Regular gamopetalous corollas are sometimes _campanulate_ or
_bell-shaped_, as in (_Campanula_) (fig. 60); _infundibuliform_ or
_funnel-shaped_, when the tube is like an inverted cone, and the limb
becomes more expanded at the apex, as in tobacco; _hypocrateriform_ or
_salver-shaped_, when there is a straight tube surmounted by a flat
spreading limb, as in primula (fig. 61); _tubular_, having a long
cylindrical tube, appearing continuous with the limb, as in _Spigelia_
and comfrey; _rotate_ or _wheel-shaped_, when the tube is very short,
and the limb flat and spreading, as in forget-me-not, _Myosotis_ (when
the divisions of the rotate corolla are very acute, as in _Galium_, it
is sometimes called _stellate_ or _star-like_); _urceolate_ or
_urn-shaped_, when there is scarcely any limb, and the tube is narrow at
both ends, and expanded in the middle, as in bell-heath (_Erica
cinerea_). Some of these forms may become irregular in consequence of
certain parts being more developed than others. Thus, in _Veronica_, the
rotate corolla has one division much smaller than the rest, and in
foxglove (_Digitalis_) there is a slightly irregular companulate
corolla. Of irregular gamopetalous corollas there may be mentioned the
_labiate_ or _lipped_ (fig. 62), having two divisions of the limb in the
form of lips (the upper one, u, composed usually of two united petals,
and the lower, l, of three), separated by a gap. In such cases the tube
varies in length, and the parts in their union follow the reverse order
of what occurs in the calyx, where two sepals are united in the lower
lip and three in the upper. When the upper lip of a labiate corolla is
much arched, and the lips separated by a distinct gap, it is called
_ringent_ (fig. 62). The labiate corolla characterizes the natural order
Labiatae. When the lower lip is pressed against the upper, so as to
leave only a chink between them, the corolla is said to be _personate_,
as in snapdragon, and some other Scrophulariaceae. In some corollas the
two lips become hollowed out in a remarkable manner, as in calceolaria,
assuming a slipper-like appearance, similar to what occurs in the
labellum of some orchids, as _Cypripedium_. When a tubular corolla is
split in such a way as to form a strap-like process on one side with
several tooth-like projections at its apex, it becomes _ligulate_ or
_strap-shaped_ (fig. 63). This corolla occurs in many composite plants,
as in the florets of dandelion, daisy and chicory. The number of
divisions at the apex indicates the number of united petals, some of
which, however, may be abortive. Occasionally some of the petals become
more united than others, and then the corolla assumes a _bilabiate_ or
_two-lipped_ form, as seen in the division of Compositae called
Labiatiflorae.
Petals are sometimes suppressed, and sometimes the whole corolla is
absent. In _Amorpha_ and _Afzelia_ the corolla is reduced to a single
petal, and in some other Leguminous plants it is entirely wanting. In
the natural order Ranunculaceae, some genera, such as _Ranunculus_,
globe-flower and paeony, have both calyx and corolla, while others, such
as clematis, anemone and _Caltha_, have only a coloured calyx. Flowers
become double by the multiplication of the parts of the corolline whorl;
this arises in general from a metamorphosis of the stamens.
[Illustration:
FIG. 61.--Flower of cowslip (_Primula veris_) cut vertically. s, Sepals
joined to form a gamosepalous calyx; c, corolla consisting of tube and
spreading limb; a, stamens springing from the mouth of the tube; p,
pistil.
FIG. 62.--Irregular gamopetalous labiate corolla of the Dead-nettle
(_Lamium album_). The upper lip u is composed of two petals united, the
lower lip (l) of three. Between the two lips there is a gap. The throat
is the part where the tube and the labiate limb join. From the arching
of the upper lip this corolla is called ringent.
FIG. 63.--Irregular gamopetalous ligulate flower of Ragwort (_Senecio_).
It is a tubular floret, split down on one side, with the united petals
forming a straplike projection. The lines on the flat portion indicate
the divisions of the five petals. From the tubular portion below, the
bifid style projects slightly.]
Certain structures occur on the petals of some flowers, which received
in former days the name of _nectaries_. The term nectary was very
vaguely applied by Linnaeus to any part of the flower which presented an
unusual aspect, as the crown (_corona_) of narcissus, the fringes of the
Passion-flower, &c. If the name is retained it ought properly to include
only those parts which secrete a honey-like substance, as the glandular
depression at the base of the perianth of the fritillary, or on the
petal of _Ranunculus_ (fig. 55), or on the stamens of Rutaceae. The
honey secreted by flowers attracts insects, which, by conveying the
pollen to the stigma, effect fertilization. The horn-like nectaries
under the galeate sepal of aconite (fig. 58) are modified petals, so
also are the tubular nectaries of hellebore (fig. 56). Other
modifications of some part of the flower, especially of the corolla and
stamens, are produced either by degeneration or outgrowth, or by
_chorisis_, or _deduplication_. Of this nature are the scales on the
petals in _Lychnis_, _Silene_ and _Cynoglossum_, which are formed in the
same way as the ligules of grasses. In other cases, as in Samolus, the
scales are alternate with the petals, and may represent altered stamens.
In _Narcissus_ the appendages are united to form a crown, consisting of
a membrane similar to that which unites the stamens in _Pancratium_. It
is sometimes difficult to say whether these structures are to be
referred to the corolline or to the staminal row.
