G2 MR JOHN AITKEN ON DEW. 



where the natural surface of the blade had been destroyed. On thinking over the 

 matter, it became evident that this property of leaves that exude drops at night 

 ought to have been foreseen by me. The fact that the emitted moisture remains 

 as a drop, shows that the surface of the leaf rejects water; if the leaf surface 

 got wetted with water, the exuded liquid would have crept outwards from the 

 exuding pore, and have wetted the leaf for some distance all around it. These 

 exuded drops behave very much in the same manner as a drop of water 

 attached to the end of a glass rod that is not very clean ; the water does not 

 wet the rod, but draws itself up into a drop. If the drop had been attached to 

 a wooden rod or a piece of thread, or anything that was easily wetted, it would 

 not have remained as a drop, but have spread itself all over the surface of the 

 body. 



When examining the plants in the garden during rain, in addition to those 

 plants which I knew exuded drops at night, I noticed a number of others 

 that rejected the rain drops, and kept their surfaces dry in the same manner as 

 the exuding plants. Amongst these were Nasturtium, some of the Brassiceae 

 family not previously observed, and also some weeds. Now, it appeared that if 

 the above reasoning is correct, these other dry-surfaced plants ought to exude 

 drops, I therefore marked them, and on afterwards experimenting found that 

 they also discharged drops like the others. 



It almost looked at first sight as if this property of repelling water was a 

 distinguishing characteristic of the leaves of all exuding plants; but on further 

 considering the matter, the idea soon suggested itself that the other class of 

 plants, the leaves of which got wetted with rain, might also exude moisture, as 

 it was evident that if they did exude the discharge would be masked, for the 

 moisture would not collect on them in drops, but sj)read itself over the leaves, 

 and so become undistinguishable from dew. It therefore seemed desirable that 

 other experiments be made on this class of plants, to see if any of them exuded 

 moisture. It was evident that special precautions would be necessary to enable 

 us to see the exuded moisture on leaves easily wetted, as it would not be so 

 easily seen as the sparkling drop on water-repelling leaves. 



For investigating this point, the most convenient plant I could find was a 

 strong growing variety of everlasting flower (Helichrysum). This plant was 

 one of those observed to have its leaves wet while it was raining, and no exuded 

 drops were observed on it at night. The first thing determined was to see if 

 there was any root pressure to cause exudation. The plant was cut across 

 at the bottom of the stem, and the pressure gauge attached near the root. The 

 root pressure was found to be 950 mm.; that is, this plant had as great an 

 internal pressure as was found in the drop-exuding plants. In order to see 

 whether it exuded when hydrostatic pressure was applied, the upper part of 

 the plant, which was cut off for taking the root pressure, was removed to the 



