360 
F. F. Blackman. 
can be protected by intake of sugar. The last named material after 
18 hours in 5 % levulose survived four hours, frozen at —2°C, while 
roots without sugar were all killed. 
Water plants fall into two sharply marked classes in this 
connection. Elodea, Cliara, and Stratiotes, which in winter vegetate 
at the bottom of streams at a temperature just above zero, contain 
starch but no sugar and are killed on exposure to —2 ft C or —4"C. 
In contrast to these we find at the edge of the stream the submerged 
wintering stumps of the plants which vegetated above the water in 
the summer, such as Menyanthes, Ranunculus lingua, Myosotis, Sium. 
These have all converted their starch to sugar for the winter and in 
correlation it is found that a temperature of —7 n C is required to 
kill them. 
Another phenomenon which seems to be explained by Lidforss’ 
work is the puzzling fact that many a plant which has survived the 
profound and prolonged cold of winter may be killed by a sharp 
night-frost in early spring (long before, of course, it has started 
any new growth). This is especially common when the night-frost 
is preceded by a spell of bright sunny radiation. The accepted 
explanation is that it is the quick change of temperature from 
warm day to cold night that injures, but all experimenters agree 
now that the rate of cooling or thawing makes very little difference. 
Lidforss finds that a succession of warm days in spring causes the 
regeneration of starch and holds that it is the disappearance of the 
protective sugar that makes the plant more susceptible. This is 
borne out by his observations that it is the well-sunned south side 
of trees like Ilex and Taxus that suffer in such weather and that 
here the sugar has gone, while the shaded north side of the tree 
still keeps its sugar and is uninjured by the spring frost. 
In Holosteum umhellatum a warm January day with a tem¬ 
perature of +5°C starts the regeneration of starch and if a cold 
spell follows, it may revert to sugar. 
Warming up a leaf in a greenhouse for a week when, as some¬ 
times happens in the early winter, it can be done without an 
appreciable fall in the sugar content, does not make the leaf more 
susceptible to cold, so that clearly it is not the direct effect of 
warmth that matters. 
In South Sweden the starch of vegetation gives place to sugar, 
rather quickly, in November, and remains thus till early or middle 
April when the reverse change takes place, but there the winter is 
more prolonged than in our climate, 