Petals are attached to the axis usually by a narrow base. When this
attachment takes place by an articulation, the petals fall off either
immediately after expansion (_caducous_) or after fertilization
(_deciduous_). A corolla which is continuous with the axis and not
articulated to it, as in campanula and heaths, may be persistent, and
remain in a withered or marcescent state while the fruit is ripening. A
gamopetalous corolla falls off in one piece; but sometimes the base of
the corolla remains persistent, as in _Rhinanthus_ and _Orobanche_.
The _stamens_ and the _pistil_ are sometimes spoken of as the essential
organs of the flower, as the presence of both is required in order that
perfect seed may be produced. As with few exceptions the stamen
represents a leaf which has been specially developed to bear the pollen
or microspores, it is spoken of in comparative morphology as a
microsporophyll; similarly the carpels which make up the pistil are the
megasporophylls (see ANGIOSPERMS). _Hermaphrodite_ or _bisexual_ flowers
are those in which both these organs are found; _unisexual_ or
_diclinous_ are those in which only one of these organs appears,--those
bearing stamens only, being _staminiferous_ or "male"; those having the
pistil only, _pistilliferous_ or "female." But even in plants with
hermaphrodite flowers self-fertilization is often provided against by
the structure of the parts or by the period of ripening of the organs.
For instance, in _Primula_ and _Linum_ some flowers have long stamens
and a pistil with a short style, the others having short stamens and a
pistil with a long style. The former occur in the so-called thrum-eyed
primroses (fig. 61), the latter in the "pin-eyed." Such plants are
called _dimorphic_. Other plants are _trimorphic_, as species of
_Lythrum_, and proper fertilization is only effected by combination of
parts of equal length. In some plants the stamens are perfected before
the pistil; these are called _proterandrous_, as in _Ranunculus repens_,
_Silene maritima_, _Zea Mays_. In other plants, but more rarely, the
pistil is perfected before the stamens, as in _Potentilla argentea_,
_Plantago major_, _Coix Lachryma_, and they are termed _proterogynous_.
Plants in which proterandry or proterogyny occurs are called
_dichogamous_. When in the same plant there are unisexual flowers, both
male and female, the plant is said to be _monoecious_, as in the hazel
and castor-oil plant. When the male and female flowers of a species are
found on separate plants, the term _dioecious_ is applied, as in
_Mercurialis_ and hemp; and when a species has male, female and
hermaphrodite flowers on the same or different plants, as in
_Parietaria_, it is _polygamous_.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.
FIG. 64.--Flower of _Paeonia peregrina_, in longitudinal section. k,
Sepal; c, petal; a, stamens; g, pistil. (1/2 nat. size.)]
Stamens.
The stamens arise from the thalamus or torus within the petals, with
which they generally alternate, forming one or more whorls, which
collectively constitute the _androecium_. Their normal position is below
the pistil, and when they are so placed (fig. 64, a) upon the thalamus
they are hypogynous. Sometimes they become adherent to the petals, or
are _epipetalous_, and the insertion of both is looked upon as similar,
so that they are still hypogynous, provided they are independent of the
calyx and the pistil. In other cases they are perigynous or epigynous
(fig. 65). Numerous intermediate forms occur, especially amongst
Saxifragaceae, where the parts are _half superior_ or _half inferior_.
Where the stamens become adherent to the pistil so as to form a column,
the flowers are said to be _gynandrous_, as in _Aristolochia_ (fig. 66).
These arrangements of parts are of great importance in classification.
The stamens vary in number from one to many hundreds. In acyclic flowers
there is often a gradual transition from petals to stamens, as in the
white water-lily (fig. 31). When flowers become double by cultivation,
the stamens are converted into petals, as in the paeony, camellia, rose,
&c. When there is only one whorl the stamens are usually equal in number
to the sepals or petals, and are arranged opposite to the former, and
alternate with the latter. The flower is then _isostemonous_. When the
stamens are not equal in number to the sepals or petals, the flower is
_anisostemonous_. When there is more than one whorl of stamens, then the
parts of each successive whorl alternate with those of the whorl
preceding it. The staminal row is more liable to multiplication of parts
than the outer whorls. A flower with a single row of stamens is
_haplostemonous_. If the stamens are double the sepals or petals as
regards number, the flower is _diplostemonous_; if more than double,
_polystemonous_. The additional rows of stamens may be developed in the
usual centripetal (acropetal) order, as in Rhamnaceae; or they may be
interposed between the pre-existing ones or be placed outside them, i.e.
develop centrifugally (basipetally), as in geranium and oxalis, when the
flower is said to be _obdiplostemonous_. When the stamens are fewer than
twenty they are said to be _definite_; when above twenty they are
_indefinite_, and are represented by the symbol [infinity]. The number
of stamens is indicated by the Greek numerals prefixed to the term
_androus_; thus a flower with one stamen is _monandrous_, with two,
three, four, five, six or many stamens, di-, tri-, tetr-, pent-, hex- or
polyandrous, respectively.
[Illustration: FIG. 65.--Flower of Aralia in vertical section. c, Calyx;
p, petal; e, stamen; s, stigmas. The calyx, petals and stamens spring
from above the ovary (o) in which two chambers are shown each with a
pendulous ovule; d, disc between the stamens and stigmas.]
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.
