30 MR JOHN AITKEN ON DEW. 



experimenting. I however removed a branch of the poppy, which, during 

 summer, had shown such a tendency to exude moisture, and connected it by 

 means of an india-rubber tube with a head of water of about one metre. 

 After placing a glass receiver over it, so as to check evaporation, it was left for 

 two or three hours, when it was found to have excreted water freely — some 

 parts of the leaves being quite wet, while drops had collected at other places. 



The broccoli plants which had excited my interest in summer were also 

 experimented with. A full-grown leaf was fitted into the apparatus, and the 

 pressure applied. In a little over an hour it also exuded water, and soon got 

 fringed with drops along its edge in exactly the same way that was observed on 

 it in summer. Another leaf from the same plant, but much younger, being 

 about one quarter grown, on being tested in the same way did not excrete at all, 

 after the pressure had been applied for twenty-four hours. Here we have the 

 same result as that noticed in summer — one leaf exudes, while another on 

 the same plant does not. 



If the water pressed into the leaf is coloured with aniline blue, the drops 

 when they first appear are colourless, but before they grow to any size, the 

 blue appears, showing that little water was held in the veins, but the whole 

 leaf got coloured of a fine deep blue-green, like that seen when vegetation is 

 very rank, showing that the injected liquid had penetrated through the 

 whole leaf. 



Most of my experiments on this subject were made with grass. I find 

 that even in the middle of October, after having been severely frosted two or 

 three times, which had probably reduced its vitality, it still exuded so abun- 

 dantly that drops collected in air which was not saturated. A turf placed in 

 a cellar, dry enough to keep glass quite free from dewy deposit, soon collected 

 drops. These drops always appear near the tips of the blades ; they are not 

 exuded from every blade, and sometimes from only one on each stalk, but 

 generally from more ; and it is always from the blades that seem to have the 

 greatest vitality, and are nearly, but not quite full grown. Sometimes it is the 

 youngest blade that exudes, but if it is very small, it is the second youngest. 

 As the blades grow old they cease to exude ; but this seems to be due to 

 some change in the blade at the point where it exuded, and not to a diminution 

 of root pressure, as it exudes freely when the tip is cut off. 



The question might be here raised, Are these drops really exuded by the 

 plant \ Are they not due to some condensing power possessed by the leaves, 

 by the presence at these points of some substance possessing an affinity for 

 water vapour, or some process by which they may extract moisture from the 

 air ? To get an answer to this question, I selected a small turf, placed 

 over it a glass receiver, and left it till drops were excreted. Removing 

 the receiver, a blade having a drop attached to it was selected. After being 



