226 VEGETABLE PHYSIOLOGY 
just underneath the surface, which constitutes what is 
called the corter, and which gives place later on to the 
complex formation that is familiar to us under the name 
of bark. 
The formation of the successive rings of cork deeper 
and deeper in the cortex, which ultimately constitute the 
bark, is attended by the same need of a continuous instead 
Fic. 106.—Srcrion or THREE-YEAR-OLD Stem or Tilia, SHOWING THE 
MEDULLARY Rays RUNNING THROUGH THE woop. x 50. (After Kny.) 
of an intermittent supply of food. We find, therefore, 
during the process of the construction of the bark, similar 
provision of food-containing tissue, which is situated near 
the cork layers. In some cases it takes the form of regular 
sheaths; in others the food is irregularly distributed through 
the cortex, which is the seat of the appearance of the for- 
mative layers of the cork. 
