662 GENERAL CONDITIONS OF PLANT-LIFE. 



very manifest. A number of more recent observations have confirmed and extended 

 the results previously obtained ^ 



(2) Variation in the action of light on plants in proportion to its ifitensiiy"-. That 

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

 with its elevation, does not admit of a doubt, and agrees with the observed facts 

 of vegetative physiology. There can scarcely be said, however, to be any exact 

 investigations on this point; and the great obstacle to their 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 refrangible rays, i.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 can always have recourse, according to the ordinary 

 method, to the sensitiveness of the eye, i. e. to brightness, which cannot be con- 

 sidered in itself an actual objective measure of the intensity of the light, though 

 it must under certain circumstances depend upon it. 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. There is one case in which this relation between the sub- 

 jective sensitiveness of the eye and the action upon vegetation of the light 

 which causes it can be very strikingly proved; Pfefifer 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 earbon dioxide''. This coincidence must however at 

 present be considered purely accidental ^^ and cannot be extended to other pheno- 

 mena. 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. 



^ I have replied, in the second part of the 'Arbeiten des botan. Inst, in Wiirzburg,' 1872, 

 to the objections urged by Prillieux to this statement, which rest on an entire confusion of the 

 ideas Intensity of Light (objective), Brightness (subjective), Refrangibility (an objective), and 

 Colour (a subjective property of light). 



^ With respect to the distinction which must here be borne in mind between the objective 

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

 referred to. 



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



* Pfeffer in vSitzungsber. der Ges. zur Beforderung der ges. Naturwiss. fiir Marburg. 1S72, 

 May 16. 



^ See note on p. 669. 



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



