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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



adds, " tended to show that the extricatioD of moisture, cceteris paribus, 

 was most abundant in proportion to the intensity of the hght admitted 

 (orange glass in general causing more moisture to be exhaled than 

 red or green), yet in some instances blue and purple glasses, and, still 

 more remarkably, bottles filled with the cupreous solution [ammonio- 

 sulphate of copper] would cause a more abundant exhalation than 

 orange or even transparent glass. Here, however, another principle 

 seems to come into play, namely, the influence of heat radiated from 

 the surface of the screen."* 



Deherain's researches alluded to above were made upon leaves in 

 a saturated atmosphere, and he came to the following conclusions, 

 amongst others : — f 



" 1°. L 'evaporation de I'eau par les feuilles est determinee par la 

 lumiere et non par la chaleur. 



" 2°. Cette Evaporation se continue dans une atmosphere saturEe. 



"3°. Les jeunes feuilles evaporent plus d'eau que les anciennes. 



" 4°. Les rayons lumineux (jaune et rouge) efficaces pour d6terminer 

 la decomposition de I'acide carbonique, sont aussi ceux[?] qui provo-. 

 quent I'evaporation la plus abondante. 



" 5°. La difference d'action des divers rayons lumineux est encore 

 sensible quand on s'efforce de les amener a une intensite lumineuse 

 egale. " 



The coloured fluids used by M. Deherain were as follows : — 



"1. Dissolution rouge de carmin dans I'ammoniaque. 



"2. Dissolution jaune de chromate neutre de potasse. 



" 3. Dissolution verte de chlorure de cuivre. 



" 4. Dissolution bleue de sulphate de cuivre ammoniacal. 



" 5. Dissolution violette d'iode dans le sulfure de carbone." 



The quantity of water " evaporated " in one hour by a leaf of 

 barley, reduced to percentages of the whole weight of the leaf, was as 

 follows: Eed 93*6 p.c, yellow 63*4, green 5-8, blue 6-3, violet -05. 



The relative intensities of the rays, however, w^ere not estimated 

 in any way. Nor were the lights tested by the spectroscope — a most 

 important and, in fact, necessary procedure, as it is pretty certain that 

 they were not monochromatic. | 



Wiesner's§ researches led him to draw different conclusions from 

 those of M. Deherain, in that it is not the (optically) brightest part 

 of the spectrum, but those parts which correspond to the absorption- 

 bands of chlorophyll, which are mainly concerned in the process. 

 Later observers, including myself, tend to confirm his views. 



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

 is the chief difficulty in carrying out experiments successfully, from 

 the eiver-varying amount of light (not pure sunshine). He illustrated 



* Phil. Trans. 1836, i. p. 160. f Ann. Sci. Nat. ser. 5, xii (1869) p. 23. 



X A violet glass of a peculiar reddish tint in my possession transmitted tlie 

 whole of the spectr^im, and was therefore useless for experimental purposes. 



§ " Recherches sur I'influence de la lumiere et de la chaleur rayonnante sur la 

 tivanspiration des plantes," par M. J. Wiesner, Ann. rJes Sci. Nat. ser. 6. iv. 

 (1876) p. 145. 



