86 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



" Yet," WiESNER concludes, " the presence of some light seems 

 essential to the due continuance of the process." 



In ascertaining the amount of transpiration induced by different 

 rays of the spectrum, Wiesner adopted two methods : first, by placing 

 the plant in certain rays of the spectrum itself ; and, secondly, under 

 coloured fluids. The results were mutually corroborative. 



The following were the results by the first method : Red gave 

 136 milligr. per hour; yellow-orange, 122; blue, 146; ultra-violet, 70; 

 obscurity, 62. He next compared these results with others from leaves 

 placed as accurately as possible in the parts of the spectrum correspond- 

 ing with the absorption-bands of chlorophyll. These latter gave the 

 following results : - 



Red (with absorption-band No. I.) . . 34"3 milligr. per hour. 



Yellow-orange (between bands II. and III.) 32*0 ,, 



Green (between IV. and V.) . . . 30'4 ,, ,, 



Blue (corresponding to band VI.) . . 38"7 ,, ,, 



From this he concludes : " II est done bien evident que ce ne sont 

 pas les rayons les plus lumineux, les rayons jaunes, qui favorisent le 

 plus la transpiration; mais que cette faculte est repartie dans tout le 

 spectre, de telle maniere que les rayons les plus actifs sont precise- 

 ment ceux qui correspondent aux sept bandes noires du spectre de la 

 chlorophylle. 



" II est curieux d 'observer la plus forte transpiration dans les rayons 

 qui correspondent a la bande VI. M. von Wolkoff a fait voir recem- 

 ment'^ que c'est dans cette partie du spectre chlorophyllien que se fait 

 la plus puissante absorption de lumiere. 



" Les parties du spectre situees entre les bandes d 'absorption, et 

 qui sont toujours plus ou moins obscurcies par le passage a travers 

 une solution de chlorophylle, ne sont pas sans action sur la transpira- 

 tion ; mais cette influence est inferieure a celle des rayons completement 

 eteints dans cette solution."! 



That the presence of chlorophyll is intimately connected with trans- 

 piration is clear from the different results obtained by Wiesner with 

 etiolated and green plants as well as from those obtained with coloured 

 flowers, to which the reader is referred for further information.! 



My object in pursuing analogous experiments to those of Wiesner 



* "Die Lichtabsorption in den Chlorophyllosungen," Heidelberg, 1876. 

 t Ann. Sci. Nat. ser. 6. iv. (1876) p. 169. 



Z Ann. Sci. Nat. ser. 6. iv. (1876) p. 157. For additional information corro- 

 borative of Wiesner the reader may consult Memorie della Beale Accademia dei 

 Lincei (1879-80), and an abstract in Com^tes Bendus, xci. p. 6, August 9th, 1880, 

 the general conclusions of which are as follows : " Apart from, and in addition 

 to, other factors which promote evaporation, the actinic influence of light largely 

 affects transpiration in plants. Plants transpire more in light than in darkness, 

 and more in proportion to the intensity of the light. The effect is therefore 

 most marked just after midday. Only the portion of light absorbed produced 

 this effect; consequently highly coloured plants are more affected than others. 

 Plants transpire least in monochromatic light than in their own colour, and most 

 in the complementary colour. Thus, a green leaf transpires least in green and 

 most in red light, other conditions being the same." {Hort. Journ. 1880.) 



