210 PHYSIOLOGY OF NUTRITION. 



and about 3 gr. in weight, are made air-tight at both ends with 

 sealing-wax, and after being accurately weighed, are laid aside 

 for twenty-four hours. We then again weigh them, and so 

 ascertain the loss by transpiration. We now seal up by means 

 of melted wax, in the one piece the lenticels, in the other patches 

 of periderm of corresponding size, at once weigh both, and then 

 again weigh them after twenty-four hours' transpiration. The 

 piece whose lenticels were sealed will have lost absolutely, or at 

 any rate per cent., less water than the other piece used for com- 

 parison. 



The epidermis of leaves, as has already been shown in 70, is 

 very generally provided with a layer of wax, which, as is well 

 known, appears in various forms. This coating of wax reduces 

 not inconsiderably the rate of transpiration in leaves, as the 

 following experiment teaches. We may use Eucalyptus globulus, 

 taking the two leaves of the same pair. One leaf is accurately 

 weighed at once, the other after the layer of wax has been wiped 

 off with a soft cloth. Weighings repeated at the end of six, 

 twelve, or twenty-fours respectively indicate that the former 

 leaf undergoes less loss by transpiration than the latter. 3 



The experiments we have made of course allow no doubt to 

 remain that in the transpiration of plants only a small amount at 

 most of the water disappearing in the form of vapour traverses 

 the cuticularised epidermis, especially if the cuticle is richly im- 

 pregnated with waxy substances. Nevertheless, the cuticle is not 

 completely impervious to water, as the following experiments 

 teach. 



We cut off an uninjured leaf of a Begonia, the upper surface of 

 which is devoid of stomata, and lay it together with an object- 

 glass in a crystallising dish. We now treat a large quantity of 

 salt with a little water, so that the salt is slightly moistened, 

 scatter a small quantity of the salt over the upper surface of the 

 leaf, and also on the slide, and then cover the dish with a glass 

 plate. The salt on the leaf rapidly deliquesces, since it attracts 

 water out of the tissue of the leaf ; this water can only pass to 

 the outside through the cuticle. The salt on the slide attracts at 

 most a small quantity of aqueous vapour from the atmospheric 

 air, and remains comparatively dry. 



We prepare a very readily liquefied mixture by melting together 

 wax and olive oil or cacao butter (a mixture of 1 part of wax and 

 3 parts of cacao butter is very good). Two leaflets as similar as 



