‘390 MR. A. J. EWART ON ASSIMILATOKY INHIBITION. 
produced in the colour of the chlorophyll grains by prolonged 
and severe exposure. This brown or yellow coloration commonly 
shown by Conifers, Buxus, &c. during winter has been most 
thoroughly investigated by Schimper, who finds that the brown 
coloration is due to a modification of the chlorophyll pigment 
-caused by the combined effect of exposure to cold and light. In 
certain cases a yellow coloration is produced, the chlorophyll 
grains being quite pale or faintly yellowish. This, according to 
Haberlandt, is due to the light decomposing the chlorophyll and 
the low temperature preventing its re-formation. Schimper finds 
that the change of colour is due solely to a modification of the 
‘chlorophyll pigment, and is not accompanied by any destruction 
or disintegration of the chlorophyll bodies. ^ Mechanical 
influences, pressure, section-cutting, or intense frost may cause 
the chlorophyll grains to disintegrate, but all such cells soon die 
and cease to be plasmolysable. One result of prolonged severe 
‘exposure to cold is to make the chlorophyll grains very liable to 
be disrupted and disintegrated. A chlorophyllous cell, which 
has been browned or turned yellow by the action of cold, can 
only become green again if the chlorophyll grains remain intact. 
Browned specimens of Buzus, Juniperus, Taxus, Abies, and 
Biota became green again after being kept in a greenhouse at 
10° C. to 15° C. for from 3 to 5 weeks. 
None of the browned parts showed any perceptible power of 
assimilation even after being for several days in a greenhouse, 
but auy parts with green and normal chlorophyll grains showed 
in from a few hours to a day a distinet power of assimilation. 
In deeply-browned specimens of Juniperus communis a weak 
evolution of oxygen can be detected as soon as the leaves begin 
to turn green, and a fairly active evolution of oxygen may be 
shown in leaves in which a slight trace of the brown colour is 
still perceptible. Pale yellow branches of Thuja orientalis begin 
to turn green in 3 weeks. After 4 weeks plenty of branches are 
still quite pale yellow and show no evolution of oxygen, but 
others though pale are distinctly green and show a weak power 
of assimilation. After 5 weeks fairly active assimilation is shown 
by leaves which are still slightly yellowish, and the chlorophyll 
grains, though green, paler than normal. 
If the exposure inducing browning be too severe and prolonged 
the power of recovery is lost, but otherwise, provided the 