FIG. 66.--Flowers of _Aristolochia Clematitis_ cut through
longitudinally. I. Young flower in which the stigma (N) is receptive and
the stamens (S) have not yet opened; II. Older flower with the stamens
(S) opened, the stigma withered, and the hairs on the corolla dried up.]
The function of the stamen is the development and distribution of the
pollen. The stamen usually consists of two parts, a contracted portion,
often thread-like, termed the _filament_ (fig. 25 f), and a broader
portion, usually of two lobes, termed the _anther_ (a), containing the
powdery _pollen_ (p), and supported upon the end of the filament. That
portion of the filament in contact with the anther-lobes is termed the
_connective_. If the anther is absent the stamen is abortive, and cannot
perform its functions. The anther is developed before the filament, and
when the latter is not produced, the anther is sessile, as in the
mistletoe.
The filament is usually, as its name imports, filiform or thread-like,
and cylindrical, or slightly tapering towards its summit. It is often,
however, thickened, compressed and flattened in various ways, becoming
_petaloid_ in _Canna_, _Marania_, water-lily (fig. 32); _subulate_ or
slightly broadened at the base and drawn out into a point like an awl,
as in _Butomus umbellatus_; or clavate, that is, narrow below and broad
above, as in _Thalictrum_. In some instances, as in _Tamarix gallica_,
_Peganum Harmala_, and _Campanula_, the base of the filament is much
dilated, and ends suddenly in a narrow thread-like portion. In these
cases the base may give off lateral stipulary processes, as in _Allium_
and _Alyssum calycinum_. The filament varies much in length and in
firmness. The length sometimes bears a relation to that of the pistil,
and to the position of the flower, whether erect or drooping. The
filament is usually of sufficient solidity to support the anther in an
erect position; but sometimes, as in grasses, and other wind-pollinated
flowers, it is very delicate and hair-like, so that the anther is
pendulous (fig. 105). The filament is generally continuous from one end
to the other, but in some cases it is bent or jointed, becoming
_geniculate_; at other times, as in the pellitory, it is spiral. It is
colourless, or of different colours. Thus in fuchsia and _Poinciana_, it
is red; in _Adamia_ and _Tradescantia virginica_, blue; in _Oenothera_
and _Ranunculus acris_, yellow.
Hairs, scales, teeth or processes of different kinds are sometimes
times developed on the filament. In spiderwort (_Tradescantia
virginica_) the hairs are beautifully coloured, moniliform or
necklace-like, and afford good objects for studying rotation of the
protoplasm. Filaments are usually articulated to the thalamus or torus,
and the stamens fall off after fertilization; but in _Campanula_ and
some other plants they are continuous with the torus, and the stamens
remain persistent, although in a withered state. Changes are produced in
the whorl of stamens by cohesion of the filaments to a greater or less
extent, while the anthers remain free; thus, all the filaments of the
androecium may unite, forming a tube round the pistil, or a central
bundle when the pistil is abortive, the stamens becoming _monadelphous_,
as occurs in plants of the Mallow tribe; or they may be arranged in two
bundles, the stamens being _diadelphous_, as in _Polygala_, _Fumaria_
and Pea; in this case the bundles may be equal or unequal. It frequently
happens, especially in Papilionaceous flowers, that out of ten stamens
nine are united by their filaments, while one (the posterior one) is
free (fig. 68). When there are three or more bundles the stamens are
_triadelphous_, as in _Hypericum aegyptiacum_, or _polyadelphous_, as in
_Ricinus communis_ (castor-oil). In some cases, as in papilionaceous
flowers, the stamens cohere, having been originally separate, but in
most cases each bundle is produced by the branching of a single stamen.
When there are three stamens in a bundle we may conceive the lateral
ones as of a stipulary nature. In Lauraceae there are perfect stamens,
each having at the base of the filament two abortive stamens or
staminodes, which may be analogous to stipules. Filaments sometimes are
adherent to the pistil, forming a column (_gynostemium_), as in
_Stylidium_, Asclepiadaceae, _Rafflesia_, and Aristolochiaceae (fig.
66); the flowers are then termed _gynandrous_.
[Illustration: FIG. 67.--Spikelet of Reed (_Phragmites communis_) opened
out. a, b, Barren glumes; c, fertile glumes, each enclosing one flower
with its pale, d; the zigzag axis (_rhachilla_) bears long silky hairs.]
[Illustration:
FIG. 68.--Stamens and pistil of Sweet Pea (_Lathyrus_). The stamens are
diadelphous, nine of them being united by their filaments (f), while one
of them (e) is free; st, stigma; c, calyx.
FIG. 69.--Portion of wall of anther of Wallflower (_Cheiranthus_). ce,
Exothecium; cf, endothecium; highly magnified.
FIG. 70.--Quadrilocular or tetrathecal anther of the flowering Rush
(_Butomus umbellatus_). The anther entire (a) with its filament; section
of anther (b) showing the four loculi.]
The anther.
The _anther_ consists of lobes containing the minute powdery pollen
grains, which, when mature, are discharged by a fissure or opening of
some sort. There is a double covering of the anther--the outer, or
_exothecium_, resembles the epidermis, and often presents stomata and
projections of different kinds (fig. 69); the inner, or _endothecium_,
is formed by a layer or layers of cellular tissue (fig. 69, cf), the
cells of which have a spiral, annular, or reticulated thickening of the
wall. The endothecium varies in thickness, generally becoming thinner
towards the part where the anther opens, and there disappears entirely.
