THE MOLECULAR FORCES IN PLANTS. 211 



possible are cut from a Mahonia ; in one leaflet we paint only the 

 upper surface with the mixture, while the other leaflet is only 

 painted on its under surface. In both cases we smear the cut 

 end of the leaf-stalk. After the layer of wax has completely 

 cooled and stiffened, we weigh the leaves, then expose them to 

 the sunlight with their free surfaces directed upwards, and 

 after some time once more weigh them. The leaflet with its 

 under surface untouched will have given off more water in the 

 form of vapour than the other, since the under side of the leaf of 

 Mahonia possesses stornata. The upper side of the Mahonia leaf, 

 which has no stomata, naturally gives off only a comparatively 

 small quantity of water to the air, and this water must traverse 

 the cuticle. Other plants (Ilex, Nerium, Begonia, Ficus, etc.), 

 whose leaves possess stomata only on the under surface, may also 

 be employed for experiments like those with Mahonia. 



In investigating the relations between the organisation of plant 

 structures on the one hand, and their transpiration on the other, 

 it must be mentioned that the leaves of many plants, and indeed 

 other organs also, possess tissues which s'erve for storing up water. 

 Here we have to do especially with plants which vegetate in com- 

 paratively dry situations, and which frequently have to endure a 

 long period of great dry ness. We may mention particularly many 

 of the Cacti, the Cactus-like Euphorbias, many Crassulaceae, and 

 species of Aloe and Peperomia. We prepare, e.g., a transverse 

 section of the leaf of Aloe soccotrina. Beneath the very strongly 

 cuticularised epidermis lies the green assimilatory tissue, the cells 

 of which contain comparatively large chlorophyll grains. The 

 middle of the leaf is occupied by a tissue, the so-called water- 

 tissue, the large cells of which are very rich in water, and contain 

 a large quantity of mucilaginous substances. Along the boundary 

 between the water tissue and the assimilatory tissue which sur- 

 rounds it, lie the vascular bundles. When Aloes, in the dry 

 season, are unable to take up large quantities of water from the 

 soil by means of their roots, use is made of the water stored 

 up in the water tissue, and they do not materially suffer from 

 a drought, which would destroy other plants. The leaf of 

 Peperomia trichocarpa similarly possesses a well-developed water 

 tissue, which is immediately recognised on examination of trans- 

 verse sections. Under the epidermis of the upper side of the leaf 

 lies a succulent tissue devoid of chlorophyll, the cells of which 

 increase in size towards the middle of the leaf. The assimilatory 



