FLOWERING PLANTS.
ORGANS OF INCREASE AND REPRODUCTION—THE FLOWER—THE CALYX—THE
COROLLA—THE STAMEN—THE PISTIL.
Some of the simplest plants are propagated by _spores_, which are
detached, and fall upon the ground to vegetate; but in the case of the
higher orders the reproduction of species is a much more elaborate
process, and is carried on by means of certain organs called flowers.
Small buds, or _ovules_, are formed, which develop into seed. Plants
also produce buds, which grow upon various parts of it, and are capable
of reproducing their species. We will first speak of FLOWERS.
Flowers are not only the lovely blossoms we cut and place in our rooms,
but the reproductive organs of plants which may be very plain and
simple or gorgeous and fragrant, and in all probability the so-called
flowers are few in comparison to the unrecognized flowers. Trees
and bushes flower equally with the rose and the pink and carnation.
Vegetables flower as well as the lily, though we do not recognize it so
well. Let us now examine the “flower.”
Flowers may consist of four parts, but it is not absolutely necessary
that they should contain more than two. The four portions of a complete
flower are—
1. THE CALYX.
2. THE COROLLA.
3. THE STAMEN.
4. THE PISTIL.
The two last mentioned are essential. The four organs are placed around
a pedicel or peduncle (flower stalk), and are known as floral whorls.
The CALYX is the outermost whorl of all when all exist. The portions of
the calyx are known as _sepals_.
The COROLLA is usually the showy portion—the attraction of the flower.
The pieces of the corolla are called _petals_. The sweet fluids of the
plant are here concealed.
These parts—the calyx and corolla—are known as the floral envelopes,
or “perianth.” The tulip has one whorl only, and it is called the
envelope.
The calyx sometimes falls before the flower is full blown, as in the
poppy. Its lower portion is the “throat,” and the shape of the organ
varies, as will be seen by the illustrations.
[Illustration: Fig. 769.—
1. Tubular.
2. Clavate, or club-shaped.
3. Tubinate, or top-shaped.
4. Campanulate, or bell-shaped.
5. Funnel-shaped.
6. Urceolate, or urn-shaped.
7. Globular.]
The sepals are usually three to five in number. The poppy has two, and
the well-known wall-flower four free—that is, disunited—sepals. The
primrose possesses five. The calyx is the outside rim of all, and we
may thus remember it, because its _sepals_ alternate with the _petals_
of the _corolla_. The petals may be formed cup-fashion, as in the lily
of the valley, and here we have these sepals and petals in groups of
three each.
[Illustration: Fig. 770.—Trimerous corolla.]
[Illustration: Fig. 771.—Tetramerous.]
[Illustration: Fig. 772.—Pentamerous corolla.]
[Illustration: Fig. 773.—Monopetalous corolla.]
The petals differ from ordinary leaves, and in them we find all the
beautiful tints and the odour we imbibe from blossoms. The forms of
corolla correspond to those of the calyx, and are called by the same
names. But when corollas are absent the petals of course cannot provide
the necessary colours for the flower. Then the calyx is gifted, and the
sepals are brilliant. Thus Nature provides for everything.
Corollas are found with five or ten petals, and sometimes with three,
six, and nine—the numbers always doubling or adding the original
number. There may be four petals or eight, as in the “tetramerous”
corolla (fig. 771). Instances of others are illustrated, and a plant
whose petals, sepals, and stamens are numerically equal, or are
multiples of each other, are termed “symmetrical.” The “regular” flower
does not vary much, as the petals are of the same size and shape,
but there are many “irregular” flowers—as the pea—in which portions
of the calyx or corolla are of different shapes. The “labiate” and
the “campanulate” are illustrated in figs. 774, 775, including the
convolvulus and the snap-dragon. These are but a few examples of
an almost endless diversity. The “regular” flowers—exemplified in
the buttercup and convolvulus—always present the same figure to the
observer.