The walls of the cells are frequently absorbed, so that when the anther
attains maturity the fibres are alone left, and these by their
elasticity assist in discharging the pollen. The anther is developed
before the filament, and is always sessile in the first instance, and
sometimes continues so. It appears at first as a simple cellular papilla
of meristem, upon which an indication of two lobes soon appears. Upon
these projections the rudiments of the pollen-sacs are then seen,
usually four in number, two on each lobe. In each a differentiation
takes place in the layers beneath the epidermis, by which an outer layer
of small-celled tissue surrounds an inner portion of large cells. Those
central cells are the mother-cells of the pollen, whilst the
small-celled layer of tissue external to them becomes the endothecium,
the exothecium being formed from the epidermal layer.
In the young state there are usually four pollen-sacs, two for each
anther-lobe, and when these remain permanently complete it is a
_quadrilocular_ or _tetrathecal_ anther (fig. 70). Sometimes, however,
only two cavities remain in the anther, by union of the sacs in each
lobe, in which case the anther is said to be _bilocular_ or _dithecal_.
Sometimes the anther has a single cavity, and becomes _unilocular_, or
_monothecal_, or _dimidiate_, either by the disappearance of the
partition between the two lobes, or by the abortion of one of its lobes,
as in _Styphelia laeta_ and _Althaea officinalis_ (hollyhock).
Occasionally there are numerous cavities in the anther, as in _Viscum_
and _Rafflesia_. The form of the anther-lobes varies. They are generally
of a more or less oval or elliptical form, or they may be globular, as
in _Mercurialis annua_; at other times linear or clavate: curved,
flexuose, or sinuose, as in bryony and gourd. According to the amount of
union of the lobes and the unequal development of different parts of
their surface an infinite variety of forms is produced. That part of the
anther to which the filament is attached is the _back_, the opposite
being the _face_. The division between the lobes is marked on the face
of the anther by a groove or _furrow_, and there is usually on the face
a _suture_, indicating the line of dehiscence. The suture is often
towards one side in consequence of the valves being unequal. The stamens
may cohere by their anthers, and become _syngenesious_, as in composite
flowers, and in lobelia, jasione, &c.
The connective.
The anther-lobes are united to the _connective_, which is either
continuous with the filament or articulated with it. When the filament
is continuous with the connective, and is prolonged so that the
anther-lobes appear to be united to it throughout their whole length,
and lie in apposition to it and on both sides of it, the anther is said
to be _adnate_ or _adherent_; when the filament ends at the base of the
anther, then the latter is _innate_ or _erect_. In these cases the
anther is to a greater or less degree fixed. When, however, the
attachment is very narrow, and an articulation exists, the anthers are
movable (_versatile_) and are easily turned by the wind, as in
_Tritonia_, grasses (fig. 105), &c., where the filament is attached only
to the middle of the connective. The connective may unite the
anther-lobes completely or only partially. It is sometimes very short
and is reduced to a mere point, so that the lobes are separate or free.
At other times it is prolonged upwards beyond the lobes, assuming
various forms, as in _Acalypha_ and oleander; or it is extended
backwards and downwards, as in violet (fig. 71), forming a
nectar-secreting spur. In _Salvia officinalis_ the connective is
attached to the filament in a horizontal manner, so as to separate the
two anther-lobes (fig. 72), one only of which contains pollen, the other
being imperfectly developed and sterile. The connective is joined to the
filament by a movable joint forming a lever which plays an important
part in the pollination-mechanism. In _Stachys_ the connective is
expanded laterally, so as to unite the bases of the anther-lobes and
bring them into a horizontal line.
Antherdehiscence.
The opening or _dehiscence_ of the anthers to discharge their contents
takes place either by clefts, by valves, or by pores. When the
anther-lobes are erect, the cleft is lengthwise along the line of the
suture--_longitudinal dehiscence_ (fig. 25). At other times the slit is
horizontal, from the connective to the side, as in _Alchemilla arvensis_
(fig. 73) and in _Lemna_; the dehiscence is then _transverse_. When the
anther-lobes are rendered horizontal by the enlargement of the
connective, then what is really longitudinal dehiscence may appear to be
transverse. The cleft does not always proceed the whole length of the
anther-lobe at once, but often for a time it extends only partially. In
other instances the opening is confined to the base or apex, each
loculament opening by a single pore, as in _Pyrola_, _Tetratheca
juncea_, Rhododendron, _Vaccinium_ and _Solanum_ (fig. 74), where there
are two, and _Poranthera_, where there are four; whilst in the mistletoe
the anther has numerous pores for the discharge of the pollen. Another
mode of dehiscence is the valvular, as in the barberry (fig. 75), where
each lobe opens by a valve on the outer side of the suture, separately
rolling up from base to apex; in some of the laurel tribe there are two
such valves for each lobe, or four in all. In some Guttiferae, as
_Hebradendron cambogioides_ (the Ceylon gamboge plant), the anther opens
by a lid separating from the apex (_circumscissile_ dehiscence).
[Illustration:
FIG. 71.--Two stamens of Pansy (_Viola tricolor_), with their two
anther-lobes and the connectives (p) extending beyond them. One of the
stamens has been deprived of its spur, the other shows its spur c.
FIG. 72.--Anther of _Salvia officinalis_. lf, fertile lobe full of
pollen; ls, barren lobe without pollen; e, connective; f, filament.
