correspond to the true state of the case. 

 The consideration of the reasons for believing that it is an advantage for the plant 

 to have the upper side of the leaf free from stomata will indeed come later, but one 

 thing must be noted here, — that the side of the leaf turned towards the ground, 

 which in most cases contains all the stomata, remains anything but dry. Of 

 course the rain-water only reaches the surface of the horizontal leaf -blade when 

 the margin is so formed that the adherent layer of water which wets the surface is 

 drawn over gradually from the upper to the under side, and that is very seldom 

 the case; but the wetting of this surface by mist and dew is all the more important 

 on this account. On taking a stroll through fields and meadows on a dewy morn- 

 ing, as a rule only the upper surfaces of the leaves come into view, and one might 

 easily be led to think that the dew is deposited only on this side. We constantly 

 use the expression that the dew " falls ". Underlying this is the idea that the dew 

 comes down like rain, and that only the upper leaf-surface becomes covered with 

 dewdrops. But one has only to turn the leaf over to convince oneself that the 

 lower surface is likewise bedewed, and on a closer examination it will even be seen 

 that dew is of more importance in connection with the lower than the upper side, 

 because it remains there so much longer. When the sun is already high in the 

 heaven, and the dewdrops have long disappeared from the upper surface, and tran- 

 spiration is in full force, the under side may still be found studded with dewdrops. 

 If in the majority of cases the stomata lie on the under side, and this side is 

 exposed to the danger of being covered with water as much as the upper one, it is 

 evident why contrivances for hindering the access of water to the stomata are 

 to be found much more abundantly on the under than on the upper side of the 

 leaf. 



The most important of these arrangements are the following: — 

 First the coating of wax. This is either in the form of a granular covering; or as 

 a fine crust which fits closely to the epidermis; or, most commonly, as a continuous 

 thin layer which is easily rubbed off, forming a delicate film popularly known as 

 " bloom ". A group of primulas, belonging to mountainous districts and to the moors 

 of low countries, of which Primula farinosa may be taken as the most widely 

 distributed and best known representative, have a rosette of leaves spreading over 

 the damp ground, and on the lower side of these leaves is a white coat, which under 

 the microscope is seen to consist of a collection of short rods and knobs of a waxy 

 nature. If such a leaf is plucked and placed in water for a short time, and then 

 withdrawn, the upper side, which is quite free from stomata, will be moistened by a 

 layer of water, while the under side, on which are the stomata protected by the 

 granular coating of wax, remains quite dry , The lower surface of the leaves is 

 covered with a fine closely adherent wax layer, in many of the willows growing in 

 damp misty places near rivers (Salix amygdalina, -purpurea, pruinosa), as well as 



