zm 
DR. A. J. EWART ON ASSIMILATORY INHIBITION. 569 
a quite distinct and from weak to moderately active power of 
assimilation is shown. In the older basal parts of the stem in 
both cortex and pith the chlorophyll grains are more or less 
undifferentiated, the cells in some cases containing irregularly 
shaped grains or almost completely diffuse green contents, and 
in others normal round pale-green grains. The amount of 
chlorophyll is less relatively to the size of the cells in the older 
than in the younger stems. In the outer cortex no evolution of 
oxygen is shown; but in the inner cortex and pith a very weak 
but distinct evolution is shown, whether the chlorophyll grains 
are more or less diffuse, or are definite *. If such stems are now 
exposed to light in ordinary air and examined after a week, in 
the cortex the chloroplastids are green, fairly large, normal, 
round, may contain starch grains and show a fairly active power 
of assimilation ; in the pith the chlorophyll grains are fewer but 
normal, round, green, and show a rather weak evolution of 
oxygen, which is, however, normal for the relative amount of 
chlorophyll present. 
The reason why in the above cases the power of assimilation 
disappears in the cortex but remains in the pith is because in 
the latter case traces of CO, derived from the respiration of the 
surrounding parts, and perhaps also directly through the stem 
from the roots along with the transpiration-current, are present 
and can be assimilated ; whereas in the outex cortex this is not 
the case, and the chlorophyll grains, exposed to light but unable 
to assimilate owing to the absence of all free CO,, fall ill and 
lose, at first temporarily but finally permanently, the power of 
assimilation. 
That the constant presence of the merest trace of CO, is 
sufficient to prevent the loss of the power of assimilation from 
taking place, is shown by the following experiment. A pot con- 
taining a large potato with a number of etiolated shoots is placed 
in a hermetically sealed bell-jar along with several vessels of 
KHO solution placed at different levels in the bell-jar, so that 
only a very small trace of the CO, evolved from the tuber and 
roots escapes absorption and reaches the leaves. Nevertheless, 
* Diffuse chlorophyll in cells capable of assimilation, though rare in Pha- 
nerogams, is present normally in parts of Cuscuta and in epidermal hairs of 
Orobanche. The above is, however, interesting as being a case in which a cell 
which normally has definite chlorophyll grains may have diffuse chlorophyll 
and yet be capable of assimilation. 
