RESTRICTION OF TRANSPIRATION BY CORK 137 



an unpeeled potato lost -0397 grams per cent, of water during twenty- 

 four hours, the corresponding amount in the case of a very thinly 

 peeled tuber was 2-5548 grams, or about G4 times as much. In the 

 course of a week the peeled potato lost 44 times as much water as the 

 intact tuber. 



In order to compare the permeability of a thin cork-film with that 

 of an average epidermal layer, the author performed the following- 

 experiment upon two-year-old shoots of certain woody plants at the 

 besinnins of December. Lengths of 20 cm. were cut from twigs of 

 Aesctdus Hippocastannm, Syringa vulgaris, Sambuciis nigra and Pirns 

 communis. In every case the two severed ends, the leaf-scars, and the 

 lenticels were carefully sealed with melted wax-mixture, and the 

 experimental twigs were then allowed to transpire for three days in 

 the laboratory, at a temperature of 17-19 C, and in an atmosphere 

 with a relative humidity of 50-60 per cent. The daily loss of weight 

 was determined by weighing. Simultaneously the evaporation from a 

 freely exposed water-surface was measured under the same conditions, 

 just as in the corresponding experiments carried out ijpon the epidermis 

 (cf. p. 100). In calculating the transpiring surface, the wax-coated 

 areas were, of course, deducted. Under these conditions the average 

 amounts of water evaporated (in grams per sq. dcm. per 24 his.) were 

 as follows : 



Aesculus Hippocastanum, - - "154 



Syringa vulgaris, - "189 



Sambuciis nigra, - - - - - - '317 



Pints communis, - ... -430 



Water-surface, - 0-9:20 



It happened that in this particular experiment the evaporation from 

 the water-surface was approximately the same as in the previously 

 described experiment, . in which the transpiration from an astomatic 

 leaf-surface was compared with the evaporation from a freely exposed 

 water-surface. Hence the rates of transpiration of the leaves and 

 twigs can be directly compared. It appears that in the Horse-chestnut 

 the transpiratory activity of a two-year-old branch (;154g.) is very 

 nearly equal to that of a leaf (*156g.); in other words, the outer 

 epidermal walls and the two-year-old cork-film exert the same restrictive 

 effect upon transpiration. In the remaining instances, however, the 

 transpiratory activity of the branches is, on the whole, considerably 

 greater than that of the astomatic leaf-surfaces of the same plant. 

 Nevertheless, the presence of periderm diminishes the loss of water 

 very appreciably ; for evaporation from a freely exposed water-surface 

 was found to exceed transpiration from the branches sixteen-fold in 



