astoris_) (Figs. 93-95).

The plant grows abundantly in waste places, and is in flower nearly
the year round, sometimes being found in flower in midwinter, after a
week or two of warm weather. It is, however, in best condition for
study in the spring and early summer. The plant may at once be
recognized by the heart-shaped pods and small, white, four-petaled
flowers. The plant begins to flower when very small, but continues to
grow until it forms a much-branching plant, half a metre or more in
height. On pulling up the plant, a large tap-root (Fig. 93, _A_, _r_)
is seen, continuous with the main stem above ground. The first root of
the seedling plant continues here as the main root of the plant, as
was the case with the gymnosperms, but not with the monocotyledons.
From this tap-root other small ones branch off, and these divide
repeatedly, forming a complex root system. The main root is very tough
and hard, owing to the formation of woody tissue in it. A
cross-section slightly magnified (Fig. 93, _M_), shows a round,
opaque, white, central area (_x_), the wood, surrounded by a more
transparent, irregular ring (_ph._), the phloem or bast; and outside
of this is the ground tissue and epidermis.

The lower leaves are crowded into a rosette, and are larger than those
higher up, from which they differ also in having a stalk (petiole),
while the upper leaves are sessile. The outline of the leaves varies
much in different plants and in different parts of the same plant,
being sometimes almost entire, sometimes divided into lobes almost to
the midrib, and between these extremes all gradations are found. The
larger leaves are traversed by a strong midrib projecting strongly on
the lower side of the leaf, and from this the smaller veins branch.
The upper leaves have frequently two smaller veins starting from the
base of the leaf, and nearly parallel with the midrib (_C_ i). The
surface of the leaves is somewhat roughened with hairs, some of which,
if slightly magnified, look like little white stars.

Magnifying slightly a thin cross-section of the stem, it shows a
central, ground tissue (pith), whose cells are large enough to be seen
even when very slightly enlarged. Surrounding this is a ring of
fibro-vascular bundles (_L_, _fb._), appearing white and opaque, and
connected by a more transparent tissue. Outside of the ring of
fibro-vascular bundles is the green ground tissue and epidermis.
Comparing this with the section of t