40 THE FLOWERING PLANT. 
out a little thought on the part of the student, and examination 
of actual specimens. 
We may now pass to the consideration of bark. A twig can 
generally be “‘ peeled” with ease, and this is owing to the delicate 
nature of the cambium ring, which is broken through in the 
process of peeling. As the wood receives annual additions on its 
outside from the cambium, so does the bast receive similar addi- 
tions, the difference being that they are very much thinner and 
formed on the inside. There is never such a regular appearance 
of annual rings as in the wood (fig. 7, K). It is evident that the 
method of increase described must subject the external tissues to 
a great deal of tension. The epidermis may for some time keep 
up with the growth in thickness, but sooner or later, in most 
cases, it bursts and is thrown off. Its place is taken by brown 
_ layers of cork cells formed in the following way. A layer of cells 
in the cortex, ze, outer part of the ground-tissue, begins to 
divide’ actively and forms a layer of meristem, known as cork 
cambium (fig. 7, I). Its cells, however, are not elongated like 
those of ordinary cambium. The cells formed on the outside by 
division of this layer become cork elements. These, as seen in 
cross-section, are rectangular in shape and arranged in radial 
rows (cf. p. 31). Their walls are thickened, brown, and com- 
posed of a substance, suberin, probably allied to cutin (p. 30). 
‘They contain no protoplasm. An examination of elder-shoots 
of different age will illustrate these points. The youngest are 
bright green, since the chlorophyll-bearing cells of the cortex 
can be seen through the transparent epidermis, but older ones 
are brown, owing to the formation of cork. This is not very 
transparent, and so does not permit the green cortical tissue to 
be clearly seen from the outside. A cross-section will show the 
relation of the different parts. If this is made through a branch 
about half an inck thick, the following points can be easily made 
out by the eye alone. A large part of the interior will be occu- 
pied by the bright, white, spongy pith, the colour of which is due 
to the fact that the cells are dead and contain air instead of pro- 
toplasm. Then follows a whitish-brown band of wood, in which 
medullary rays are plainly seen. The cambium forms an ex- 
tremely narrow band, the position of which is shown by the fact 
that the outer part of the stem can be peeled off at this point. 
Next comes a narrow greenish-white zone, the bast, and this is 
succeeded by a narrower dark-green portion, the chlorophyll- 
bearing cortex. This is invested by. a thin papery layer of 
whitish-brown cork, which can easily be detached, owing to the 
delicate nature of the cork cambium. That it is subjected to 
tension may be seen by the presence of numerous little fissures, 
