MR JOHN AITKEN ON DEW. 29 



blades showed that they also tend to have large drops t attached to them, 

 while the rest of the blade is dry, and these drops were always found, to be 

 situated at certain definite points; they were always near the tips of the blades. 

 These large drops seen on plants at night are therefore not dew at all, but are 

 watery juices exuded by the plants. 



Now this excretion of water by the leaves of growing plants is not a new 

 discovery — it has been long well known. But what seems extremely curious is, 

 that its relation to dew has never been recognised, at least so far as I am 

 aware, and. it must be admitted that it is one of considerable importance. 



It is well known that plants transpire from their leaves an immense amount 

 of moisture, which passes off in an invisible form. Prof. J. Boussingault found 

 that mint transpired 82 grammes of water per square metre in sunshine, and 

 36 grammes in shade; but if the roots of the plants were removed, they only 

 transpired 16 and 15 grammes respectively. This simple experiment proves 

 that the root sends into the stem of the plant a supply of water, that it acts as 

 a kind of force-pump, and keeps up a pressure inside the tissues of the plant. 

 This supply sent in by the root is in most conditions removed by means of 

 transpiration from the surface of the leaves. 



Now what will be the result if transpiration is checked, while the root con- 

 tinues to send forward supplies? It will evidently depend on two things — 

 first, the pressure the root is capable of exerting before its action is stopped; 

 and second, the freeness with which the water can escape from the leaves. If 

 the root pressure is small, it will cease with the transpiration ; but if it is great, 

 the sap will be forced into the plant, and if nature has provided any outlets it 

 will escape at these openings. 



Dr J. W. Mool* has given great attention to the subject, and has experi- 

 mented on a number of plants. The method he employed in his researches 

 was to place the leaves under the most favourable conditions for the excretion 

 of drops, by diminishing the transpiration as far as possible, and by supplying 

 them with water. He substituted for root pressure, a pressure produced by a 

 column of mercury. Out of 60 plants experimented on by Dr Mool, he found 

 that the leaves of 29 excreted drops without being injected, 13 leaves became 

 injected and excreted drops, and 18 became injected and did not excrete at 

 all. He says that the excretion takes place by water-pores, and by ordinary 

 stomata, while in some cases it occurs at surfaces possessing neither of 

 these organs. 



I have recently made a few experiments on this subject in its relation to 

 dew. As, however, the season was far advanced before the experiments were 

 begun, but little could be accomplished, for the activity of the plants was nearly 

 over, and grass was almost the only plant possessing sufficient vitality for 



* Nature, vol. xxii. p. 403. 