To the petals the beautifully-varied colours of plants are due, and
though it is not possible to enter upon the subject here, we may
conclude that the various beauties of the colours of flowers are owing
to light and air acting upon the various “colouring matters” contained
in the plant. Seeds planted in a dark cellar will spring up pale;
admit light, and they will become green, for light thus acts upon the
chlorophyl. But the flowers of the plant are not so dependent upon
light, as can easily be proved. Many interesting experiments have been
made upon flowers by acids and gases.[36]
[Illustration: Fig. 774.—Labiate corolla.]
[Illustration: Fig. 775.—Campanulate.]
The STAMENS are the next in order for our consideration. They are found
within the petals (or the calyx if no petals be present). Stamens vary
very much in different plants both in number and general features—but,
as in the case of petals, they keep, as a rule, to certain numbers
and doubles of them. The stamen consists of two portions—a lower,
thread-like part called the filament, and an oblong bag or head, termed
the _anther_. This contains a powdery matter called pollen, and is the
essential part of the stamen. The filament, which corresponds to the
petiole of the leaf, may be absent, in which case the anther is called
_sessile_. A lily will show the stamens perfectly, the anther being
prominent in many other plants also, such as daffodils and fuchsias.
The stamens are very important organs with regard to the classification
of plants—for number, length, and position, whether free or united, are
all characteristic features. The length of filaments is always the same
in the same kind of plant, and therefore is a very palpable test.
The _anther_ contains the _pollen_, a powdery matter, usually
yellow-coloured, but sometimes also red, brown, violet, or
green-coloured. Pollen-grains vary from 1/20 to 1/300 of a line in
diameter. Under a powerful microscope they appear as ellipsoidal, or
sometimes spherical, triangular, polyhedral vesicles, filled with a
granular semi-fluid matter. To effect fecundation, the pollen-grains
must come into contact with a certain part of the plant which is
intended to receive them, and which is called the _ovule_, and is found
in the fourth or innermost verticil of the flower, the _pistil_. Of the
further development of the ovule, we shall have occasion to speak in
the paragraph treating of the _seed_.
At the proper time the anther opens and discharges its contents, the
pollen-grains, some of which reach the place of their destination. The
position of the stamens to the pistil is usually such that the latter
can readily receive the pollen-grains. In many plants, however, the
stamens are too short to reach the pistils; or the two essential organs
of reproduction are in separate flowers, or even on different plants.
In such cases, the conveyance of the pollen from the anthers to the
pistils is effected by the agency of the wind, or by that of insects,
and more particularly by the bee. If the anthers are removed from the
flower previously to their opening, no fruit is produced.
Varieties of flowers and fruits are produced artificially, by shaking
the pollen of one plant upon the flowers of another, deprived of the
stamens. Many esteemed sorts of stock-gilliflowers and pinks have been
produced in this way.
The pistil constitutes the fourth and innermost whorl, and occupies
accordingly the centre of the flower and the apex of the axis, whose
growth is terminated with the production of the fruit.
[Illustration: Fig. 776.—Pistil.]
The pistil also is formed by one or several modified leaves, called
_carpels_, in this part of the flower, and which exhibit a more marked
resemblance in colour and structure to the ordinary leaves than the
stamens and petals do. The formation of the pistil from the leaf may be
considered to proceed in this manner: that the edges of the leaf are
folded inwards and unite, whilst the mid-rib is prolonged upwards (fig.
776A). The place where the margins of the folded leaves are united is
called _suture_ or _seam_ (_ventral suture_, in contradistinction to
the mid-rib, which is called the _dorsal suture_); and it is here that
the seed-buds or _ovules_ are developed.
The pistil consists of two parts—viz., the _ovary_ or _germen_, which
contains the ovules or young seeds, _a_, and the _stigma_, _b_, either
placed upon the ovary, or upon the _style_, or stalk, which is between
the stigma and the ovary.
A _pistil_ may be of one _carpel_ (simple), or of more than one
(compound). The carpels or the carpellary leaves are the “ovaries.”
The pistil is a very important test for the classification of plants;
some trees have no pistils, and the ovules are consequently naked. Such
plants are called _gymnospermæ_. The coniferæ (firs and pines) are thus
recognizable, and the position of the ovule is very much that of the
ordinary bud.
FOOTNOTES:
[35] Respecting artificial colouring of flowers, _see_ page 329.