TRANSPIRATION 75 



estimating transpiration which have been employed in modern 

 times, namely^ (i) weighing, (ii) a rough sort of potometer, 

 (iii) enclosing a branch in a glass balloon and collecting the 

 precipitated moisture, the well-known plan followed by various 

 French observers. 



He {Vegetable Staticks, p. 51) concluded his balance of loss 

 and gain in transpiring plants by estimating the amount of 

 available water in the soil to a depth of three feet, and calcu- 

 lating how long his sunflower would exist without watering. 

 He further concludes (p. 57) that an annual rainfall (of 22 inches) 

 is " sufficient for all the purposes of nature, in such flat countries 

 as this about Teddington." 



He constantly notes small points of interest, e.g. (p. 82) that 

 with cut branches the water absorbed diminishes each day and 

 that the former vigour of absorption may be partly renewed by 

 cutting a fresh surface 1 . 



He also showed (p. 89) that the transpiration current can 

 flow perfectly well from apex to base when the apical end is 

 immersed in water. 



These are familiar facts to us, but we should realise that it is 

 to the industry and ingenuity of Hales that we owe them. In 

 a repetition (p. 90) of the last experiment, we have the first 

 mention of a fact fundamentally important. He took two 

 branches (which with a clerical touch he calls M and N) and 

 having removed the bark from a part of the branch dipped the 

 ends in water, N with the great end downwards, but M upside 

 down. In this way he showed that the bark was not necessary 

 for the absorption or transmission of water 2 . I suspect that 

 one branch was inverted out of respect for the hypothesis of 

 sap-circulation. He perhaps thought that water could travel 

 apically by the wood, but only by the bark in the opposite 

 direction. 



Later in his book (pp. 128 and 131) he gives definite argu- 

 ments against the hypothesis in question. 



Next in order (p. 95) comes his well-known experiment on 

 the pressure exerted by peas increasing in size as they imbibe 



1 Compare F. von Hohnel, Bot. Zeititng, 1879, p. 318. 



2 This is also shown by experiment xc, Vegetable Staticks, p. 123. 



