740 GENERAL CONDITIONS OF PLANT- LIFE. 



the action of light on plants varies with its intensity, as that of temperature with its 

 elevation, does not admit of a doubt, and is obvious in all physiological observations. 

 There can scarcely be said, however, to be any exact investigations on this point ; 

 and the great obstacle to iheir accomplishment is that we have at present no method 

 of measuring the intensity of rays of light of any particular refrangibility in terms of 

 a fixed unit which can be applied to plants. As far as concerns the highly refran- 

 gible rays, t. e. those which have the greatest mechanical effect, we are compelled to 

 adopt the photo-chemical method of Bunsen and Roscoe^ which however gives no 

 information respecting the different intensity of the red, orange, and yellow light, 

 and can only be applied with great difficulty to experiments on vegetation. In the 

 photometry of the less refrangible rays, on the contrary, we must always have 

 recourse, according to the ordinary method, to the sensitiveness of the eye, t. e. to 

 brightness, which cannot be considered in itself to be an actual objective measure of 

 the intensity of the light, though it may be assumed under certain circumstances 

 that increase or diminution of subjective brightness corresponds to increase or 

 diminution of objective intensity. In describing the relation between the intensity 

 of light and vegetation, we have therefore at present, with a few exceptions, to 

 employ the ordinary expressions dark, dull, bright, dazzlingly bright, &c., and to 

 assume that they correspond to certain objective intensities. There is one case in 

 which this relation between the subjective sensitiveness of the eye and the action 

 upon vegetation of the light which causes it can be very strikingly proved ; Pfeffer 

 has shown that the curve of the subjective sensitiveness of the eye for the colours of 

 the solar spectrum coincides exactly with the curve expressing the power of different 

 regions of the spectrum in decomposing carbon dioxide ^. This coincidence must 

 however at present be considered purely accidental^, and cannot be extended to 

 other phenomena. If the sunlight or diffused daylight which reaches the observer 

 were always of the same intensity, it would be easy to regulate artificially, according 

 to definite gradations, the intensity of the light that acts on the plant. But since the 

 light of incandescent bodies (such as the Drummond's light*) contains the same 

 rays as sunlight and acts similarly on the functions of plants, constant sources 

 of light of a definite intensity can in this way be arranged, which will admit of 

 gradual adjustment, in order to study the influence on vegetation of light of different 

 intensities. 



If we now turn to the observations on record, those of Wolkoff are the only 

 ones in which actual measurements have been made. With the assistance of 

 the photometric method contrived by Bunsen and Roscoe^, he showed first of 

 all that changes in the intensity of the highly refrangible light do not stand in 

 any appreciable relation to the exhalation of gas by water-plants. This is an 



intensity of light and its brightness to the eye, see the paper quoted above and the literature there 

 referred to. 



^ See the admirable paper by Wolkoff in the Jahrb. fiir wiss. Bot. vol. V. p. i. 



^ Pfeffer in Sitzungsber. der Ges. zur Beforderung der ges. Naturwiss. fiAr Marburg, 1872, 

 May 16. 



^ vSee note on p. 747. 



* See Herve Mangon, Comp. rend. 1861, p. 243. — Prillieux, ibid. 1869, p. 408. 



^ Bunsen and Roscoe, Fogg. Ann. vol. 108. 



