Sunlight upon Aquatic Plants . 369 
tinuance of these vital activities that the production of heat 
depends. Thus cell-division ceases to take place at zero 
or a few degrees above it 1 ; C 0 2 -assimilation ceases in 
tropical plants between 4 0 C. and 8° C., in warm, temperate, 
sub-tropical and water-plants between o° C. and 2 0 C., while 
in cool, temperate, arctic and alpine plants assimilation only- 
ceases when the plants are frozen, i. e., at a few degrees 
below zero 2 . Similarly rotation gradually diminishes as 
the temperature falls below the optimum, and after remain- 
ing for a prolonged period at a temperature at or near 
zero may cease, temporarily or permanently 3 , while faint 
respiration may go on even below zero until a tempera- 
ture of — io°C. is reached 4 . It is evident that even when 
a plant is surrounded by a medium the temperature of 
which is several degrees below zero, since faint respiration 
may, in very resistant plants (many Conifers, Mosses, Lichens, 
Schizophyta, and unicellular Algae), still continue, the interior 
of a morphological or accidental cell-aggregate may be at 
a slightly higher temperature, provided the loss of heat 
by conduction, radiation, or transpiration is slight or absent. 
Though a trifling, this is by no means a negligible quantity, 
and may, since the production of heat goes on continually, 
be one of considerable importance, when all transpiration 
is prevented and the plant is frozen in ice and surrounded 
by an insulating jacket of air, or is covered by snow. Fresh- 
water plants, especially if massed together, are frequently 
found to be covered by bubbles of gas when the water in 
which they lie freezes. Moreover, the water around the 
plant may be melted or absorbed so that the bubbles 
coalesce, and the plant or portions of it come to lie in 
insulating air chambers, which form an admirable protection 
from the outside cold. In such a chamber, when exposed 
to ordinary light, the heat-rays which penetrate the ice will 
1 Strasburger, Zellbildung und Zelltheilung, 1880, p. 171. 
2 Ewart, Assim. Inhib., Journ. Linn. Soc. Bot., Vol. xxxi, p. 401. 
3 Cf. Ewart, 1 . c., p. 394. 
4 Jumelle, Rev. Gen. de Bot., Vol. iv, 1892, p. 114 seq. 
