74 STEPHEN HALES 



thus the day transpiration was roughly seven times the nocturnal 

 rate. This difference may be accounted for by the closure of the 

 stomata at night. 



Hales of course knew nothing of stomata, but it is surprising 

 to find Sachs in 1865 discussing the problem of transpiration 

 with hardly a reference to the effect of stomatal closure. 



Hales^ notes another point which a knowledge of stomatal 

 behaviour might have explained, viz. that with "scanty watering 

 the perspiration much abated," he does not attempt an explanation 

 but merely refers to it as a " healthy latitude of perspiration in 

 this Sunflower." 



In the course of his work on sunflowers he notices that the 

 flower follows the sun, he says however that it is " not by 

 turning round with the sun," i.e. that it is not a twisting of the 

 stalk, and goes on to call it nutatioti which must be the locus 

 classicus for the term used in this sense. 



An experiment^ that I do not remember to have seen quoted 

 elsewhere is worth describing. It is one of the many experiments 

 that show the generous scale on which his work was planned. 

 An apple bough five feet long was fixed to a vertical glass tube 

 nine feet long. The tube being above and the branch hanging 

 below the pressure of the column of water would act in concert 

 with the suck of the transpiring leaves instead of in opposition 

 to this force. He then cut the bare stem of his branch in two, 

 placing the apical half of the specimen (bearing side branches 

 and leaves) with its cut end in a glass vessel of water, the basal 

 and leafless half of the branch remained attached to the vertical 

 tube of water. In the next 30 hours only 6 ounces dripped 

 through the leafless branch, whereas the leafy branch absorbed 

 18 ounces. This, as he says, shows the great power of perspira- 

 tion. And though he does not pursue the experiment, it is 

 worthy of note as an attempt like those of Janse^ and others to 

 correlate the flow of water under pressure with the flow due to 

 transpiration. 



It is interesting to find that Hales used the three methods of 



^ Vegetable Staticks, ^. H- ^ Vegetable Staticks, p. ^i. 



* Janse in Pringsheinis yahrb. xviii. p. 38. The later literature is given by 

 Dixon in Progressus Rei Bot. in., 1909, p. 58. 



