442 Mr. D. Thoday. Experimental Researches on [Mar. 1, 



resistance must be overcome by the leaf in absorbing water from the con- 

 ducting tracts immediately below its petiole. 



When, on the other hand, the leaf is cut from the stem and the cut end of 

 its petiole immersed in water, that portion of the resistance which was due to 

 tension disappears altogether. The absorption of water therefore becomes 

 easy, so long as other sources of resistance do not come into play. 



In actual experience it is difficult to avoid for long the blocking of the 

 vessels. Even when the petiole is cut through while immersed in water, air is 

 apt to enter the vessels from the air spaces. When this does not occur the 

 vessels sooner or later become clogged with mucilaginous substances from the 

 cut cells. When ordinary water containing air is supplied, air-bubbles are 

 liberated in the water channels and block them completely, especially when 

 the leaf is exposed to the sun. 



It appears, then, that (1) initially, under the most favourable circum- 

 .stances, detached leaves experience relatively little resistance to the 

 absorption of water, and are therefore likely to open their stomata more 

 •widely than leaves still on the plant, where the water supply is obtained 

 •with greater difficulty ; but that (2) sooner or later, owing to the blocking of 

 their water channels, they are likely to close their stomata, and so come to 

 be at a disadvantage compared with attached leaves. 



Brown and Escombe's experiments with Catalpa, already quoted, illustrate 

 the initial phenomenon, and I have obtained confirmatory evidence with 

 leaves of the same plant by means of the horn hygroscope. Leaves which 

 had been detached since 7 A.M., and exposed to sunlight since 10.20 A.M., 

 gave readings at noon ranging between 67 and 58, whereas attached leaves 

 gave readings between 49 and 53. In the afternoon, however, several 

 of the same detached leaves gave very low readings, having begun to lose 

 turgor, tlius illustrating the second ])henonienon. 



Tiie relative duration and importance of these two relations must vary 

 with the nature of the plant and the conditions of experiment. The initial 

 •opening of the stomata will be most marked in the case of trees and in bright 

 sunshine ; it will be less marked or absent on moist dull days, and, with 

 many herbaceous plants, the water supply to leaves on the plant may 

 be abundant enough to allow the stomata, even in sunshine, to open to their 

 widest extent.* Some observations which I liave made indicate tliat this is 

 tru(! for Jfclianfhiin annnnM. 



Two leaves were detached aliout 9.30 A.M. on September 18, 1909, and a 



* When root i)rc!S8in'e leaclu^H as far im llie leaves, detachment would reduce the 

 efficiency of the water HUpply, and niiglit be followed by some reduction of the atoniatal 

 aperturcH. Such phenomena are especially probable in low herbaceous ])lants. 



