In Winter Woods 387 
times the cell walls are very thick, sometimes 
they are thin, and sometimes they are pitted or 
barred or ringed so that under the microscope 
they show patterns of great beauty. 
A tiny sliver of wood may be made up of many 
kinds of cells, which are alike only in one respect 
—that the tree has had a use for them all. 
Toward the centre of the trunk and larger 
branches lie the oldest cells, whose work is nearly 
or entirely done. 
They form the ‘‘ heart-wood’’ which, as every 
cabinetmaker knows, is darker in color and closer 
in texture than the younger ‘‘ sap-wood’’ which 
surrounds it. 
Outside the ‘‘sap-wood’’ there is in spring a 
layer of young growing cells which are building up 
new bark and new wood. In April we shall find 
this forming tissue lying just below the bark, be- 
tween it and the wood. 
But at this season no active growth is going 
forward, and no delicate new cells are forming 
and swelling between the tree and its bark. 
In the bark itself, at varying depths according 
to the kind of tree, lie several, layers of cork-cells. 
In summer this cork undergarment covers all 
the tree except the tops of its tenderest twigs and 
