MR. A. J. EWART ON ASSIMILATORY INHIBITION. 441 
| Pyrus Tor- Leaves exposed, attached to the parent plant, to full sunlight | 
| ringo; till 4 P.M. show no assimilation though living, green, and | 
ZEsculus with normal chlorophyll grains. After 2 h. in diffuse | 
flava ; light still no assimilation, but next day the same prepara- | 
| Ficus tions show, if living, a weak but distinct power of assimi- | 
elastica. lation, and fresh preparations made from the same leaves | 
| now show a fairly active power of assimilation. 
That the stoppage of assimilation is a direct effect of the 
exposure to sunlight, and is not due to any accumulation of 
carbohydrates, is shown by the following facts:—(1) On a leaf 
exposed to bright diffuse daylight no perceptible inhibitory 
influence is produced, though the actual amount of assimilation 
must be as great as, if not greater than, in a leaf exposed to full 
sunlight. (2) The recovery and return of assimilation after 
insolation will take place in diffuse daylight, and this although 
the leaf has been separated from the parent plant. (3) After 
recovery assimilation may remain fairly active for several days 
in separated leaves kept exposed to diffuse daylight, whereas if 
the stoppage were due to an excessive accumulation of carbo- 
hydrates it should only be weak and should soon cease again; 
i. e. as soon as the amount of sugar &c. removed by respiration 
during the night is replaced by the renewed assimilation. Nor 
is the stoppage the result of the heating effect of the sun's rays, 
for in Pyrus and ZEsculus transpiration suffices to keep the leaf 
cool during the exposure, and though the thick leaves of Ficus 
may be slightly warmed by the sun's rays, the rise in temperature 
is only slight and does not approach anywhere near the critical 
point. 
Ulmus scabra var. pyramidalis, Acer tataricum, and Fagus 
sylvatica var. atropurpurea seem to be more resistant than the 
plants previously mentioned. Attached leaves exposed during 
a cloudless day to full sunlight till 5 p.m. show a quite distinct 
but weakened power of assimilation, which is after a few hours' 
exposure to diffuse light, or, in some cases, not till the next day, 
quite active. The retention of the power of assimilation in such 
leaves may be due to the fact that for a considerable portion of 
the day they have their edges to the sun or are exposed only to 
oblique illumination. Hence it is seldom that under normal 
conditions, except in shade-loving plants, that even an entire 
day’s exposure to full sunlight causes anything more than a 
weakening of assimilation, In young leaves, in which assimi- 
