_The embryo has only one cotyledon; the leaves are as a rule scattered,
with parallel venation; the vascular bundles of the stem are closed,
there is no increase of thickness. The flower is typically constructed
of five 3-merous whorls, placed alternately._
THE EMBRYO is generally small in proportion to the abundant endosperm
(exceptions, see _Helobieæ_), and its single cotyledon is often
sheath-like, and very large. On the germination of the seed either the
entire cotyledon, or its apex only, most generally remains in the seed
and absorbs the nutritive-tissue, while the lower portion elongates
and pushes out the plumule and radicle, which then proceed with their
further growth. The primary root in most cases soon ceases to grow,
but at the same time, however, numerous lateral roots break out from
the stem, and become as vigorous as the primary root, or even more so.
Increase in thickness does not take place in these roots; they branch
very little or not at all, and generally die after a longer or shorter
time.
THE STEM is frequently a corm, bulb, or other variety of underground
stem, as the majority of the Monocotyledons are perennial, herbaceous
plants; it has scattered, closed vascular bundles (Fig. 276), and no
cambium by which a continuous thickening may take place. The stem of
the Palms, however, attains a very considerable thickness, which is due
to the meristem of its growing-point continually increasing in diameter
for a lengthened period (often for many years), until it has reached a
certain size. In this condition the growing-point has the form of an
inverted cone, and it is only when this cone has attained its requisite
size that the formation of a vertical cylindrical stem commences.
Certain tree-like Liliaceæ, as _Dracæna_, _Aloe_, etc., have a
continuous increase in thickness; this is due to a meristematic layer,
which arises in the cortex, outside the original vascular bundles,
which were formed at the growing-point of the stem. This meristem
continues to form thick-walled parenchyma and new, scattered vascular
bundles. The primary vascular bundles, in the Palms and others, run
in a curved line from their entrance into the stem at the base of
the leaf, towards the centre of the stem, and then bend outwards and
proceed downwards in a direction more parallel to the sides of the stem
(Fig. 277). The bundles formed later, in those stems which increase in
thickness, are not continued into the leaves.
THE BRANCHING as a rule is very slight, the axillary buds of the
majority of the leaves never attaining development, _e.g._ in the
Palms, bulbous plants and others. As the cotyledon arises singly,
the succeeding leaves also must be scattered, but they are frequently
arranged in two rows (Grasses, Iris, etc). _The first leaf borne on a
branch_ (the “Fore-leaf,”[24]--the bracteole, if on a floral shoot)
has generally, in the Monocotyledons, a characteristic form and
position, being situated on the posterior side of its own shoot, and
hence turned towards the main axis; it is sometimes provided with two
laterally-placed keels (Figs. 279 _f_, 290 _øi_), but the midrib is
often absent. It arises in some cases from two primordia, which at the
beginning are quite distinct, and thus has been regarded as formed by
two leaves. It is, however, only one leaf, a fact which is evident from
several circumstances, one being that it never supports more than one
shoot, and this stands in the median plane (Fig. 279).
[Illustration: FIG. 276.--Transverse section of the stem of a
Palm: _v v_ is the wood portion, _b b_ the bast portion of
the vascular bundled.]
[Illustration: FIG. 277.--Diagrammatic representation of the
course of the vascular bundles, from the stem into the leaves in
a Monocotyledon.]
THE LEAVES are _amplexicaul_, and have a large sheath but no stipules;
the blade is most frequently long, ligulate, or linear, entire, with
parallel venation, the veins being straight or curved (Figs. 300, 309).
Connecting the large number of veins which run longitudinally, there
are as a rule only weak transverse ones. It is very rarely that other
forms of leaves are found, such as cordate (Figs. 302, 312), or that
the blade is branched, or the venation is, for example, pinnate or
palmate (Figs. 225, 298); these deviations are especially found in the
Araceæ, the Palms, the Scitamineæ (Fig. 308), the Dioscoreaceæ, and in
several aquatic plants. The incisions in the Palm-leaf are derived by
the splitting of an originally entire leaf.
THE STRUCTURE OF THE FLOWER is generally as follows: Pr3 + 3, A3 + 3,
G3, rarely S3 + P3 with the other members unchanged.[25] Instead of 3,
the numbers 2 and 4 may occur; rarely others. In all these instances
there are 5 whorls, which regularly alternate with one another, most
frequently in the 3-merous flower, as in the diagram (Fig. 278). This
diagram is found in the following orders: Liliaceæ, Convallariaceæ,
Juncaceæ, Bromeliaceæ, Amaryllidaceæ, Dioscoreaceæ, Palmæ, some
Araceæ, and in some small orders, and may be considered as the typical
structure and also the starting point for the exceptional orders. The
ovary in many Monocotyledons has many ovules, and the fruit becomes
a many-seeded berry or capsule; this form is no doubt the oldest. In
others the number of seeds becomes reduced to 1, and the fruit then
becomes a cypsela, or a drupe (_e.g. Gramineæ_, _Cyperaceæ_, _Palmæ_,
etc).
[Illustration: FIG. 278.--Diagram of the ordinary, regular flower
in the Monocotyledons: _s_ is the bract.]
[Illustration: FIG. 279.--Diagram of _Iris_: _f_ the bracteole;
in its axil is a shoot with its bracteole.]
[Illustration: FIG. 280.--Diagram of _Orchis_: _l_ the lip; σ σ
the two staminodes.]
Deviations from this typical floral structure in some instances may
be traced to _suppression_, very rarely to a _splitting_ of certain
members, the typical relative positions not being changed. Thus,
the Iridaceæ, the Cyperaceæ, most of the Gramineæ and some Juncaceæ
deviate in having only 3 stamens (Fig. 279), the inner whorl (indicated
by *) not becoming developed. The Musaceæ differ in the posterior
stamen not being developed; _Zingiberaceæ_ (Fig. 314), _Marantaceæ_,
and _Cannaceæ_, in the fact that only 1 of all the stamens bears an
anther, and the others are either suppressed or developed into petaloid
staminodes, with some perhaps cleft in addition. The Orchideæ deviate
in having, generally, only the anterior stamen of all the 6 developed
(Fig. 280). In this, as in other instances, the suppression of certain
parts of the flower is often connected with _zygomorphy_ (_i.e._
symmetry in _one_ plane), chiefly in the inner perianth-whorl, but also
in the other whorls. In the Orchids, the perianth-leaf (the labellum,
Fig. 280 _l_) which is directly opposite the fertile stamen, is larger
and altogether different from the others. The perianth-leaves may also
be suppressed; see, for example, the two diagrams of the Cyperaceæ
(Fig. 284). In some orders the suppression of these leaves, which form
the basis of the diagram, is so complete that it is hard to reduce the
actual structure of the flower to the theoretical type, _e.g._ the
Grasses (Fig. 290) and _Lemna_ (Fig. 303). In the first family, which
especially comprises water-plants, a somewhat different structure is
found; thus Fig. 282 differs somewhat from the ordinary type, and other
flowers much more so; but the floral diagrams which occur in this
family may perhaps be considered as the most probable representatives
of an older type, from which the ordinary pentacyclic forms have taken
their origin. In favour of this theory we have the larger number of
whorls, the spiral arrangement of some of these in the flower, with
a large and indefinite number of stamens and carpels, the perfectly
apocarpous gynœceum which sometimes occurs, etc., etc.