86 EEV. U. HENSLOVV ON THE 



Wiesiier with etiolated and green plants as well as from tliose 

 obtained with coloured flowers, to Avhicli the reader is referred 

 for further information*. 



My object in pursuing analogous experiments to those of 

 Wiesner was to try to test his conclusions, using, however, 

 coloured glasses instead of fluids ; and the results I have obtained 

 all tend to corroborate his. 



It is impossible to ascertain absolutely the amount of water 

 transpired which is due to any particular band of colour in the 

 solar spectrum ; for the difficulties are insuperable. To say 

 nothing of other influences at work to aid in the elimination ot 

 water, such as external and internal variations of temperature, 

 dryness of the air, and above all, variations in the intensity of 

 sunlight, it is impossible to get glasses monochroniatically pure 

 in any colour excepting red. Hence the results cannot be more 

 than relative or approximatel}- true. Such as they are, how- 

 ever, I find, like Wiesner, that the largest amount of water 

 transpired is coincident with tliose parts of the spectroscope 

 wherein lie the strongest absorption-bands of chlorophyll. 



Chakacter of tue CoLOUREn Glasses. — Before giving the 



o-^'-^'-o 



results of my experiments, it "will be desirable to describe the 

 character of the glasses employed. 



^ed. This is a pure monochromatic ruby-red, which transmits 

 no other rays w hatever besides red light ; and it is that portion 

 of the spectrum which contains the very strong chlorophyllian 

 absorptfon-bands Nos. I. and II. The thickness of the glass is 

 one tenth of an inch. 



YeUoio, This glass passes not only the yellow, but all the red 

 and green rays as well, up to F in the blue ; but stops the rest 



* Ann. Sci. Nat. ser. 0, iv. (1870), p. 157. For additional information 

 corroboratiye of Wiesner the reader may consult * Memorie della Eeale 

 Accadcmia dei Lincei ' (1879-80), and an abstract in 'Comptes Eendu?/ xci. 

 p. 6, xVug. yth, 18S0, the general conclusions of which are as follows : 

 *^Apart from, and in addition to, other factors which promote evaporation, 

 the actinic iiifluence of hglit largely affects transpiration in plants. Plants 

 trans{)ire more in liglit 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.) 



