162 VEGETABLE PHYSIOLOGY 
movement only in the presence of free oxygen. He placed 
a filament of a green alga upon a glass slide in a fluid con- 
taining a number of the bacteria, covered it with a glass 
cover-slip, and sealed it with wax. He kept it in darkness 
till the microbes had come to rest, and then by the aid of a 
microspectroscope he threw an image of the solar spectrum 
upon the filament and observed in what parts of it the 
bacteria accumulated as soon as they began to move. These 
places corresponded with the positions of the absorption 
bands which we have seen to be characteristic of the chloro- 
phyll spectrum, the maximum effect being produced by the 
deep band in the red region. These were evidently the 
places at which the chlorophyll apparatus of the filament 
was at work, the movements of the bacteria showing that 
oxygen was liberated there. Timiriazeff proved the same 
thing by throwing the spectrum of solar light upon a darkened 
leaf, when he found that starch was produced only in the 
positions of those same absorption bands, indicating that 
those were the only places of photosynthetic activity. 
The process of photosynthesis has been found to proceed 
under certain circumstances in light which is too feeble in 
intensity to cause the development of chlorophyll. It is 
effected in these cases by the etiolin, which we have seen 
to be present under such conditions. The photosynthetic 
power of etiolin is, however, exceedingly small. 
The percentage of carbon dioxide admitted to the chloro- 
plasts has some influence upon the activity of the process. 
Normal air contains a mere trace of the gas, less than 8 parts 
in 10,000. A more copious supply is, however, distinctly 
advantageous, and the activity increases as the percentage 
rises, though the plant does not gain in weight proportion- 
ately. The optimum quantity appears to be about 10 per 
cent. with light of the ordinary intensity. More than this 
gradually exerts a paralysing influence on the chloroplast, 
and consequently sets up an inhibition of the apparatus. 
The optimum amount of carbon dioxide varies, however, 
considerably with the intensity of the illumination and 
