8. =Growth from the Seed.= Phanerogamous plants grow from seed, and
their flowers are destined to the production of seeds. A seed has a
rudimentary plant ready formed in it,--sometimes with the two most
essential parts, i. e. stem and leaf, plainly discernible; sometimes
with no obvious distinction of organs until germination begins. This
incipient plant is called an EMBRYO.
9. In this section the Flax-plant is taken as a specimen, or type, and
the development and history of common plants in general is illustrated
by it. In flax-seed the embryo nearly fills the coats, but not quite.
There is a small deposit of nourishment between the seed-coat and the
embryo: this may for the present be left out of the account. This embryo
consists of a pair of leaves, pressed together face to face, and
attached to an extremely short stem. (Fig. 2-4.) In this rudimentary
condition the real nature of the parts is not at once apparent; but when
the seed grows they promptly reveal their character,--as the
accompanying figures (Fig. 5-7) show.
[Illustration: Fig. 1. Pod of Flax. 2. Section lengthwise, showing two
of the seeds; one whole, the other cut half away, bringing contained
embryo into view. 3. Similar section of a flax-seed more magnified and
divided flatwise; turned round, so that the stem-end (caulicle) of the
embryo is below: the whole broad upper part is the inner face of one of
the cotyledons; the minute nick at its base is the plumule. 4. Similar
section through a seed turned edgewise, showing the thickness of the
cotyledons, and the minute plumule between them, i. e. the minute bud on
the upper end of the caulicle.]
10. Before the nature of these parts in the seed was altogether
understood, technical names were given to them, which are still in use.
These initial leaves were named COTYLEDONS. The initial stem on which
they stand was called the RADICLE. That was because it gives rise to the
first root; but, as it is really the beginning of the stem, and because
it is the stem that produces the root and not the root that produces the
stem, it is better to name it the CAULICLE. Recently it has been named
_Hypocotyle_; which signifies something below the cotyledons, without
pronouncing what its nature is.
[Illustration: Fig. 5. Early Flax seedling; stem (caulicle), root at
lower end, expanded seed-leaves (cotyledons) at the other: minute bud
(plumule) between these. 6. Same later; the bud developed into second
pair of leaves, with hardly any stem-part below them; then into a third
pair of leaves, raised on a short joint of stem; and a fifth leaf also
showing. 7. Same still older, with more leaves developed, but these
singly (one after another), and with joints of stem between them.]
11. On committing these seeds to moist and warm soil they soon sprout,
i. e. _germinate_. The very short stem-part of the embryo is the first
to grow. It lengthens, protrudes its root-end; this turns downward, if
not already pointing in that direction, and while it is lengthening a
root forms at its point and grows downward into the ground. This root
continues to grow on from its lower end, and thus insinuates itself and
penetrates into the soil. The stem meanwhile is adding to its length
throughout; it erects itself, and, seeking the light, brings the seed up
out of the ground. The materials for this growth have been supplied by
the cotyledons or seed-leaves, still in the seed: it was the store of
nourishing material they held which gave them their thickish shape, so
unlike that of ordinary leaves. Now, relieved of a part of this store of
food, which has formed the growth by which they have been raised into
the air and light, they appropriate the remainder to their own growth.
In enlarging they open and throw off the seed-husk; they expand, diverge
into a horizontal position, turn green, and thus become a pair of
evident leaves, the first foliage of a tiny plant. This seedling,
although diminutive and most simple, possesses and puts into use, all
the ORGANS of VEGETATION, namely, root, stem, and leaves, each in its
proper element,--the root in the soil, the stem rising out of it, the
leaves in the light and open air. It now draws in moisture and some
food-materials from the soil by its root, conveys this through the stem
into the leaves, where these materials, along with other crude food
which these imbibe from the air, are assimilated into vegetable matter,
i. e. into the material for further growth.
