Chapter X — 187 — Loss and Retention 



numerous methods include weighing of cut out leaf tracings on paper of 

 known weight per unit area, calculating area by photoelectric means, leaf 

 matching, and other methods. Miller (1938) discusses these various 

 techniques at some length. 



Three general methods of recording water loss have been used: i) by 

 expressing the weight of water lost per unit time per plant unit, such as 

 unit of area, green weight, or dry weight, 2) by comparing the water lost 

 by unit plant area with that lost from a similar area of water, and J) by 

 expressing the total amount of water lost over an extended period of time 

 per unit weight of dry substance produced by the plant for the same time 

 period. These three means of expressing transpiration have been termed 

 respectively : 1 ) intensity of transpiration, rate of transpiration, absolute 

 transpiration, 2) relative transpiration, transpiration capacity, transpiring 

 power, transpiration resistance, and 3) water requirement, efficiency of 

 transpiration, coefificient of transpiration, 



Guticular Transpiration : — Not only the thickness but also the com- 

 position of the cuticle on the surface of epidermal cells varies considerably 

 with species, age, position, and condition of the plant organ. An example 

 of the influence of the environment on cuticle formation may be found in 

 seedlings of citrus grown under humid conditions. Such plants were sub- 

 ject to sun scorch due to the little cuticular protection afforded and to poor 

 stomatal response (Nigam, 1934). In fruit the cutin may so completely 

 permeate the epidermal and hypodermal layer as to completely imbed the 

 epidermal cells. Cuticular transpiration though a function of the per- 

 meability may not be directly correlated with cuticular thickness. Cutin 

 is a heterogeneous substance composed of several wax-and-oil-like frac- 

 tions. Markley and Sando (1931) studying the wax-like coating on the 

 surface of apples separated the petroleum ether soluble fraction from the 

 one that was petroleum ether insoluble. The former was composed chiefly 

 of the hydrocarbon triacontane (C30H62) with small amounts of hepta- 

 cosanol (C27H56O) and traces of true oils, while the latter was identified 

 as ursolic acid C30H48O3. These two fractions increased with age and 

 development of the fruit but not uniformly. The oily petroleum ether solu- 

 ble fraction increased faster rendering the layer more impervious to water. 



PiENiAZEK (1944) found that when lenticular transpiration, which 

 amounted to from 8 to 25 per cent of the total water loss, was excluded, 

 by covering each lentical with paraffin, the rate of transpiration from sev- 

 eral varieties of apples was not directly related to thickness of the cuticle 

 itself but that varieties with a large wax deposit on the surface of the 

 cuticle lost water most slowly. This wax deposit on apples has different 

 chemical and physical characteristics from the cuticle (Markley and 

 Sando, 1931). 



Diurnal fluctuations have been observed in the physical properties of 

 leaf cuticle. By measuring the contact angle of water drops on leaves of 

 Brassica sinapis and Triticum vulgar e, Fogg (1944) studied the wettabil- 

 ity of the cuticle. A diurnal change in the contact angle was observed 

 (Figure 50) . This would imply that there is a diurnal fluctuation in cuticu- 

 lar transpiration in normal plants. Wilting caused a marked reversible 

 rise in contact angle which was not, according to Fogg, a mechanical 

 stretching effect. 



Cuticular transpiration is most conveniently studied in hypostomatal 

 leaves where the stomata can be covered by vaseline or other water im- 



