where growth ceased at the end of the season, the cells becoming
smaller as growth was feebler. The following year when growth
commenced again, the first wood cells formed by the cambium were
much larger, as growth is most vigorous at this time, and the wood
formed of these larger cells is softer and lighter colored than that
formed of the smaller cells of the autumn growth.

The wood is mainly composed of tracheids, there being no vessels
formed except the first year. These tracheids are characterized by
the presence of peculiar pits upon their walls, best seen when thin
longitudinal sections are made in a radial direction. These pits
(Fig. 76, _D_, _p_) appear in this view as double circles, but if
cut across, as often happens in a cross-section of the stem, or in a
longitudinal section at right angles to the radius (tangential),
they are seen to be in shape something like an inverted saucer with
a hole through the bottom. They are formed in pairs, one on each
side of the wall of adjacent tracheids, and are separated by a very
delicate membrane (_F_, _p_, _G_, _y_). These "bordered" pits are
very characteristic of the wood of all conifers.

The structure of the root is best studied in the seedling plant, or
in a rootlet of an older one. The general plan of the root is much
like that of the pteridophytes. The fibro-vascular bundle (Fig. 75,
_M_, _fb._) is of the so-called radial type, there being three xylem
masses (_x_) alternating with as many phloem masses (_ph._) in the
root of the seedling. This regularity becomes destroyed as the root
grows older by the formation of a cambium ring, something like that
in the stem.

The development of the sporangia is on the whole much like that of
the club mosses, and will not be examined here in detail. The
microspores (pollen spores) are formed in groups of four in
precisely the same way as the spores of the bryophytes and
pteridophytes, and by collecting the male flowers as they begin to
appear in the spring, and crushing the sporangia in water, the
process of division may be seen. For more careful examination they
may be crushed in a mixture of water and acetic acid, to which is
added a little gentian violet. This mixture fixes and stains the
nuclei of the spores, and very instructive preparations may thus be
made.[11]

[11] See the last chapter for details.

[Illustration: FIG. 77.--Scotch pine