12. =Further Growth= soon proceeds to the formation of new
parts,--downward in the production of more root, or of branches of the
main root, upward in the development of more stem and leaves. That from
which a stem with its leaves is continued, or a new stem (i. e. branch)
originated, is a BUD. The most conspicuous and familiar buds are those
of most shrubs and trees, bearing buds formed in summer or autumn, to
grow the following spring. But every such point for new growth may
equally bear the name. When there is such a bud between the cotyledons
in the seed or seedling it is called the PLUMULE. This is conspicuous
enough in a bean (Fig. 29.), where the young leaf of the new growth
looks like a little plume, whence the name, _plumule_. In flax-seed this
is very minute indeed, but is discernible with a magnifier, and in the
seedling it shows itself distinctly (Fig. 5, 6, 7).
13. As it grows it shapes itself into a second pair of leaves, which of
course rests on a second joint of stem, although in this instance that
remains too short to be well seen. Upon its summit appears the third
pair of leaves, soon to be raised upon its proper joint of stem; the
next leaf is single, and is carried up still further upon its supporting
joint of stem; and so on. The root, meanwhile, continues to grow
underground, not joint after joint, but continuously, from its lower
end; and commonly it before long multiplies itself by branches, which
lengthen by the same continuous growth. But stems are built up by a
succession of leaf-bearing growths, such as are strongly marked in a
reed or corn-stalk, and less so in such an herb as Flax. The word
"joint" is ambiguous: it may mean either the portion between successive
leaves, or their junction, where the leaves are attached. For precision,
therefore, the place where the leaf or leaves are borne is called a
NODE, and the naked interval between two nodes, an INTERNODE.
[Illustration: Fig. 8. Upper part of Flax-plant in blossom.]
14. In this way a simple stem with its garniture of leaves is developed
from the seed. But besides this direct continuation, buds may form and
develop into lateral stems, that is, _into branches_, from any node. The
proper origin of branches is from the AXIL of a leaf, i. e. the angle
between leaf and stem on the upper side; and branches may again branch,
so building up the herb, shrub, or tree. But sooner or later, and
without long delay in an annual like Flax, instead of this continuance
of mere vegetation, reproduction is prepared for by
15. =Blossoming.= In Flax the flowers make their appearance at the end
of the stem and branches. The growth, which otherwise might continue
them farther or indefinitely, now takes the form of blossom, and is
subservient to the production of seed.
[Illustration: Fig. 9. Flax-flowers about natural size. 10. Section of a
flower moderately enlarged, showing a part of the petals and stamens,
all five styles, and a section of ovary with two ovules or rudimentary
seeds.]
16. =The Flower= of Flax consists, first, of five small green leaves,
crowded into a circle: this is the CALYX, or flower-cup. When its
separate leaves are referred to they are called SEPALS, a name which
distinguishes them from foliage-leaves on the one hand, and from petals
on the other. Then come five delicate and _colored_ leaves (in the Flax,
blue), which form the COROLLA, and its leaves are PETALS; then a circle
of organs, in which all likeness to leaves is lost, consisting of
slender stalks with a knob at summit, the STAMENS; and lastly, in the
centre, the rounded body, which becomes a pod, surmounted by five
slender or stalk-like bodies. This, all together, is the PISTIL. The
lower part of it, which is to contain the seeds, is the OVARY; the
slender organs surmounting this are STYLES; the knob borne on the apex
of each style is a STIGMA. Going back to the stamens, these are of two
parts, viz. the stalk, called FILAMENT, and the body it bears, the
ANTHER. Anthers are filled with POLLEN, a powdery substance made up of
minute grains.
17. The pollen shed from the anthers when they open falls upon or is
conveyed to the stigmas; then the pollen-grains set up a kind of growth
(to be discerned only by aid of a good microscope), which penetrates the
style: this growth takes the form of a thread more delicate than the
finest spider's web, and reaches the bodies which are to become seeds
(OVULES they are called until this change occurs); these, touched by
this influence, are incited to a new growth within, which becomes an
embryo. So, as the ovary ripens into the seed-pod or capsule (Fig. 1,
etc.) containing seeds, each seed enclosing a rudimentary new plantlet,
the round of this vegetable existence is completed.