MR JOHN AITKEN ON DEW. 57 



rising from the soil underneath. If any drops now appear on the leaves, it is 

 evident the air cannot be the source of the moisture. 



When tested in this way, it will be found that any exuding plant will soon 

 become studded with drops, and present exactly the appearance it would at 

 night. The time the drops take to appear depends on certain conditions ; 

 amongst these are — 1st, the kind of plant experimented on; 2nd, the state of 

 vital activity in the plant at the time ; and 3rd, the degree of dryness of the 

 leaves at the time it is put under the receiver. If the leaves are very dry, it 

 will take some time to fill the tissues of the plant with sap before the surplus 

 begins to exude. A broccoli plant in fair health and condition may be expected 

 to show drops in less than an hour. These drops are small at first, and 

 gradually grow so large that they fall off by their own weight. 



An experiment of this kind is so easily made by any one, that the interest and 

 the information gained is ample reward for the little trouble taken in making 

 it. Instead of using a metal plate as above described for isolating the plant 

 from the damp soil, a simpler plan is to use a piece of sheet india-rubber, with 

 a radial slit in it, for slipping it round the plant, the two edges being joined 

 with india-rubber solution, the glass receiver being, as before, placed over the 

 plant, and its edges resting on the rubber. 



The evidence advanced in the first part of this paper in support of the 

 statement that the " dew-drop " is exuded by the plant is — 1st, that the drop 

 at the tip of a blade of grass grows as quickly when isolated from all supply of 

 moist air as when exposed to saturated air ; 2nd, the blades of those plants 

 which have drops attached to them on dewy nights exhibit no drops when 

 separated from the root, even when supplied with water and placed in 

 saturated air ; and 3rd, when hydrostatic pressure is applied to the stalk of 

 the leaf of any plant that has drops attached to it at night, it causes the leaf 

 to exude at exactly the same points as the drops appeared on dewy nights. 

 Although the above evidence is fairly conclusive, yet there is a point where it 

 might be strengthened by the addition of a link to the chain. While it is 

 shown that hydrostatic pressure will produce the same effects as are seen on 

 plants at nights, yet no evidence is adduced to show that there is any internal 

 pressure in those plants on which " dew-drops " have been observed. This 

 omission has now been corrected, and some experiments made in connection 

 with this point. The plants selected for experiment were of the same kinds 

 as those which were observed to have " dew-drops " on them at night, and 

 which showed exuded drops when subjected to hydrostatic pressure, such as 

 broccoli, cauliflower, poppy, &c. For convenience, some plants were grown in 

 flower pots, and the experiments made while they were still small. 



For measuring the pressure inside the plants, U-shaped tubes half filled 

 with mercury were used, and the pressure measured by the height to which 



VOL. XXXIII. PART I. H 