FIG. 73.--Stamen of Lady's Mantle (_Alchemilla_), with the anther
opening transversely.
FIG. 74.--Stamen of a species of Nightshade (_Solanum_), showing the
divergence of the anther-lobes at the base, and the dehiscence by pores
at the apex.
FIG. 75.--The stamen of the Barberry (_Berberis vulgaris_), showing one
of the valves of the anther (v) curved upwards, bearing the pollen on
its inner surface.]
The anthers dehisce at different periods during the process of
flowering; sometimes in the bud, but more commonly when the pistil is
fully developed and the flower is expanded. They either dehisce
simultaneously or in succession. In the latter case individual stamens
may move in succession towards the pistil and discharge their contents,
as in _Parnassia palustris_, or the outer or the inner stamens may first
dehisce, following thus a centripetal or centrifugal order. These
variations are intimately connected with the arrangements for
transference of pollen. The anthers are called _introrse_ when they
dehisce by the surface next to the centre of the flower; they are
_extrorse_ when they dehisce by the outer surface; when they dehisce by
the sides, as in _Iris_ and some grasses, they are _laterally_
dehiscent. Sometimes, from their versatile nature, anthers originally
introrse become extrorse, as in the Passion-flower and _Oxalis_.
The usual colour of anthers is yellow, but they present a great variety
in this respect. They are red in the peach, dark purple in the poppy and
tulip, orange in _Eschscholtzia_, &c. The colour and appearance of the
anthers often change after they have discharged their functions.
Stamens occasionally become sterile by the degeneration or
non-development of the anthers, when they are known as _staminodia_, or
rudimentary stamens. In _Scrophularia_ the fifth stamen appears in the
form of a scale; and in many Pentstemons it is reduced to a filament
with hairs or a shrivelled membrane at the apex. In other cases, as in
double flowers, the stamens are converted into petals; this is also
probably the case with such plants as _Mesembryanthemum_, where there
is a multiplication of petals in several rows. Sometimes, as in _Canna_,
one of the anther-lobes becomes abortive, and a petaloid appendage is
produced. Stamens vary in length as regards the corolla. Some are
enclosed within the tube of the flower, as in _Cinchona_ (_included_);
others are _exserted_, or extend beyond the flower, as in _Littorella_
or _Plantago_. Sometimes the stamens in the early state of the flower
project beyond the petals, and in the progress of growth become
included, as in _Geranium striatum_. Stamens also vary in their relative
lengths. When there is more than one row or whorl in a flower, those on
the outside are sometimes longest, as in many Rosaceae; at other times
those in the interior are longest, as in _Luhea_. When the stamens are
in two rows, those opposite the petals are usually shorter than those
which alternate with the petals. It sometimes happens that a single
stamen is longer than all the rest. A definite relation, as regards
number, sometimes exists between the long and the short stamens. Thus,
in some flowers the stamens are _didynamous_, having only four out of
five stamens developed, and the two corresponding to the upper part of
the flower longer than the two lateral ones. This occurs in Labiatae and
Scrophulariaceae (fig. 76). Again, in other cases there are six stamens,
whereof four long ones are arranged in pairs opposite to each other, and
alternate with two isolated short ones (fig. 77), giving rise to
_tetradynamous_ flowers, as in Cruciferae. Stamens, as regards their
direction, may be erect, turned inwards, outwards, or to one side. In
the last-mentioned case they are called _declinate_, as in amaryllis,
horse-chestnut and fraxinella.
[Illustration: FIG. 76.--Corolla of foxglove (_Digitalis purpurea_), cut
in order to show the didynamous stamens (two long and two short) which
are attached to it.]
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.
FIG. 77.--Tetradynamous stamens (four long and two short) of wallflower
(_Cheiranthus Cheiri_).]
The pollen-grains or microspores contained in the anther consist of
small cells, which are developed in the large thick-walled mother-cells
formed in the interior of the pollen-sacs (microsporangia) of the young
anther. These mother-cells are either separated from one another and
float in the granular fluid which fills up the cavity of the pollen-sac,
or are not so isolated. A division takes place, by which four cells are
formed in each, the exact mode of division differing in dicotyledons and
monocotyledons. These cells are the pollen-grains. They increase in size
and acquire a cell-wall, which becomes differentiated into an outer
cuticular layer, or _extine_, and an inner layer, or _intine_. Then the
walls of the mother-cells are absorbed, and the pollen-grains float
freely in the fluid of the pollen-sacs, which gradually disappears, and
the mature grains form a powdery mass within the anther. They then
either remain united in fours, or multiples of four, as in some acacias,
_Periploca graeca_ and _Inga anomala_, or separate into individual
grains, which by degrees become mature pollen. Occasionally the membrane
of the mother-cell is not completely absorbed, and traces of it are
detected in a viscid matter surrounding the pollen-grains, as in
Onagraceae. In orchidaceous plants the pollen-grains are united into
masses, or _pollinia_ (fig. 78), by means of viscid matter. In orchids
each of the pollen-masses has a prolongation or stalk (_caudicle_) which
adheres to a prolongation at the base of the anther (_rostellum_) by
means of a viscid gland (_retinaculum_) which is either naked or
covered. The term _clinandrium_ is sometimes applied to the part of the
column in orchids where the stamens are situated. In some orchids, as
_Cypripedium_, the pollen has its ordinary character of separate grains.
