8 INTRODUCTION 
longer than the other, so that only one of a pair is cut 
across in a transverse section of some cushions. 
Immediately inside the cushions is a continuous cork 
layer, which kills the outer cortex and epidermis, and 
inside this again is the inner cortex composed of live cells, 
with intercellular spaces, somewhat thick cellulose walls, 
and living contents with chloroplasts. A narrow continuous 
layer of phloem surrounds the wood cylinder, and imme- 
diately outside the 
phloem are a few nearly 
‘spherical resin cysts, 
which look like resin 
ducts in section, but are 
not extended longitudi- 
nally. The wood cylin- 
der may be 4 or 5 mm.in 
breadth, and contains 
a few resin ducts; and 
in the centre is a star- 
shaped medulla. 
Sections of older 
Fig. 2.—Diagram of a transverse section stems show progressive 
of a one-year-old stem of larch. ca.,cam- gtages of thickening. 
bium; co., cork; i.c., inner cortex; /.r., i 
foliar resin duct; »., medulla or pith ; m.r., Theleaf-cushionsgradu- 
medullary ray; 0.¢., outer cortex; ., ally disintegrate and fall 
phloem; 7.c., resin cyst; 7.d., resin duct: . 
Ewe 4 an. woes off, and are usually in- 
distinguishable afterthe 
fifth year. New cork layers are formed immediately under 
the first one. These layers are not always complete, and, 
in the second year especially, partial layers are frequently 
formed stretching under the furrows. In general a new 
cork layer is formed each year. The inner cortex remains 
living for an indefinite time, but loses its chlorophyll after 
a while. The phloem becomes thicker, and the outer first- 
made elements are so stretched tangentially as to leave 
numerous large intercellular spaces. A fresh ring of wood 
is put on each year. 
Detailed structure of tissues : epidermis. The cells measure 
