MR JOHN AITKEN ON DEW. . 63 



laboratory, where it was connected by means of an india-rubber tube with a 

 head of water of about 15 metres, and surrounded with saturated air. After a 

 time drops appeared at the tips of most of the leaves, and also at some other 

 points on them ; but these drops were quite unlike those on grass, broccoli, and 

 other water-repelling plants ; they spread themselves on the leaves, and adhered 

 to them, no reflection being given from the back of the flattened drop. It 

 could, however, be easily seen, when the experiment was made in this way, that 

 moisture is exuded from the plant, whereas at night no exuded moisture is 

 perceptible. The reason for this is, that under the condition of the experiment, 

 the exuded drop only spreads to a certain extent, and the outline of the wetted 

 surface is defined, because the whole surface of the leaf is not wet ; but at night 

 the surface of the leaf is wet with dew, and the exuded drop spreads and 

 thins away by imperceptible degrees into the dewed surface. This was 

 illustrated in the above experiment by breathing on the leaf, so as to bring it 

 into the same condition it is on dewy nights, the drop was then seen to spread 

 rapidly outwards. 



We see from the above that a plant may be exuding, and yet we may not be 

 able to notice it. This is specially the case while dew is forming, that is under 

 natural conditions ; for dew is very generally forming while plants are exuding, 

 and it is difficult to tell from an examination made at night whether any plant 

 whose leaves have an affinity for water is exuding or not. It is therefore much 

 better to test the plants under artificial conditions, by placing them in saturated 

 air, but where no dew can be formed on their surfaces. This can be done by 

 placing them at night under hand-glasses, and well protected from radiation, or 

 even during the day under metal boxes, and well shaded. In this way a few 

 plants, whose leaves got wet with rain, were tested, and all were found to exude 

 if the evaporation from the leaves was stopped long enough, and time given for 

 the tissues to get filled with sap. In all cases the exuded moisture adhered to 

 the leaf and formed a wet patch. The plants tested were helichrysum, stocks, 

 asters, mignonette, foxglove, celery, lettuce, turnips. 



The plants were taken at hazard, and while some, such as mignonette and 

 stocks, exuded little, the others discharged a good deal. The root pressure of a 

 stock was measured, and found to be only about one-half that of the more freely 

 exuding Helichrysum. The root pressure will, however, be only one factor in 

 determining the amount exuded, as it is evident the rate of supply sent in by 

 the root will be of as much importance ; but no measurements of quantity have 

 been made by me. It may be as well to note here, that though the few plants, 

 taken at hazard, all showed powers of exuding, yet we must not therefore 

 conclude that all plants have this property. 



It is interesting to note the effects of these two ways in which the surface 

 of leaves behave towards their exuded sap and water. Take the different kinds 



