THE WINTER LIFE OF PLANTS. 
207 
flowering-plants manifest no active life-processes during- their 
winter’s slumber, one process is as absolutely necessary to their 
preservation as to their growth — there must be an interchange 
of matter between the air and the body of the plant. This 
exchange may be small, but it is more than probable that 
it to some extent exists; for it is a law of plant-life that it 
can only exist so long as evaporation and breathing are carried 
on. In winter the inside of a tree, examined with a thermo- 
meter, shows a higher temperature than that of the sur- 
rounding atmosphere, and this warmth is a proof of vital 
changes. Although in the development of most deciduous- 
leaved trees and perennial herbaceous plants, winter is a state 
of rest, yet they retain their vital properties, and it is no more 
a state of absolute inactivity than the healthy sleep of the 
animal body, with which this winter’s repose of vegetation may 
be very properly compared. 
Even those trees which are apparently devoid of foliage, still 
possess leaves. We refer to the scales or covering leaves 
which constitute the outer envelopes of the buds. These are 
the winter leaves of trees, and each tree has them quite as 
characteristic of its species as its summer leaves. That the 
scales of buds are only modified leaves, every botanist will 
readily allow. In many plants the transition of the bud-scales 
into ordinary green leaves is well marked ; and that they are 
varieties only of the same common leaf-forms, is also indicated by 
their spiral arrangement, and by the fact that they are alternate 
or opposite in position about their rudimentary axis, exactly as 
the leaves may happen to be arranged on the fully developed 
shoot. These bud-scales or covering-leaves must be regarded 
as the very lowest type of leaf. They begin to form in spring, 
and continue growing through the whole of the summer. 
During the active period of vegetation the current of sap is 
diverted away from them by the summer leaves, so that they 
cannot make much progress. In autumn the sap gradually 
ceases its flow towards the summer leaves, and its last move- 
ment, before it stagnates in the tissues, is probably to these 
winter leaves or bud-scales ; for it is then that the buds are 
matured, just before the trees shed their summer leaves in 
autumn. Hence physiologists have very properly designated 
as autumnal sap this peculiar flow of the stagnating nutritive 
current. The winter leaves being formed under such circum- 
stances, are necessarily circumscribed to the smallest space, and 
as they do not take the conspicuous forms of the summer 
leaves, they escape vulgar observation. 
These winter leaves of trees, so apparently insignificant, will, 
however, richly repay investigation. The tree requires protec- 
tion, not nutrition, in winter, and these leaves are organized 
