MR. A. J. EWART ON ASSIMILATORY INHIBITION. 889 
Cold and Assimilation. 
The lowest temperature at which determinations can be made 
by the Bacterium method is just above zero at about 1°C. In 
all cases, unless otherwise mentioned, the frozen plants are thawed 
and examined at this temperature and then brought to a tem- 
perature of from 15° to 20°C. The following plants taken during 
January after 3 weeks continuous frost, the temperature often 
falling below —15° C., showed when examined no power of 
assimilation, though most of the leaves remained living :—Zlex 
Aquifolium, Buxus sempervirens var. arborescens, Pinus montana, 
Taxus baccata, Thuja occidentalis, Juniperus Sabina, Cheiranthus 
Cheiri, Ulex europeus, Cytisus austriacus. After 2 to 4 hours 
at 15? C. weak assimilation, in parts moderately active, is 
shown, and in from 8 hours to 1 day assimilation is quite active. 
A long latent period of recovery indicates that the exposure has 
very nearly reached the plant's limit of resistance. 
Many herbaceous plants are able, if covered by snow, to retain 
last year's leaves alive and green during the entire winter. A 
series of such plants were examined in the middle of winter after 
having been covered by from 1 to 2 feet of snow for 4 weeks, when 
in all eases at first no assimilation was shown, weak to moderately 
active after 3-5 hours at 15° C., and quite active in 1 day :—Pri- 
mula marginata, Helleborus lividus and H. guttatus, Poa pratensis, 
Catharinea undulata, Ceratodon purpureum, Hypnum triquetrum, 
Secale cereale, Triticum sativum, Opuntia Rafinesquii. Certain 
herbaceous plants are able to withstand prolonged and severe 
exposure without being markedly affected. Thus the leaves of 
flowering plants of Crocus, after being kept frozen for a week at 
from —2? C. to —5? C., showed on examination fairly active 
assimilation. After further exposure for 3 days to —5? C. to 
—12° C. they remained living, showed at first no assimilation, 
but after 1-2 hours at 15^ C. weak, in 3 hours fairly, and in 
5 hours quite active assimilation. Similarly, plauts of Semper- 
vivum Braunii, after being exposed for 3 weeks to a temperature 
varying from —5? C. to —15? C. were living but showed no 
assimilation, none after 3 hours at 15? C., faint in 5 hours, and 
fairly active in 1 day. 
The stoppage of assimilation caused by exposure to cold is not 
necessarily accompanied by any visible change in either ehloro- 
phyll or protoplasm, though changes may in certain cases be 
