On a Method of Studying Transpiration. 



271 



p.m. Transpiration. 



12.20 285 

 34 64-7 



43 Surface of branch vaselined. 



59 47-3 



4.36 181 

 Oct. 3— 



A.M. 



10.23 Fresh surface cut to branch. 



11.21 10-9 



32 Four incisions made per leaf, i.e. two on each side of midrib. 



36 94-3 



P.M. 



12.5 One more slit per side. 



7 196 



12 One more slit, making four per side 



44 255 

 Oct. 4— 



A.M. 



10.11 Fresh surface cut to branch. 



11.5 234 



It will be seen that the coating of vaseline on the leaves and surface of 

 the branch does not completely check transpiration. Thus, as above 

 mentioned, on the second day (October 3), when the original negative 

 pressure must have disappeared, transpiration had only been reduced from 

 379 to 10'9 or from 100 to 2 - 88. This fact is in the present instance of 

 little importance, as my object is to illustrate the effect of incisions on 

 the transpiration rate. 



It is obvious (i) that when the lamina is cut into strips the transpiration 

 rises with great rapidity ; (ii) that although in this instance it does not 

 obtain the rate of transpiration observed when the stomata were open, the 

 two are comparable for practical purposes. 



In the case of these slit leaves it is of some interest to know the amount 

 of connection between the external air and the intercellular spaces. This 

 was estimated from the observations on the laurel twig (Experiment LO 2) 

 just described. Each leaf had eight incisions (four per side), varying in 



0-00708 c.c. The figure 439 is the reciprocal of 22'8 multiplied by 10,000. To convert 

 the number 439 into cubic centimetre it is only necessary to multiply it by 2 - 55 mm. 3 

 which gives the rate, in this case 1'12 c.c. per hour per 600 cm. 2 or 18"7 c.c. per square 

 metre of stoma-bearing area. 



VOL. LXXXVII. — B. X 



