8o6 
Irving . —Photosynthesis and 
It was thought that in such a case the whole assimilatory apparatus 
might be efficiently developed, except the green pigment, and as this in¬ 
creased by degrees, so the power of photosynthesis would increase. 
Thus the amount of chlorophyll present would then be the limiting factor 
for assimilation, and interesting data might be looked for relating the 
amount of the pigment present to the amount of photosynthesis that could 
be affected. 
Contrary to expectation it was found that not only etiolated shoots of 
a deep orange yellow possessed no measurable power of assimilation, but 
shoots which had developed quite a considerable depth of green had not 
yet attained this power. 
Only when a full grass-green colour was attained was the assimilation 
of C0 2 sufficient to cause the amount of C0 2 given out in the light to be 
unmistakably less than the amount given out in the dark. 
In searching for the first faint signs of assimilation it is unnecessary to 
add C0 2 to the air passing over the plant; the C0 2 supplied by the plant’s 
own respiration will serve as material for its photosynthetic activity. This 
source of C0 2 has the further advantage of being produced within the plant 
itself, so that there are no difficulties of diffusion to be overcome, such as 
might arise with a cuticularized organ in which the stomata were in¬ 
sufficiently developed to freely admit the passage of C0 2 from the air 
surrounding the plant. The experiments carried out with whole seedlings 
of Barley may first be described and then those more critical ones done 
with cut shoots of Barley and Vida Faba. 
For these first experiments Barley was germinated in a glass cell 
through which a current of air could be drawn and the output of C0 2 
measured continuously. The arrangement is shown in Fig. i. The glass 
