MR JOHN AITKEN ON DEW. 31 



carefully dried, the tip of the blade was placed in a small glass receiver, so 

 as to isolate it from the damp air of the larger receiver. This small 

 covering glass measured about 10 mm. in diameter by about 15 mm. in 

 height. Its open end was closed by means of a very thin plate of metal 

 cemented to it. In the centre of this plate was pierced a small opening, of 

 the same size and shape as the selected blade of grass. The tip of the blade 

 was entered about 5 mm. into this small receiver, and to prevent moisture 

 entering and coming in contact with the tip of the blade, an air-tight joint between 

 the blade and the metal was made with india-rubber solution. The tip of 

 the blade was thus isolated inside the small receiver in which the air was 

 dry. The large glass receiver was then placed over the turf to prevent 

 evaporation from the lower part of the blade, or the experiment was made in a 

 room where the air was not very dry. After a time, generally some hours, the 

 turf was examined. A drop was always found to have formed on the tip of 

 the blade inside the small receiver, and this drop was, as nearly as could be 

 judged, always as large as the drops formed in the moist air under the large 

 receiver. It would thus appear that these drops are really exuded by the 

 plant, and not extracted from the air. 



These exuded drops seem to be almost entirely the result of root pressure, 

 because if we cut off the roots, and place the stems in water, putting over all 

 a glass receiver standing in water so as to saturate the air, and as a test 

 that the conditions are favourable, placing a small turf alongside the 

 cut grass under the receiver, we shall find that scarcely any drops make 

 their appearance on the rootless stems, while those with roots have drops 

 attached to them. Again, if we take one of these rootless stems, and attach it 

 by means of the india-rubber tube to a head of water, it is found to exude 

 drops at the tips of its blades in moist air in the same way as when it was 

 attached to its roots. 



These excreted drops are formed on grass on other than dewy nights. 

 After rain, if there has been no wind, and the air near the ground becomes 

 saturated, a rearrangement of the drops takes place. Some time after the rain 

 has ceased, most of the blades will be found to be tipped with a drop at the 

 same point as the exuded drop appeared at night — a position which no falling 

 rain drop could keep. This tendency of plants to exude moisture explains 

 why the grass is almost always wet during autumn. At that season evapora- 

 tion is slow, and as the plants are constantly pouring in supplies to the drops, 

 it takes a long time for the slow evaporation to overcome the wetting effect 

 and dry up the grass. 



The question as to what degree of humidity in the air is necessary before 

 plants will exude drops, would seem to be greatly determined by the rate at 

 which the supply is sent into the leaf. If the supply is greater than the 



