140 Dr. F. Darwin and Miss D. F. M. Pertz. [June 15, 



ago* on withering leaves, in which the stomata were apparently open long 

 after the leaf ceased to react to the horn hygroscope. 



The great advantage of Lloyd's microscopic method is that it gives 

 (assuming it to be trustworthy) absolute quantities, i.e., it gives the actual 

 size of the stomatal pore. The porometer only gives relative sizes. Lloyd's 

 method suffers from the fact that, on a given leaf at a given moment, stomata 

 are found varying from 10 to 1 units in diameter. And, since it is impossible 

 to give unlimited time to each reading, it follows that his determinations of 

 stomatal sizes are rather rough. The porometer, on the other hand, auto- 

 matically strikes an average of many hundred stomata at each reading, and 

 this we believe to be a great advantage over the microscopic method. 



A cognate fact is that, on a given branch at a given time, the different 

 leaves may have stomata in very different conditions as to aperture. The 

 following figures illustrate the differences observed in the laurel : — 



Experiment 78. — P. laurocerasus. October 18, 1910. 

 The leaves were numbered from the apex downwards, and were all of the 

 current year except Nos. 12 and 13, which were unhealthy-looking leaves of 

 1909. The fall of the column was timed for 4 cm., i.e., 22-18 cm. 



Leaf No 2 



Times in seconds 6 "9 



The observations lasted from 10.50 to 11.40 a.m. It will be seen that the 

 rates of flow of the 1910 leaves varied from 5"6 to 19'5, or from 1 to 3'5. 

 The stomata on the old leaves, Nos. 12 and 13, are considerably more closed. 

 If this fact is true for other plants (and we have reason to believe it is so), it 

 is clear that any comparison between transpiration and the aperture of the 

 stomata, as estimated by the rate of flow through a single leaf, is not neces- 

 sarily trustworthy, because the transpiration of a branch must depend on 

 the average aperture of the stomata on a number of leaves, while the rate of 

 flow is dependent on the behaviour of a single leaf. From this point of 

 view Lloyd's method must be commended, it being part of his technique that 

 many leaves should be examined. f 



8 j 12 13 

 19-5 140-0 78-0 



* F. Darwin ('98), p. 547. 

 t Lloyd ('08), p. 23. 



