eal ll 4 4 PY a. ‘> A i 4 L i, - pa - 
_ ’ ' ) . e - . b, 3 ri 
; Ne 74 “eo Wie Win a WP it A % 
é é ¥ » f 7 ‘ ae) / 
Nawions o> ee AW : ' Caps Pipe Peay We 4 
hf * d ¢ 4 - < . 
4 . 7 i / ~ ' : : 4 
normal size; the leaves of ferns do not even unfold, and attain only, 
1 
Tas OF EVEN yG55 of their normal superficial development in the light. 
White daylight consists of a mixture of rays of light, 
differmg in their colour and their refrangibility ; and it 
might be expected that the action of the different rays on 
the vital processes of the plant would not be the same. 
This is illustrated by the accompanying diagram (Fig. 373). 
ee ee ee ee, 
Brightness. : 
bp eee ose emer A ccimilation 
Fic. 373.—Diagrammatic representation of the efficacy of rays of light of different 
refrangibility in causing evolution of oxygen, compared with their brightness and 
heating power. ‘The solar spectrum A H serves as a base on which Fraunhofer’s 
lines are indicated ; A and B lie in the red light, c on the border of the orange, pb 
in the yellow, # in the green, F in the blue on the border of the green, G on the 
border of the indigo and violet, H in the violet. The height of the three 
curves, of Heat, Brightness, and Assimilation, at any point, represents the inten- 
sity of the action. Thus assimilation is still slight at G on the border of the violet 
-and indigo, and attains its maximum between D and E on the border of the yellow 
and green, then diminishing, and ceasing at a on the border of the red light. 
It is seen that the process of assimilation neatly coincides with the subjective — 
brightness as shown in the colours of the spectrum, while the curve of heating: 
power takes a totally different course and attains its maximum at a point where 
the eye is unable to perceive any light, and where no process of assimilation goes 
on in the plant. ! 
The most important chemical process in the plant, the decomposition 
_ of carbon dioxide in the cells which contain chlorophyll and the elimina- 
tion of oxygen resulting from it, proceeds almost as energetically as in 
white daylight, in a mixture of orange, yellow, and green light which 
acts only slightly and very slowly on photographic paper. Blue, violet, 
and the ultra-violet rays, on the contrary, which at once colour photo- 
graphic paper a deep brown, have no or very little effect on the elimina- 
tion of oxygen.’ ‘The same relationship towards light has been shown 
1 It is an established mode of expression to speak of the blue, violet, 
and ultra-violet rays as the chemically active, because they are efficient, 
| ‘ as 3 he Life of the Plant. . 213 
1 . 
/ 
f 
