pteridophytes.
These cells finally form a firm tissue, whose cells are filled with
starch that forms a reserve supply of food for the embryo plant when
the seed germinates. The embryo (Fig. 81, _K_, _em._, Fig. 82, _J_),
even when the seed is ripe, remains very small, and shows scarcely
any differentiation. It is a small, pear-shaped mass of cells, the
smaller end directed toward the upper end of the embryo sac.

The integuments grow with the embryo sac, and become brown and hard,
forming the shell of the seed. The stalk of the ovule also enlarges,
and finally forms the peculiar, spongy appendage of the seeds already
noticed (Fig. 81, _J_, _K_).

CHAPTER XVI.

CLASSIFICATION OF THE MONOCOTYLEDONS.

In the following chapter no attempt will be made to give an exhaustive
account of the characteristics of each division of the monocotyledons,
but only such of the most important ones as may serve to supplement
our study of the special one already examined. The classification
here, and this is the case throughout the spermaphytes, is based
mainly upon the characters of the flowers and fruits.

The classification adopted here is that of the German botanist
Eichler, and seems to the author to accord better with our present
knowledge of the relationships of the groups than do the systems that
are more general in this country. According to Eichler's
classification, the monocotyledons may be divided into seven groups;
viz., I. _Liliiflorae_; II. _Enantioblastae_; III. _Spadiciflorae_;
IV. _Glumaceae_; V. _Scitamineae_; VI. _Gynandrae_; VII. _Helobiae_.

ORDER I.--_Liliiflorae_.

The plants of this group agree in their general structure with the
adder's-tongue, which is a thoroughly typical representative of the
group; but nevertheless, there is much variation among them in the
details of structure. While most of them are herbaceous forms (dying
down to the ground each year), a few, among which may be mentioned the
yuccas ("bear grass," "Spanish bayonet") of our southern states,
develop a creeping or upright woody stem, increasing in size from year
to year. The herbaceous forms send up their stems yearly from
underground bulbs, tubers, _e.g._ _Trillium_ (Fig. 83, _A_), or
thickened, creeping stems, or root stocks (rhizomes). Good examples of
the last are the Solomon's-seal (Fig. 83, _B_), _Medeola_ (_C_, _D_),
and iris (Fig. 84 _A_). One family, the yams (_Dioscoreae_), of which
we have one common nat