THE LEAP 187 
plished is through the growth of a corky layer of loose 
cells that forms at the base of the petiole and cuts it away 
from the stem, leaving a smooth, clean scar. Tear some 
fresh young leaves from a growing twig and compare the 
scars with those on a winter bough. Do you see any 
difference? This corky layer can be made to form in 
some plants artificially, by depriving them of working ma- 
terial. (Exp. 75.) 
204. The protection of winter-green leaves. — A great 
many, perhaps the majority of broad-leaved evergreens, 
bear no obvious protection against cold, while a large pro- 
portion, such as chickweed, violet, fumitory, groundsel 
(Senecio), and dead nettle (Lamium), would seem peculiarly 
unfitted, by their delicate structure, to withstand it. But 
recent investigations by the Swedish botanist, Lidforss, 
have shown that all winter-green leaves, with the exception 
of those on submerged water plants, which are sufficiently 
protected by the medium in which they live, lose their 
starch in winter and contain instead an increased percentage 
of sugar. The same is true of other vegetable structures 
also, where starch is present, such as roots, stems, tubers, 
and winter fruits—nuts, haws, persimmons, and the like, 
which, as every schoolboy knows, become perceptibly sweeter 
after frost. 
The presence of certain substances, of which sugar is the 
most frequent, enables plants to withstand a greater degree 
of cold than they could otherwise endure (Exp. 76). This 
effect, as shown by Lidforss’s experiments, is due to the 
action of sugar in counteracting, or retarding, the “ salting 
out ” of proteins by cold, as explained in 33. 
As sugar is readily reconverted into starch by exposure to 
a moderately high temperature for even a few days, we may 
find here an explanation of the fact that plants which have 
survived the prolonged cold of winter are often killed by a 
single sharp night frost following a few warm days in early 
spring, before the tender new growth has appeared. The 
