270 



Sir F. Darwin. 



the wounds, whereas greased leaves without such artificial stomata formed 

 none or hardly any. 



It may be objected that the stomata are not completely or uniformly 

 closed by greasing, that some remain open, and that it is to the opening and 

 closing of these in light and darkness that the rise and fall of the transpira- 

 tion of the incised leaves is due. I find it difficult to believe that the 

 general objection here discussed is sound, because experiments with the 

 porometer* have convinced me that even a careless application of vaseline 

 absolutely closes the stomata. It may be urged that in Experiment LO 2 

 (p. 271) the effect of grease is only to reduce transpiration from 379 to 

 10'9, i.e., from 100 to 2 - 88. It must be remembered, however, that fatty 

 substances are not impermeable to water, and that at any rate part of the 

 2'88 per cent, must be due to cuticular transpiration. 



Another source of error should be guarded against. Mr. Blackmanf has 

 shown that a process of healing occurs in wounded laurel leaves. The 

 beginning of the process is, however, marked by the edges of the wounds 

 becoming translucent. As soon as this occurs the specimen should be dis- 

 carded or fresh incisions made. 



The following experiment, LO 2, October 2, 1912, gives an idea of the 

 effect of greasing and slitting. It seems clear that the result is com- 

 parable (as far as magnitude is concerned) with normal stomatal transpira- 

 tion : — 



Experiment LO 2. October 2, 1912. P. Imcrocerasus. 



A laurel branch cut under water with 10 leaves (one being small) having 

 a stoma-bearing area of 600 cm. 2 . 



Fitted to a potometer (diameter of tube - 95 mm.). At a north window, 

 where the temperature during the observations varied between 13 - 6° and 

 15 - 2° C. and the relative humidity between 59 and 69 per cent. In the 

 following abbreviated record of the experiment the potometer readings 

 are corrected for differences in relative humidity. 



A.M. Transpiration. 

 11.10 439+ 

 27 427 

 53 379 



P.M. 



12.18 Finished vaselining leaves on both surfaces. 



* See F. Darwin and D. F. M. Pertz, ' Boy. Soc. Proc.,' 1911, B, vol. 84, p. 137, for a 

 description of the porometer. 



t F. Blackman and G. Matthsei, ' Annals of Botany,' 1901, vol. 16. 



J The figure 439 is obtained from the number of seconds (viz. 22-8) in which the 

 column of water in the potometer tube travels 1 cm., which means the absorption of 



