ened
rings or with pits in the walls. The small, spirally-marked vessels
are nearest the centre, and are the first to be formed in the young
bundle.

The epidermis of the leaves is composed of irregular cells with wavy
outlines like those of the ferns. Breathing pores, of the same type
as those in the ferns and monocotyledons, are found on both
surfaces, but more abundant and more perfectly developed on the
lower surface of the leaf. Owing to their small size they are not
specially favorable for study. The epidermis is sparingly covered
with unicellular hairs, some of which are curiously branched, being
irregularly star-shaped. The walls of these cells are very thick,
and have little protuberances upon the outer surface (Fig. 93, _E_).

Cross-sections of the leaf may be made between pith as already
directed; or, by folding the leaf carefully several times, the whole
can be easily sectioned. The structure is essentially as in the
adder-tongue, but the epidermal cells appear more irregular, and the
fibro-vascular bundles are better developed. They are like those of
the stem, but somewhat simpler. The xylem lies on the upper side.

The ground tissue is composed, as in the leaves we have studied, of
chlorophyll-bearing, loose cells, rather more compact upon the upper
side. (In the majority of dicotyledons the upper surface of the
leaves is nearly or quite destitute of breathing pores, and the
cells of the ground tissue below the upper epidermis are closely
packed, forming what is called the "palisade-parenchyma" of the
leaf.)

[Illustration: FIG. 95.--_A-D_, successive stages in the development
of the flower of _Capsella_, x 50. _A_, surface view. _B-D_, optical
sections. _s_, sepals, _p_, petals. _an._ stamens. _gy._ pistil. _E_,
cross-section of the young anther, x 180. _sp._ spore mother cells.
_F_, cross-section of full-grown anther. _sp._ pollen spores, x 50.
_F'_, four young pollen spores, x 300. _F"_, pollen spores germinating
upon the stigma, x 300. _pt._ pollen tube. _G_, young pistil in
optical section, x 25. H, cross-section of a somewhat older one. _ov._
ovules. _I-L_, development of the ovule. _sp._ embryo sac
(macrospore). _I-K_, x 150. _L_, x 50. _M_, embryo sac of a full-grown
ovule, x 150. _Sy._ _Synergidae_. _o_, egg cell. _n_, endosperm
nucleus. _ant._ antipodal cells. _N-Q_, development of the embryo,
x 150. _sus._ suspensor.]

The shepherd's-pur