0\ THE TKAXSPIRA.TIOX OF PLA.NT3. 81 



A Contribution to the Study of tlip Eelative Effects of Different 



Parts of the Solar SpectruiV;'n the Transpiration of Plants. 

 By the Kev. Gteorge Hen^ow, F.L.S. 



[Read 3rd December, 1885.] 



IxTRODUCTiox. — 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 twenty years that 

 satisfactory experiments have been made to ascertain the relative 

 effects of different rays of light upon transpiration. Sachs, in 

 his ' PbysiologieVegetale'*, says : — "La lumiere est un des agenta 

 qui agit le pltis efficacement sur la transpiration. 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 ' t : — " It ia 

 still doubtful whether light, /. e, radiation as such, independently 

 of the elevation of temperature caused by it, influences transpi- 

 ration." He subjoins the footnote, " Deherain's researches % do 

 not decide the question." 



Prof. Daubeny in 183(5 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 absorption 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" i|. 



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 the latest observers, 

 e.g. Wiesner ; yet he thinks that they are exceptional instead of 

 being the rule, as the latter believes them to be. ** Kow, although 

 the experiment," he adds, " tended to show that the extrication 



* P. 250 (1868). t Second English ed. (1882) p. 678. 



{ Ann. des Sci. Nat. s6t. 5, xii. (1869), p. 1. 



§ Sachs remarks as follows on Dauben^a experiments : — ** Ch. Daubeny, qui 

 8*e8t ocoup^ de cette question, ne serprime quavec une extreme prudence, et 

 pes observations ne paraissent pas I'avoir conduit a dea r^sultats poHitifB.' 

 Phys. V^g. p. 251, 



i Phil. Trans. 18:36, i. p. 159. 



LINK. JOURN. — BOTANY, VOL. XXIX. « 