The number of pollinia varies; thus, in _Orchis_ there are usually two,
in _Cattleya_ four, and in _Laelia_ eight. The two pollinia in _Orchis
Morio_ contain each about 200 secondary smaller masses. These small
masses, when bruised, divide into grains which are united in fours. In
Asclepiadaceae the pollinia are usually united in pairs (fig. 79),
belonging to two contiguous anther-lobes--each pollen-mass having a
caudicular appendage, ending in a common gland, by means of which they
are attached to a process of the stigma. The pollinia are also provided
with an appendicular staminal covering (fig. 80). The exine is a firm
membrane, which defines the figure of the pollen-grain, and gives colour
to it. It is either smooth, or covered with numerous projections (fig.
81), granules, points or crested reticulations. The colour is generally
yellow, and the surface is often covered with a viscid or oily matter.
The intine is uniform in different kinds of pollen, thin and
transparent, and possesses great power of extension. In some aquatics,
as _Zostera_, _Zannichellia_, _Naias_, &c., only one covering exists.
[Illustration:
FIG. 78.--Pollinia, or pollen-masses, with their retinacula (g) or
viscid matter attaching them at the base. The pollen masses (p) are
supported on stalks or caudicles (c). These masses are easily detached
by the agency of insects. Much enlarged.
FIG. 79.--Pistil of _Asclepias_ (a) with pollen-masses (p) adhering to
the stigma (s). b, pollen-masses, removed from the stigma, united by a
gland-like body. Enlarged.
FIG. 80.--Stamen of _Asclepias_, showing filament f, anther a, and
appendages p. Enlarged.]
[Illustration: FIG. 81.--Pollen of Hollyhock (_Althaea rosea_), highly
magnified.]
[Illustration: _From Vines' Students' Text-Book of Botany_, by
permission of Swan Sonnenschein & Co.
FIG. 82.--Germinating pollen-grain of Epilobium (highly mag.) bearing a
pollen-tube s; e, exine; i, intine; abc, the three spots where the exine
is thicker in anticipation of the formation of the pollen-tube developed
in this case at a.]
[Illustration: FIG. 83.--Male flower of Pellitory (_Parietaria
officinalis_), having four stamens with in-curved elastic filaments, and
an abortive pistil in the centre. When the perianth (p) expands, the
filaments are thrown out with force as at a, so as to scatter the
pollen.]
Pollen-grains vary from 1/300 to 1/700 of an inch or less in diameter.
Their forms are various. The most common form of grain is ellipsoidal,
more or less narrow at the extremities, which are called its _poles_, in
contradistinction to a line equidistant from the extremities, which is
its equator. Pollen-grains are also spherical; cylindrical and curved,
as in _Tradescantia virginica_; polyhedral in Dipsacaceae and
Compositae; nearly triangular in section in Proteaceae and Onagraceae
(fig. 82). The surface of the pollen-grain is either uniform and
homogeneous, or it is marked by folds formed by thinnings of the
membrane. There are also rounded portions of the membrane or pores
visible in the pollen-grain; these vary in number from one to fifty, and
through one or more of them the pollen-tube is extended in germination
of the spore. In Monocotyledons, as in grasses, there is often only one,
while in Dicotyledons they number from three upwards; when numerous, the
pores are either scattered irregularly, or in a regular order,
frequently forming a circle round the equatorial surface. Sometimes at
the place where they exist, the outer membrane, in place of being thin
and transparent, is separated in the form of a lid, thus becoming
_operculate_, as in the passion-flower and gourd. Within the
pollen-grain is the granular protoplasm with some oily particles, and
occasionally starch. Before leaving the pollen-sac a division takes
place in the pollen-grain into a vegetative cell or cells, from which
the tube is developed, and a generative cell, which ultimately divides
to form the male cells (see ANGIOSPERMS and GYMNOSPERMS).
Pollination.
When the pollen-grains are ripe, the anther dehisces and the pollen is
shed. In order that fertilization may be effected the pollen must be
conveyed to the stigma of the pistil. This process, termed _pollination_
(see POLLINATION), is promoted in various ways,--the whole form and
structure of the flower having relation to the process. In some plants,
as _Kalmia_ and Pellitory (fig. 83), the mere elasticity of the
filaments is sufficient to effect this; in other plants pollination is
effected by the wind, as in most of our forest trees, grasses, &c., and
in such cases enormous quantities of pollen are produced. These plants
are _anemophilous_. But the common agents for pollination are insects.
To allure and attract them to visit the flower the odoriferous
secretions and gay colours are developed, and the position and
complicated structure of the parts of the flower are adapted to the
perfect performance of the process. It is comparatively rare in
hermaphrodite flowers for self-fertilization to occur, and the various
forms of dichogamy, dimorphism and trimorphism are fitted to prevent
this.
[Illustration: FIG. 84.--Flower of Tree Paeony (_Paeonia Moutan_),
deprived of its corolla, and showing the disk in the form of a fleshy
expansion (d) covering the ovary.]
Disk.
