8 oS 
Irving.—Photosynthesis and 
at night. On August 20 the leaf-tips of bright green protruded about half 
an inch through the sheath, but there was no sign of appreciable reduction 
of the respiration by photosynthesis. On August 21 the green leaves were 
inches long and the photosynthesis was slight but certain, the C 0 2 falling 
to 0-004 gram in the middle of the day. On August 25 the leaves were 
3 inches long and could assimilate about °f the respiration. This was 
the maximum activity that this particular arrangement of apparatus allows, 
and on August 25, when the leaves were 5 inches high, some reaching to the 
glass roof, the photosynthesis was not found to be any greater. 
The general teaching of the experiment is that photosynthetic power 
lags behind greenness in natural development more than might have been 
expected, only catching up the respiration on the third day after the green 
leaf begins to show. In the bacterial test for photosynthesis it is this 
moment, when carbon-dioxide fixation overpowers carbon-dioxide pro¬ 
duction, that brings on the motile 
response of the bacterium, but in the 
present experiments the chloroplast 
activity has to contend with the car¬ 
bon-dioxide production of the whole 
plant, not of its particular mesophyll 
cell only. 
The second experiment of this 
type was designed to see if there 
would be a similar lag in photosyn¬ 
thetic activity when leaves that are 
fully protruded but etiolated in dark¬ 
ness are turning green in diffuse light. 
The barley seedlings were therefore allowed to develop in complete 
darkness until the protruded leaves were about 5 inches long, reaching 
nearly to the top of the cell. The cell was closed, placed in position in the 
window, and estimations started at 5 p. m. on August 29, being, however, 
completely darkened by a cloth till night had set in. On August 30, the 
first day of illumination, the bright yellow etiolated leaves developed a 
considerable tint of green except at the leaf-tips, but Fig. 3 shows that 
there was no diminution of the respiratory output of C 0 2 until the subse¬ 
quent day, when the photosynthetic power quickly became equal to 
assimilating ££ of the respiration. 
It appears, therefore, that there is the same lag of photosynthetic 
activity behind the development of chlorophyll in the illuminated etiolated 
leaves as in the younger leaves developing in natural illumination. The only 
previous analytical data on this subject with which I am acquainted are the 
measurements made by Wiesner 1 thirty-three years ago. He seems to have 
iesner : Die Entstehung des Chlorophylls. Wien, 1877, PP* 107-m. 
Fig. 3. 
