MR. A. J. EWART ON ASSIMILATORY INHIBITION, 391 
chlorophyll grains remain uninjured and intact, the re-formation 
of the green pigment soon takes place, and this occurs more 
rapidly in entire plants than in experiments as above with 
isolated branches. As might be expected, the return of assimi- 
lation is coincident with the re-formation of the chlorophyll, and 
begins before the formation of the latter is complete. In many 
plants, however prolonged the exposure may be, no change of 
colour is produced in the chlorophyll so long as the cells remain 
living. Thus in Ulex and Ilex, at the end of winter all the: 
exposed leaves or spines were brown, but the browning is here a 
post-mortem change and is a sign of death. lt frequently 
happens that the different parts or even different layers of the 
same leaf may be differently affected. Thus in Buzus, at the end 
of winter many leaves have in the middle mesophyll layers normal 
green ehlorophyll grains and show a distinet power of assimi- 
lation, those of the upper layer of palisade parenchyma being 
quite brown aud showing no power of assimilation, whilst the 
cells of the lower spongy mesophyll, forming about one-third the 
thickness of the leaf, are all dead and shrunken. Such leaves in 
spring soon die and drop off. In most leaves the upper outer 
layers of the palisade parenchyma contain brown or greenish- 
brown chlorophyll graius, and show none or only very faint 
power of assimilation, whilst the less exposed tissue on the under- 
side of the leaf has distinctly green chlorophyll grains and shows 
a fairly active power of assimilation. 
In order to compare the effect of prolonged exposure upon the 
subsequent powers of respiration of the plant with that upon 
assimilation, a few experiments, the results of which are given 
on pp. 392-393 were performed. The CO, given off is absorbed 
by KHO, the decrease in the volume of air contained in the 
receiver giving approximately the amount of CO, evolved. The 
readings are taken at the same temperature and pressure through- 
out, and the experiments are continued until the vitality of the 
specimens begins to diminish and the amount of CO, evolved to 
decrease. 