Under the term _disk_ is included every structure intervening between
the stamens and the pistil. It was to such structures that the name of
_nectary_ was applied by old authors. It presents great varieties of
form, such as a ring, scales, glands, hairs, petaloid appendages, &c.,
and in the progress of growth it often contains saccharine matter, thus
becoming truly nectariferous. The disk is frequently formed by
degeneration or transformation of the staminal row. It may consist of
processes rising from the torus, alternating with the stamens, and thus
representing an abortive whorl; or its parts may be opposite to the
stamens. In some flowers, as _Jatropha Curcas_, in which the stamens are
not developed, their place is occupied by glandular bodies forming the
disk. In Gesneraceae and Cruciferae the disk consists of tooth-like
scales at the base of the stamens. The parts composing the disk
sometimes unite and form a glandular ring, as in the orange; or they
form a dark-red lamina covering the pistil, as in _Paeonia Moutan_ (fig.
84); or a waxy lining of the hollow receptacle, as in the rose; or a
swelling at the top of the ovary, as in Umbelliferae, in which the disk
is said to be epigynous. The enlarged torus covering the ovary in
_Nymphaea_ (_Castalia_) and _Nelumbium_ may be regarded as a form of
disk.
The pistil.
The pistil or _gynoecium_ occupies the centre or apex of the flower, and
is surrounded by the stamens and floral envelopes when these are
present. It constitutes the innermost whorl, which after flowering is
changed into the fruit and contains the seeds. It consists essentially
of two parts, a basal portion forming a chamber, the _ovary_, containing
the ovules attached to a part called the _placenta_, and an upper
receptive portion, the _stigma_, which is either seated on the ovary
(_sessile_), as in the tulip and poppy, or is elevated on a stalk called
the _style_, interposed between the ovary and stigma. The pistil
consists of one or more modified leaves, the _carpels_ (or
_megasporophylls_). When a pistil consists of a single carpel it is
_simple_ or monocarpellary (fig. 85). When it is composed of several
carpels, more or less united, it is _compound_ or _polycarpellary_ (fig.
86). In the first-mentioned case the terms carpel and pistil are
synonymous. Each carpel has its own ovary, style (when present), and
stigma, and may be regarded as formed by a folded leaf, the upper
surface of which is turned inwards towards the axis, and the lower
outwards, while from its margins are developed one or more _ovules_.
This comparison is borne out by an examination of the flower of the
double-flowering cherry. In it no fruit is produced, and the pistil
consists merely of sessile leaves, the limb of each being green and
folded, with a narrow prolongation upwards, as if from the midrib, and
ending in a thickened portion. In _Cycas_ the carpels are ordinary
leaves, with ovules upon their margin.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.
FIG. 85.--Pistil of Broom (_Cytisus_) consisting of ovary o, style s,
and stigma t. It is formed by a single carpel.
FIG. 86.--Vertical section of the flower of Black Hellebore (_Helleborus
niger_). The pistil is apocarpous, consisting of several distinct
carpels, each with ovary, style and stigma. The stamens are indefinite,
and are inserted below the pistil (hypogynous).
FIG. 87.--Fruit of the Strawberry (_Fragaria vesca_), consisting of an
enlarged succulent receptacle, bearing on its surface the small dry
seed-like fruits (achenes).
FIG. 88.--Fruit of _Rosa alba_, consisting of the fleshy hollowed axis
s', the persistent sepals s, and the carpels fr. The stamens (c) have
withered. (After Duchartre.)
FIG. 89.--Pistil of _Ranunculus_. x, Receptacle with the points of
insertion of the stamens a, most of which have been removed.
FIG. 90.--Syncarpous Pistil of Flax (_Linum_), consisting of five
carpels, united by their ovaries, while their styles and stigmas are
separate.]
A pistil is usually formed by more than one carpel. The carpels may be
arranged either at the same or nearly the same height in a verticil, or
at different heights in a spiral cycle. When they remain separate and
distinct, thus showing at once the composition of the pistil, as in
_Caltha, Ranunculus_, hellebore (fig. 86), and _Spiraea_, the term
_apocarpous_ is applied. Thus, in Sedum (fig. 22) the pistil consists of
five verticillate carpels o, alternating with the stamens e. In magnolia
and _Ranunculus_ (fig. 89) the separate carpels are numerous and are
arranged in a spiral cycle upon an elongated axis or receptacle. In the
raspberry the carpels are on a conical receptacle; in the strawberry, on
a swollen succulent one (fig. 87); and in the rose (fig. 88), on a
hollow one. When the carpels are united, as in the pear, arbutus and
chickweed, the pistil becomes _syncarpous_. The number of carpels in a
pistil is indicated by the Greek numeral. A flower with a simple pistil
is monogynous; with two carpels, digynous; with three carpels,
trigynous, &c.
The union in a syncarpous pistil is not always complete; it may take
place by the ovaries alone, while the styles and stigmas remain free
(fig. 90), and in this case, when the ovaries form apparently a single
body, the organ receives the name of _compound_ ovary; or the union may
take place by the ovaries and styles while the stigmas are disunited; or
by the stigmas and the summit of the style only. Various intermediate
states exist, such as partial union of the ovaries, as in the rue, where
they coalesce at their base; and partial union of the styles, as in
Malvaceae. The union is usually most complete at the base; but in
Labiatae the styles are united throughout their length, and in
Apocynaceae and Asclepiadaceae the stigmas only. When the union is
incomplete, the number of the parts of a compound pistil may be
determined by the number of styles and stigmas; when complete, the
external venation, the grooves on the surface, and the internal
divisions of the ovary indicate the number.
The placenta.
[Illustration: FIG. 91.--Pistil of Pea after fertilization of the
ovules, developing to form the fruit. f, Funicle or stalk of ovule (ov);
pl, placenta; s, withered style and stigma; c, persistent calyx.]
