much-shortened branches and under favorable circumstances
give rise to new stems. They have a hard, brown rind, and are composed
within mainly of a firm, white tissue, filled with starch.

The surface of the stem is marked with furrows, and a section across
it shows that corresponding to these are as many large air spaces that
traverse the stem from joint to joint. From the joints numerous roots,
quite like those of the ferns, arise.

If the stem is dug up in the late fall or winter, numerous short
branches of a lighter color will be found growing from the joints.
These later grow up above ground into branches of two sorts. Those
produced first (Fig. 72, _A_), in April or May, are stouter than the
others, and nearly destitute of chlorophyll. They are usually twenty
to thirty centimetres in height, of a light reddish brown color, and,
like all the stems, distinctly jointed. The sheaths about the joints
(_L_) are much larger than in the others, and have from ten to twelve
large black teeth at the top. These sheaths are the leaves. At the top
of the branch the joints are very close together, and the leaves of
different form, and closely set so as to form a compact cone (_x_).

A cross-section of the stem (_D_) shows much the same structure as the
underground stem, but the number of air spaces is larger, and in
addition there is a large central cavity. The fibro-vascular bundles
(_f.b._) are arranged in a circle, alternating with the air channels,
and each one has running through it a small air passage.

The cone at the top of the branch is made up of closely set,
shield-shaped leaves, which are mostly six-sided, on account of the
pressure. These leaves (_F_, _G_) have short stalks, and are arranged
in circles about the stem. Each one has a number of spore cases
hanging down from the edge, and opening by a cleft on the inner side
(_G_, _sp._). They are filled with a mass of greenish spores that
shake out at the slightest jar when ripe.

The sterile branches (_B_) are more slender than the spore-bearing
ones, and the sheaths shorter. Surrounding the joints, apparently just
below the sheaths, but really breaking through their bases, are
circles of slender branches resembling the main branch, but more
slender. The sterile branches grow to a height of forty to fifty
centimetres, and from their bushy form the popular name of the plant,
"horse-tail," is taken. The surface of the plant is hard and rough,
due to the presence