106 DERMAL SYSTEM 



factor must also be eliminated before the restrictive effect of the 

 cell-wall can be accurately determined. 



In view of these facts the author lias carried out the following 

 experiment with leaves of Ficus elastica, Hedera Helix and Aesculus 

 Hippocastamtm. In each case one of a pair of leaves (or leaflets) was 

 killed with chloroform vapour, while its fellow was used in the living 

 condition. The lower surface to which the stomata are restricted in 

 all three species was then coated with a layer of cacao-wax (a mixture 

 of one part by weight of beeswax with three parts of cacao-butter, 

 recommended for this purpose by Stahl), so that evaporation was 

 thenceforward confined to the upper or astomatic surface ; the whole 

 of the petiole, including the cut end, was similarly waxed in every 

 case. The experimental leaves were then allowed to transpire in a 

 shaded portion of the laboratory, in an atmosphere with a relative 

 humidity of 68-75 per cent., at temperatures ranging from 19 C. to 

 23 C. The loss of water due to evaporation was estimated by weighing 

 the leaves daily at the same hour. For the sake of comparison a 

 similar record was kept of the evaporation which took place under 

 similar conditions from the freely exposed surface of water contained 

 in a shallow vessel. After three days the average loss by evaporation 

 in grams per sq. dcm. per twenty-four hours, was found to be as follows : 



Ficus elastica, ------- 



Hedera Helix, ------- 



Aesculus Hippocastanum, ----- 



Water-surface, ------- 



All the leaves were still quite fresh at this stage, those which had been 

 killed being discoloured, as at the beginning of the experiment. 



The above experiment shows in the first place that the smallness of 

 the evaporation from the leaves, as compared with a freely exposed 

 water-surface, is very largely attributable to the influence of the 

 cuticle and cutinised layers, and not to that of the living protoplasm, 

 although water vapour does escape somewhat more rapidly from dead 

 leaves than it does from live ones. In the case of Ficus elastica, for 

 example, the quantity of water retained mainly through the action of 

 the outer epidermal wall amounts to 6'922 minus "056, or 6'866 grams, 

 and is thus 286 times as great as the small additional quantity ("024 

 grams) which is prevented from escaping by the plasmatic membranes 

 of the living leaf. In every instance the amount of water lost by the 

 dead leaves is only a small fraction of the quantity that evaporates 

 from an equal area of the water-surface, namely ^jth in the case of 

 Aesculus, xYl^rd in that of Ficus and y^-th in that of Hedera. These 

 figures afford a most striking proof of the high efficiency of the 



