Fig. 475. Diagram of a cross section of a very young
exogenous stem, showing six woody bundles or wedges. 476. Same later,
with wedges increased to twelve. 477. Still later, the wedges filling
the space, separated only by the thin lines, or medullary rays, running
from pith to bark.]

429. When such a stem grows on from year to year, it adds annually a
layer of wood outside the preceding one, between that and the bark.
This is exogenous growth, or outside-growing, as the name denotes.

[Illustration: Fig. 478. Piece of a stem of Soft Maple, of a year old,
cut crosswise and lengthwise.]

[Illustration: Fig. 479. A portion of the same, magnified.]

[Illustration: Fig. 480. A small piece of the same, taken from one side,
reaching from the bark to the pith, and highly magnified: _a_, a small
bit of the pith; _b_, spiral ducts of what is called the _medullary
sheath_; _c_, the wood; _d_, _d_, dotted ducts in the wood; _e_, _e_,
annular ducts; _f_, the liber or inner bark; _g_, the green bark; _h_,
the corky layer; _i_, the skin, or epidermis; _j_, one of the medullary
rays, or plates of silver grain, seen on the cross-section.]

430. Some new bark is formed every year, as well as new wood, the former
inside, as the latter is outside of that of the year preceding. The ring
or zone of tender forming tissue between the bark and the wood has been
called the _Cambium Layer_. _Cambium_ is an old name of the
physiologists for nutritive juice. And this thin layer is so gorged with
rich nutritive sap when spring growth is renewed, that the bark then
seems to be loose from the wood and a layer of viscid sap (or _cambium_)
to be poured out between the two. But there is all the while a
connection of the bark and the wood by delicate cells, rapidly
multiplying and growing.

431. =The Bark= of a year-old stem consists of three parts, more or less
distinct, namely,--beginning next the wood,--

1. The LIBER or FIBROUS BARK, the _Inner Bark_. This contains some
wood-cells, or their equivalent, commonly in the form of bast or
bast-cells (411, Fig. 444), such as those of Basswood or Linden, and
among herbs those of flax and hemp, which are spun and woven or made
into cordage. It also contains cells which are named _sieve_-cells, on
account of numerous slits and pores in their walls, by which the
protoplasm of contiguous cells communicates. In woody stems, whenever a
new layer of wood is formed, some new liber or inner bark is also forme