it occupies the whole cavity of the seed, the young
stalk (_st._) being bent down against the back of one of the
cotyledons (_f_).

[Illustration: FIG. 94.--_A_, cross-section of the stem of the
shepherd's-purse, including a fibro-vascular bundle, x 150. _ep._
epidermis. _m_, ground tissue. _sh._ bundle sheath. _ph._ phloem.
_xy._ xylem. _tr._ a vessel. _B_, a young root seen in optical
section, x 150. _r_, root cap. _d_, young epidermis. _pb._ ground.
_pl._ young fibro-vascular bundle. _C_ cross section of a small root,
x 150. _fb._ fibro-vascular bundle. _D_, epidermis from the lower side
of the leaf, x 150. _E_, a star-shaped hair from the surface of the
leaf, x 150. _F_, cross-section of a leaf, x 150. _ep._ epidermis.
_m_, ground tissue. _fb._ section of a vein.]

A microscopic examination of a cross-section of the older root shows
that the central portion is made up of radiating lines of
thick-walled cells (fibres) interspersed with lines of larger, round
openings (vessels). There is a ring of small cambium cells around
this merging into the phloem, which is composed of irregular cells,
with pretty thick, but soft walls. The ground tissue is composed of
large, loose cells, which in the older roots are often ruptured and
partly dried up. The epidermis is usually indistinguishable in the
older roots. To understand the early structure of the roots, the
smallest rootlets obtainable should be selected. The smallest are so
transparent that the tips may be mounted whole in water, and will
show very satisfactorily the arrangement of the young tissues. The
tissues do not here arise from a single, apical cell, as we found in
the pteridophytes, but from a group of cells (the shaded cells in
Fig. 94, _B_). The end of the root, as in the fern, is covered with
a root cap (_r_) composed of successive layers of cells cut off from
the growing point. The rest of the root shows the same division of
the tissues into the primary epidermis (dermatogen) (_d_), young
fibro-vascular cylinder (plerome) (_pl._), and young ground tissue
(periblem) (_pb._). The structure of the older portions of such
a root is not very easy to study, owing to difficulty in making
good cross-sections of so small an object. By using a very
sharp razor, and holding perfectly straight between pieces of pith,
however, satisfactory sections can be made. The cells contain so
much starch as to make them almost opaq