370 PHYSIOLOGY 
The bands 1-3, and possibly 4, belong to chlorophyllin, while the indefinite three, 
7-7/7, belong to carotin. These three are much better seen in the absorption 
spectrum of carotin alone (fig. 648, 7J>). 
Fluorescence. Chlorophyll has another physical character, which it shares with 
some other dyes ; its solution is fluorescent. When a strong solution in alcohol is 
held between the eye and the light, the color is a vivid green; but if examined by 
bright reflected light, it appears deep blood-red. While this is a useful recognition 
mark, the physiological significance of fluorescence, if any, cannot be explained. 
The absorbed energy. The energy that drives the machinery is de- 
rived from light, for if a green plant be kept in darkness, it is entirely 
unable to make any carbohydrates. Furthermore, it is only the chloro- 
plast directly illuminated that receives this energy. A lighted portion 
of a leaf cannot communicate the energy to a darkened area. If a por- 
tion of a leaf be covered with an opaque plate, while CO 2 is allowed free 
access, the rest of the leaf may show evidence of active photosynthesis, 
but the darkened area shows none. Moreover, it is the energy absorbed 
by the chlorophyll that does the work. 
The following experiment shows this: A plant was kept in the dark until its 
leaves showed no trace of starch. Then on a sunny day a spectrum of sunlight, as 
bright as possible, was cast on a leaf and kept steadily in the same place for some 
hours. Thus the chlorophyll could absorb energy only in those regions along the 
band of light where fell the waves of lengths that it can stop; on the leaf these re- 
gions of course corresponded in position to the absorption bands before described. 
If, therefore, the leaf works with the absorbed energy, photosynthesis can occur only 
in these strips and not elsewhere. After the exposure, on testing the leaf for starch 
(the accumulation of which is a mark of active photosynthesis), it was found in 
abundance where lay absorption band i (fig. 648), and scantily in others; but 
it was wholly lacking in other parts of the spectrum. 
This is what would be expected; but there was once an idea that 
chlorophyll acted merely as a screen, shading the protoplasm from harm- 
ful rays of light; and that the protoplasm could work properly only 
behind such a screen. There is now evidence that the protoplasm is 
unnecessary in the first stages of carbohydrate synthesis, those strictly 
called photosynthesis. It is probably light transformed to electricity 
that reduces the H 2 CO 3 to formaldehyde (see p. 375), which then con- 
denses into more complex carbohydrates. 
Exposure to light. Plainly the light which has passed through a 
chloroplast is unlike that which has not; and the more chloroplasts it 
passes through, the more complete is the absorption of effective waves. 
The upper cells of a leaf, therefore, are in a more favorable position with 
respect to light than the lower, especially in weak or diffuse light ; but 
