458 MR. A. J. EWART ON ASSIMILATORY INHIBITION. 
phenomenon may, however, also be shown by the leaves of certain 
Phanerogams. 
Betula lenta.—Exposed but folded leaves. L. 3, B. yo. Green marginal 
leaf-teeth contain fairly large and green chlorophyll grains and show a 
weak evolution of O; whereas the central part of the leaf is red owing to 
the presence of a red sap in the epidermal cells which has disappeared 
from the more exposed epidermal cells of the leaf-teeth, shows no evolu- 
tion of O., and the chlorophyll bodies are yellowish-green and from 4 to 
3 the size of those in the mesophyll of the leaf-teeth. With leaves, L. 3, 
. 1, in which the lamina is still pleated, the same relative differences in 
the size of the chlorophyll grains of the two parts are still shown, and 
whilst the body of the lamina shows ouly a very weak evolution of O, 
from the leaf-teeth this is moderatively active. With fully expanded 
leaves, L. 4. B. 4, in which the red colour is present only in the epidermal 
cells over the veins, the difference between the chlorophyll grains is no 
longer perceptible, and all parts of the leaf show a quite active power of 
assimilation. 
Corylus Avellana.—Leaves, L. 35, B. ys, folded, hairy, and yellowish 
green, No evolution of O. Tip of leaf greener and very weak evolution 
of O. L. 3, B. Az; still folded and yellowish green. Tips of the teeth 
at the ends of the main veins with larger and greener chlorophyll grains 
and a moderately active evolution of O. Central part, chlorophyll grains 
smaller and more yellowish ; evolution of O absent or barely perceptible, 
but in parts (over the veins and near the margin) a weak but distinct 
evolution of O. L.4, B. i, P. à. Leaf expanded. Teeth green, and 
show active evolution of O. Rest of leaf still slightly yellowish, and 
chlorophyll grains small and somewhat pale; moderately active evolu- 
tion of O, which is in part weak. L. 2, B. 4, P. 2. Chlorophyll grains 
| equally large and green, and quite active assimilation in all parts. 
In Betula lenta there is a very close correspondence between 
the commencement of assimilation and the disappearance of the 
protective red dye. In each part of the leaf, as the chlorophyll 
grains become adult and commence to assimilate, the red dye 
fades and disappears from the sap of the epidermal cells. In 
Corylus, assimilation appears sooner over the veins than in the 
rest of the lamina, because these parts are, in the folded con- 
dition of the leaf with the ventral surface outwards, more 
exposed to light and to contact with air than the depressed 
parts, covered thickly by hairs, lying between the ribs. 
It does not necessarily follow that all power of assimilation is 
absent because no power of evolving oxygen is shown under 
optimal illumination, for it is always possible that a slight power 
of assimilation may be present, but is overpowered and masked 
by the active respiration of the young leaf. To determine 
whether or not this formed a serious source of error, comparative 
determinations were made vf the amounts of CO, given off in 
