USE OF THE SPECTROSCOPE IN STUDY OF PLANT LIFE. 83 



favourable to the process of Assimilatio7i. I gave statistics of my 

 experiments on this process of plant life only."^^ 



On the present occasion I propose recording some experiments on 

 Transpiration and comparing that with the loss of water by the pm^ely 

 mechanical effects of Evaporation by heat. Bespiration is, as will be 

 'seen, dependent on the heat-rays rather than the luminous ones, upon 

 which the first two mentioned are dependent. 



The following, therefore, may be regarded as a contribution to the 

 study of the relative effects of different parts of the solar spectrum on 

 the transpiration of plants. 



The conviction that light is an important factor among the causes 

 of the phenomena of plant-growth has long been held, but it is only 

 within the last half-century that satisfactory experiments have been 

 made to ascertain the relative effects of different rays of light upon 

 transpiration. Sachs, in his " Physiologic Veg6tale,"i says: "La 

 lumiere est un des agents qui agit le plus efticacement sur la transpira- 

 tion. Mais on ne pent pas dire positivement si elle agit par elle-meme, 

 ou par son union intime avec une elevation de temperature. " 



That author repeats this opinion in his " Text-book : " It is 

 still doubtful whether light, i.e. radiation as such, independently of 

 the elevation of temperature caused by it, influences transpiration." 

 He subjoins the footnote, " Deherain's researches§ do not decide the 

 question. 



Peof. Daubeny in 1836 carried out some experiments with coloured 

 lights, but forbore to give any numerical results, as he met with some 

 apparent anomalies. || He came, however, to the conclusion that " the 

 processes [the exhalation of moisture from the leaves and the absorp- 

 tion of it by the roots] are probably dependent on the combined action 

 of heat and light, coupled with those mechanical influences which 

 operate upon dead as well as upon living organic matter."^ 



Although the glasses used in his experiments were not tested by 

 the spectroscope, he appears to have come to some conclusions very 

 nearly the same as those arrived at by later observers, e.g. Wiesner; 

 yet he thinks that they are exceptional instead of being the rule, as the 

 latter believes them to be. "Now, although the experiment," he 



Since the above paper was published an interesting analogy has been shown 

 to exist between the formation of formaldehyde by means of a weak electric 

 current on carbonic acid dissolved in water, and its formation in leaves, before 

 its change into starch. The amount varies with the light and the amount of 

 carbonic acid present, the strength of the electric currents in green organs varvinff 

 with the intensity of the light. 



From Klebs' researches it would seem that red light (band No. I. ?) is most 

 concerned in the making of flowers, not the ultra-violet only, as was supposed 

 (see Eev. Gen. de Bot., vol. xxi. pp. 431, 432, and vol. xxii. p. 95ff.). 



t p. 250 (1868). ^ ' 



t Second English ed. (1882), p. 678. 



§ Ann. des Sci. Nat. ser. 5, xii. (1869), p. 1. 

 ^ II Sachs remarks as follows on Daubeny's experiments : " Ch. Daubeny, qui 

 s est occupe de cette question, ne s'exprime qu'avec une extreme prudence, et ses 

 observations ne paraissent pas I'avoir conduit a des resultats positifs."— P7iv5 

 Veg. p. 251. r ■/ . 



H Phil. Trans. 1836, i. p. 159. *\ 



G 2 



