442 MR. A. J. EWART ON ASSIMILATORY INHIBITION. 
lation is not yet fully active and the tissues and cuticle thin and 
transparent, and in leaves several months old which have passed 
the zenith of their vitality, a temporary stoppage of assimilation 
is much more readily produced by a full day’s insolation, and the 
leaves may become paler or even yellowish. In normal adult 
leaves it is best produced by removing all but one or two leaves 
from a terminal leafy branch, and keeping the surfaces of these 
perpendicular to the sun’s rays for an entire unclouded day. 
The leaves remain fresh and green, but assimilation is either 
weak or absent. After 2-3 hours in weak diffuse light, a faint 
power of assimilation may be shown in the latter, but generally 
still none. Next day the separated leaves, kept under a bell-jar,. 
with the petioles in water, show active powers of assimilation. 
Quercus pedunculata, Tilia europea, and Prunus avium, of 
temperate plants, and Griselinia littoralis, Bambusa gracilis, and 
Euonymus fimbriata, of warm temperate and tropical plants,. 
gave similar results. In exposed cut leaves, with their petioles 
in water, the stoppage is more readily produced; but here the 
conditions are abnormal, and the leaf may be more or less wilted. 
after a full day’s insolation. Also, even when the leaf remains 
fresh and green, assimilation may be permanently affected, only 
a weak evolution of oxygen being shown on recovery. In 
exposed adult leaves, taken from a variety of plants after a full 
day’s sunlight, assimilation was always present though often. 
weak. In nature leaves are exposed to the full force of the 
sun’s perpendicular rays only for a short time daily. A very 
slight protection is in most cases sufficient to prevent the stoppage 
of assimilation, such as a slight clouding over of the sun, con- 
tinuously or at intervals, or covering the leaf with a film of 
cigarette-paper, or allowing the edge of the leaf to be turned to- 
the sun. Hence in nature a stoppage of assimilation as a direct 
effect of sunlight is of very rare occurrence in actively assimi- 
lating leaves, though a weakening must often be produced. 
The stoppage of assimilation is not necessarily associated with 
any visible change in either the chlorophyll or the plasma. 
When green leaves are exposed to sunlight the decomposition of 
chlorophyll goes on more rapidly than its production, though the 
amount of chlorophyll decomposed is insufficient to cause a 
change in coloration visible to the naked eye. Keeble * has 
* Keeble, ‘‘ Hanging Foliage of certain Tropical Trees,” in Annals of Botany, 
Yol. ix. 1895, pp. 59-93. 
