56 MR JOHN AITKEN ON DEW. 



the supply of vapour to form dew would depend on the evaporation of the 

 dew, and on what was brought in by the winds. 



The "Dew-Drop." 



The statement that the "dew-drops" formed on plants at night is not dew 

 at all, but is formed of the exuded sap of the plant, has been rejected by some 

 on account of its being contrary to all accepted ideas on the subject, while some 

 who have accepted it, have given only an indifferent assent. It has therefore 

 seemed desirable that further evidence be advanced in support of the state- 

 ment, and also that some simpler methods be devised for studying the pheno- 

 mena connected with the exudation of moisture by plants, so that those not 

 accustomed to making difficult experiments may be able to demonstrate the 

 point for themselves. 



One of the simplest experiments of this kind is to cut a piece of turf, or, 

 better still, lift a single grass plant with a clod of earth attached to it, which 

 can generally be easily found in any garden. The leaves and stems of a single 

 plant being separate and open, the phenomena are more easily observed on it 

 than in the confused vegetation of a turf. Place the plant on a plate, and 

 invert a tumbler or other vessel over it, so as to enclose the plant and rest it on 

 the plate. This should be done when the soil is not too dry, otherwise water 

 will require to be given to the plant. After it has been kept in moist air 

 for about an hour drops will begin to exude, and the tip of nearly every 

 blade will be found to be studded with a diamond-like drop. 



In the above simple experiment there is nothing to tell us where the 

 moisture came from to form the drops. It might be contended that it was 

 condensed by the plant out of the moist air. It has, however, been shown in 

 the first part of this paper, that when the tip of the blade is isolated from 

 all supply of moist air, the drop at the end grows as quickly as the drops 

 at the ends of the blades exposed to saturated air. This experiment is, 

 however, a somewhat difficult one for any one not accustomed to work of 

 this kind. The point may, however, be proved in a much simpler way. Take 

 any exuding plant with a single stem, such as a broccoli or poppy, if it is grow- 

 ing in a pot so much the better, as it is more convenient both for making and 

 seeing the results of the experiment. Prepare a circular disc of metal — say tin- 

 plate — with a hole in its centre large enough for the stem of the plant to pass 

 through, then cut the disc in two through the centre. Now place the disc on 

 the pot with the stem of the plant passing through the hole, and join the two 

 halves of the disc, either by soldering, or by cementing over the joint a strip of 

 sheet india-rubber. A large glass receiver is now placed over the plant with 

 its edges resting on the metal plate. In this way the plant is isolated in air 

 from which it can extract no moisture, the metal plate preventing vapour 



