8 



INTRODUCTION 



longer than the other, so that only one of a pair is cut 

 across in a transverse section of some cushions, 



ImniecHately 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- stages of thickening, 



bium; co., cork; i.e., inner cortex; f.r., rpi i j: i • a 



foliar resin duct ; m., medulla or pith ; m.r., Ineleat-CUSlnonsgraclU- 



medullarjr ray; o.c, outer cortex; p., ally disintegrate and fall 



phloem; r.c, resin cyst; r.d., resin duct ^^ ^^^ ^^^ ^^^^^jj .^^_ 

 ni wood ; w., wood. ' -^ 



distinguishable after the 

 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 



