TRANSPlKAXrON OF PLANTS. 83 



AViosner's * researches led him to draw different conclusions 



from those of M. Deherain, in that it is not the (oi)tically) brij^ditest 



part of the spectrum, but those parts which correspond to the 



absorptiou-bands of chloropbyll, which are maiuly concerned in 



the process. Later observers, including myself, tend to confirm 

 his views. 



A fact of importance, to which lie early calls the reader's 

 attention, is the chief difficulty iu carrying out experiments suc- 

 cessfully, from the ever-varying amount of liglit (not pure sun- 

 shine). He illustrated tliis by the following experiment. He 

 chose young plants of Maize, haviug their roots in water, pro- 

 tected above by a layer of oil to prevent evaporation. He placed 

 them in one scale of a balance in equilibrium. He theu removed 

 10 milligrammes ; aud as soon as the scales were again in equili- 

 brium lie removed 10 more, and so on. Equilibrium was succes- 

 sively restored after the periods of Gmin. 15 sec; 7,15; 4,30; 4,20 ; 

 7,45 ; and 5,10. The temperature and humidity were constaut* 

 Hence the variations of transpiration could only be accounted for 

 by the unequal illumination of the sky caused by passing clouds. 

 He also adds several experiments showing the great differences 

 which result between the effects of bright sunshine, diffused liglit, 

 gas, and in obscurity, and concludes with the remarks : — " Dans 

 tons les cas, ces quelques experiences montrent qu'ou s'expose a 

 de graves erreurs quand on etudie Taction de la lumicre sur la 

 trans2)iration sans tenir compte des changements de Teclairage." 

 Unfortunately the intensity of liglit is just the one thing which 

 at present it is impossible to estimate. 



The next point of importance to which Wiesner draws atten- 

 tion is tlie part which the ultra-red calorific rays play as a cause of 

 transpiration. Alluding to M. Deherain's experiments he saysf: 

 *' M. Deherain a expose une plaute dans Tair satare derriere uue 

 solution d'iode dans le sulfure de carbone qui ne laisse passer 

 que les rayons calorifiques obscurs, et il en conclut que dans ces 

 conditions, les rayons obscurs sont pour ainsi dire sans effet sur 

 la transpiration. 



*' J'ai fait moi-meme deux series d'experiences avec de jeunea 

 MaVs et des rameaux d'lf- Seize experiences a la lumiere solairc 



* *' Recherches sur I'influence de l;i lumiere et de la chakur rayonnauto sur 

 la transpiration des plantes," par M. J. Wiesner, Aun. des Sci. Nat. str. C, 

 iv. (1870), p. 145. 



t Ann. Sci. Nat. si't. 0, iv. (1876) p. 15*J. 



o2 



