6 Proceedings. 



and turgescence /or some time by the water which is taken up 

 by the immersed branch. Other experimental proofs might 

 be adduced to prove the absorption of water, but those I have 

 adduced, with the following demonstrations of the same fact 

 by living plants, will be sufficient. 



It is well known that plants discharge a large amount of 

 water into the air. This exhalation takes place in part 

 through the cells of the epidermis, but chiefly through the 

 stomata, or special organs provided for this purpose. The 

 amount of this exhalation varies much in diilerent plants, in 

 different parts of the same plant, and in the same plant at 

 different times. It is also greatly influenced by the number 

 and distribution of the stomata, the thickness of the cuticle, 

 the amount of chlorophyll in the leaf, the presence, intensity, 

 and colour of the light, and the moisture or dryness of the 

 soil in which the roots are placed. 1 mention the influence 

 of these circumstances on the exhalation of water because 

 they have a direct bearing on the amount of its absorption — 

 one action being compensatory to the other. For instance, 

 under the heat and light of a midsummer day the water 

 exhaled by the leaves often exceeds that which is absorbed 

 by the roots, and consequently the leaves lose their turge- 

 scence, become flaccid, and di"oop ; but when a shower 

 occurs or when dew falls in the evening these flaccid leaves, 

 by recovering their wonted firmness and turgescence, show 

 the effects, and afford a proof of the absorption of water. 

 The same thing is proved by observing the condition of trees 

 during the dry season in tropical climates. They are then 

 destitute of leaves, but before any rain falls on the ground 

 many of them burst into flower, when the prevalence of 

 moisture-laden winds which precede the rainy season gives 

 them the opportunity of absorbing water. Lastly, on this 

 head I would wish you to observe the relation which seems 

 to exist between the fibrill* by which roots take up water 

 from the earth and the hairs, scales, and other asperities by 

 which leaves can obtain it from the air. Where one is largely 

 developed the other is proportionately deficient. As we all 

 know, some of our native plants have simple tap-roots, with 



