erry. Others are woody stemmed
plants, persisting from year to year, and often becoming great trees
that live for hundreds of years. Still others are climbing plants,
either twining their stems about the support, like the morning-glory,
hop, honeysuckle, and many others, or having special organs (tendrils)
by which they fasten themselves to the support. These tendrils
originate in different ways. Sometimes, as in the grape and Virginia
creeper, they are reduced branches, either coiling about the support,
or producing little suckers at their tips by which they cling to walls
or the trunks of trees. Other tendrils, as in the poison ivy and the
true ivy, are short roots that fasten themselves firmly in the
crevices of bark or stones. Still other tendrils, as those of the
sweet-pea and clematis, are parts of the leaf.

The stems may be modified into thorns for protection, as we see in
many trees and shrubs, and parts of leaves may be similarly changed,
as in the thistle. The underground stems often become much changed,
forming bulbs, tubers, root stocks, etc. much as in the
monocotyledons. These structures are especially found in plants which
die down to the ground each year, and contain supplies of nourishment
for the rapid growth of the annual shoots.

[Illustration: FIG. 93.--_A_, base of a plant of shepherd's-purse
(_Capsella bursa-pastoris_), x 1/2. _r_, the main root. _B_, upper part
of the inflorescence, x 1. _C_, two leaves: i, from the upper part;
ii, from the base of the plant, x 1. _D_, a flower, x 3. _E_, the
same, with sepals and petals removed, x 3. _F_, petal. _G_, sepal.
_H_, stamen, x 10. _f_, filament. _an._ anther. _I_, a fruit with one
of the valves removed to show the seeds, x 4. _J_, longitudinal
section of a seed, x 8. _K_, the embryo removed from the seed, x 8.
_l_, the first leaves (cotyledons). _st._ the stem ending in the root.
_L_, cross-section of the stem, x 20. _fb._ fibro-vascular bundle.
_M_, a similar section of the main root, x 15. _N_, diagram of the
flower.]

The structure of the tissues, and the peculiarities of the flower and
fruit, will be better understood by a somewhat careful examination of
a typical dicotyledon, and a comparison with this of examples of the
principal orders and families.

One of the commonest of weeds, and at the same time one of the most
convenient plants for studying the characteristics of the
dicotyledons, is the common shepherd's-purse (_Capsella
bursa-p