larger
size and thinner walls, which are thickened only in certain parts so as
to have peculiar markings, and which often are seen to be made up of a
row of cylindrical cells, with the partitions between absorbed or broken
away, are called DUCTS, or sometimes VESSELS. There are all gradations
between wood-cells and ducts, and between both these and common cells.
But in most plants the three kinds are fairly distinct.

[Illustration: Fig. 444. Magnified wood-cells of the bark (bast-cells)
of Basswood, one and part of another. 445. Some wood cells from the wood
(and below part of a duct); and 446, a detached wood-cell of the same;
equally magnified.]

[Illustration: Fig. 447. Some wood cells from Buttonwood, Platanus,
highly magnified, a whole cell and lower end of another on the left; a
cell cut half away lengthwise, and half of another on the right; some
pores or pits (_a_) seen on the left; while _b b_ mark sections through
these on the cut surface. When living and young the protoplasm extends
into these and by minuter perforations connects across them. In age the
pits become open passages, facilitating the passage of sap and air.]

409. The proper cellular tissue, or _parenchyma_, is the ground-work of
root, stem, and leaves; this is traversed, chiefly lengthwise, by the
strengthening and conducting tissue, wood-cells and duct-cells, in the
form of bundles or threads, which, in the stems and stalks of herbs are
fewer and comparatively scattered, but in shrubs and trees so numerous
and crowded that in the stems and all permanent parts they make a solid
mass of wood. They extend into and ramify in the leaves, spreading out
in a horizontal plane, as the framework of ribs and veins, which
supports the softer cellular portion or parenchyma.

410. =Wood-Cells, or Woody Fibres=, consist of tubes, commonly between
one and two thousandths, but in Pine-wood sometimes two or three
hundredths, of an inch in diameter. Those from the tough bark of the
Basswood, shown in Fig. 444, are only the fifteen-hundredth of an inch
wide. Those of Buttonwood (Fig. 447) are larger, and are here highly
magnified besides. The figures show the way wood-cells are commonly put
together, namely, with their tapering ends overlapping each
other,--spliced together, as it were,--thus giving more strength and
toughness. In hard woods, such as Hickory and Oak, the walls of these
tubes are very thick, as well as dense; while in soft woods, such as
White