276 HOW CROPS GROW. 
and deep longitudinal rifts, and it gradually decays or 
drops away exteriorly as the newer bark forms within. 
Cork is one form which the epidermal cells assume on 
the stem of the cork oak, on the potato tuber, and many 
other plants. 
Pith Rays. — Those portions of the first-formed cell- 
tissue which were interposed between the young and orig- 
inally ununited wood-fibers remain, and connect the pith 
with the rind. In hard stems 
they become flattened by the 
pressure of the fibers, and are 
readily seen in most kinds of wood 
when split lengthwise. They are 
especially conspicuous in the oak 
and maple, and form what is com- 
monly known as the silver-grain. 
The botanist terms them pith-rays 
or medullary rays. 
Fig. 51 exhibits a section of a 
bit of wood of the Red Pine, 
(Pinus picea,) magnified 200 di- 
SS 
ON 
— 
~=eveeeee> 
4 
ameters. The section is made 
tangential to the stem and length- 
Y\\| wise of the wood-cells, four of 
; which are in part represented, A ; 
see it cuts across the pith-rays, whose 
cell-structure and position in the wood are seen at m, n. 
Cambium of Exogens.—The growing part of the exog- 
enous stem is thus found between the wood and the bark, 
or rather between the fully formed wood and the mature 
bark. There is, in fact, no definite limit where wood ceases 
and bark begins, for they are connected by the cambial or 
formative tissue, from which, on the one hand, wood-fibers, 
and on the other, bast-fibers, or the tissues of the bark, 
rapidly develope. In the cambium, likewise, the pith-rays 
