USE UF THE SPECTROSCOPE IN STUDY OF PLANT LIFE. 85 



this b}' the following experiment : He chose young plants of Maize, 

 having their roots in water, protected above by a layer of oil to prevent 

 evaporation. He placed them in one scale of a balance in equilibrium. 

 He then removed 10 milligrammes; and as soon as the scales were 

 again in equilibrium he removed ten more, and so on. Equilibrium 

 was successively restored after the periods of 6 min. 15 sec; 7,15; 

 4,30; 4,20; 7,45; and 5,10. The temperature and humidity were 

 constant. 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 betw^een the effects of bright sunshine, diffused light, gas, 

 and in obscurity, and concludes with the remarks: "Dans tons les 

 cas, ces quelques experiences montrent qu'on s 'expose a de graves 

 erreurs quand on ^tudie Taction de la lumiere sur la transpiration sans 

 tenir compte des changements de r^clairage." 



Unfortunately the intensity of light is just the one thing which 

 at present it is impossible to estimate with great accuracy. 



The next point of importance to which Wiesner draws attention 

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

 piration. 



" J'ai fait moi-meme deux series d 'experiences avec de jeunes Mais 

 et des rameaux d'If. Seize experiences a la lumiere solaire et a la 

 lumiere du gaz m'ont conduit k ce resultat : que les rayons calorifiques 

 ohscurs agisse7it tr^s-fortement sur la transpiration, et que cette 

 influence, relativement a celle des autres rayo7is du spectre, est plus 

 grande quand on se sert de la lumiere du gaz que quand on opere a 

 la lumiere solaire.''^ 



After detailing his methods of experiment he concludes : " II y a 

 entre la valeur calculee de la transpiration k I'obscurite et la valeur 

 observee une difference de 11 pour 100. 



" En passant sur ces erreurs d 'experiences, on trouve d'une maniere 

 approximative que pour 100 d'eau transpiree, 79 reviennent aux rayons 

 lumineux et ultra-violets, et 27 aux rayons calorifiques obscurs." 



It is worth w^hile to recall the fact that Daubeny in 1836 perceived 

 the importance of heat-rays. He attributed the excess of transpiration 

 under blue, purple, and the cupreous solution, in great part to the heat 

 radiated from the screen, for " a bottle filled with water, blackened 

 with ink to such a degree as to transmit just as much light so far as 

 could be measured by the eye, as that filled with the copper solution 

 was found to do, caused an equally considerable amount of water to 

 be evolved by the plant. . . . Now as water, with the addition of a 

 little ink, is known to absorb the rays proceeding from all parts of 

 the spectrum in an equal ratio, it follows that the effect produced in 

 either instance must be ascribed to the heat radiated, and not to any 

 peculiar virtue of the violet extremity in stimulating the vegetative 

 functions." 



* " La flamme du gaz donne bien plus de rayons calorifiques que de rayons 

 lumineux (Tyndall)." 



