THE CHLOROPHYLL APPARATUS 1657 
views of Usher and Priestley should prove to be correct, part 
of it is applied directly to the original decomposition of the 
absorbed carbon dioxide without the intervention of the 
living substance of the plastid. A very ingenious method of 
demonstrating that the energy is derived from the rays of 
light absorbed by the pigment was devised by Engelmann. 
He observed that certain bacteria were excited to active 
movement only in the presence of free oxygen. He placed 
a filament of a green alga upon a glass slide in a fluid 
containing 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 a 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 chlorophyll 
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. Tmiriazeff 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 pro- 
ceed 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 the antecedent of chlorophyll. The photo- 
synthetic power of etiolin is, however, exceedingly small. 
The percentage of carbon dioxide admitted to the 
chloroplasts has some influence upon the activity of the 
process. Normal air contains a mere trace of the gas, less 
than 4 parts in 10,000. A more copious supply is, how- 
ever, distinctly advantageous, and the activity increases as 
