MR JOHN AITKEN ON DEW. . 61 



moisture in the ground, so that the same plant may emit droj)s at one time and 

 not at another. We have seen that even so free an exuding plant as grass may 

 cease to discharge ; others cease with a less degree of dryness, and with a 

 less decrease in vital activity. The number that exudes under favourable 

 conditions is, however, much greater than we might at first imagine. In the 

 south of France, in spring, a very great number of plants were observed to 

 exude on dewy nights ; even roses had their leaves fringed with drops, a 

 condition in which I have never seen these plants in this country ; but the 

 activity of vegetable life in spring is very much greater in the south than with us. 

 This question of root pressure in plants is one of vast interest ; so much 

 still remains to be known about it. How is it that one plant must have the 

 soil in which it grows full of water, while another requires it to be only damp % 

 Another seems to be able to grow on nearly dry soil, whilst another still can by 

 means of its air-roots extract moisture from air that is not saturated. What is 

 the source of energy called into action by this latter class to enable it to 

 condense the vapour in the air ? Is it a chemical process ? or a purely physical 

 one, like the condensation of vapour by Professor Tait's hygrometer when it 

 is falling % or is it some unknown function of vitality % These questions, how- 

 ever, open up a field much too wide to be considered here. 



28th July 1886. — After I had written the above paragraph, and as I supposed 

 had closed the paper, it slowly dawned upon me that the surface of the leaves of 

 all the different kinds of plants that have been observed to exude drops behaved 

 themselves in a particular manner towards water. None of them seemed to be 

 wetted by it. The glistening rain-drop on the grass shows that the blades of 

 that plant are not wetted by water, the glistening being due to the reflection 

 from the inside of the drop, where it rests on the blade, but does not touch it. 

 But do all the other exuding plants repel water in the same manner ? As it 

 was raining while these thoughts passed through my mind, a visit to the garden 

 was at once made, and the broccoli, poppy, and all the other exuding plants were 

 examined. Every one of them was found to behave towards the rain-drops in 

 the same manner as the grass. The rain-drops slipped off their surfaces " like 

 water off a duck's back ;" and where water collected in the hollows of the blades, 

 the reflection from its internal surface showed it was not in contact with them. 



The other plants — cultivated and uncultivated — in the garden were then 

 examined, when most of them were found to be quite wet. The difference 

 in their appearance from the exuding ones was very marked. At first sight 

 the leaves of plants that got wet, like potatoes, beans, &c, looked almost as if 

 they were dry, but in reality the water wetted them so perfectly all over, that 

 it ran off, leaving only a thin and even film on their surfaces ; whereas all 

 the plants that exuded drops had their surfaces dry, save certain small areas 