[Illustration:
FIG. 92.--Trilocular ovary of the Lily (_Lilium_), cut transversely. s,
Septum; o, ovules, which form a double row in the inner angle of each
chamber. Enlarged.
FIG. 93.--Diagrammatic section of a quinquelocular ovary, composed of
five carpels, the edges of which are folded inwards, and meet in the
centre forming the septa, s. The ovules (o) are attached to a central
placenta, formed by the union of the five ventral sutures. Dorsal
suture, l.
FIG. 94.--Diagrammatic section of a five-carpellary ovary, in which the
edges of the carpels, bearing the placentas and ovules o, are not folded
inwards. The placentas are parietal, and the ovules appear sessile on
the walls of the ovary. The ovary is unilocular.]
[Illustration:
FIG. 95.--Diagrammatic section of a five-carpellary ovary, in which
the septa (s) proceed inwards for a certain length, bearing the
placentas and ovules (o). In this case the ovary is unilocular, and the
placentas are parietal. Dorsal suture, l.
FIG. 96.--Pistil of Pansy (_Viola tricolor_), enlarged. 1, Vertical;
2, horizontal section; c, calyx; d, wall of ovary; o, ovules; p,
placenta; s, stigma.
FIG. 97.--Transverse section of the fruit of the Melon (_Cucumis
Melo_), showing the placentas with the seeds attached to them. The three
carpels forming the pepo are separated by partitions. From the centre,
processes go to circumference, ending in curved placentas bearing the
ovules.
FIG. 98.--Diagrammatic section of a compound unilocular ovary, in
which there are no indications of partitions. The ovules (o) are
attached to a free central placenta, which has no connexion with the
walls of the ovary.]
[Illustration:
FIG. 99.--Pistil of _Cerastium hirsutum_ cut vertically. o, Ovary; p,
free central placenta; g, ovules; s, styles.
FIG. 100.--The same cut horizontally, and the halves separated so as
to show the interior of the cavity of the ovary o, with the free central
placenta p, covered with ovules g.]
The ovules are attached to the _placenta_, which consists of a mass of
cellular tissue, through which the nourishing vessels pass to the ovule.
The placenta is usually formed on the edges of the carpellary leaf (fig.
91)--_marginal_. In many cases, however, the placentas are formations
from the axis (axile), and are not connected with the carpellary leaves.
In marginal placentation the part of the carpel bearing the placenta is
the _inner_ or _ventral suture_, corresponding to the margin of the
folded carpellary leaf, while the _outer_ or _dorsal suture_ corresponds
to the midrib of the carpellary leaf. As the placenta is formed on each
margin of the carpel it is essentially double. This is seen in cases
where the margins of the carpel do not unite, but remain separate, and
consequently two placentas are formed in place of one. When the pistil
is formed by one carpel the inner margins unite and form usually a
common marginal placenta, which may extend along the whole margin of the
ovary as far as the base of the style (fig. 91), or may be confined to
the base or apex only. When the pistil consists of several separate
carpels, or is apocarpous, there are generally separate placentas at
each of their margins. In a syncarpous pistil, on the other hand, the
carpels are so united that the edges of each of the contiguous ones, by
their union, form a _septum_ or _dissepiment_, and the number of these
septa consequently indicates the number of carpels in the compound
pistil (fig. 92). When the dissepiments extend to the centre or axis,
the ovary is divided into cavities or _cells_, and it may be
_bilocular_, _triloculur_ (fig. 92), _quadrilocular_, _quinquelocular_,
or _multilocular_, according as it is formed by two, three, four, five
or many carpels, each carpel corresponding to a single cell. In these
cases the marginal placentas meet in the axis, and unite so as to form a
single _central_ one (figs. 92, 93), and the ovules appear in the
central angle of the loculi. When the carpels in a syncarpous pistil do
not fold inwards so that the placentas appear as projections on the
walls of the ovary, then the ovary is _unilocular_ (fig. 95) and the
placentas are _parietal_, as in _Viola_ (fig. 96). In these instances
the placentas may be formed at the margin of the united contiguous
leaves, so as to appear single, or the margins may not be united, each
developing a placenta. Frequently the margins of the carpels, which fold
in to the centre, split there into two lamellae, each of which is curved
outwards and projects into the loculament, dilating at the end into a
placenta. This is well seen in Cucurbitaceae (fig. 97), _Pyrola_, &c.
The carpellary leaves may fold inwards very slightly, or they may be
applied in a valvate manner, merely touching at their margins, the
placentas then being parietal (fig. 94), and appearing as lines or
thickenings along the walls. Cases occur, however, in which the
placentas are not connected with the walls of the ovary, and form what
is called a _free central placenta_ (fig. 98). This is seen in many of
the Caryophyllaceae and Primulaceae (figs. 99, 100). In Caryophyllaceae,
however, while the placenta is free in the centre, there are often
traces found at the base of the ovary of the remains of septa, as if
rupture had taken place, and, in rare instances, ovules are found on the
margins of the carpels. But in Primulaceae no vestiges of septa or
marginal ovules can be perceived at any period of growth; the placenta
is always free, and rises in the centre of the ovary. Free central
placentation, therefore, has been accounted for in two ways: either by
supposing that the placentas in the early state were formed on the
margins of carpellary leaves, and that in the progress of development
these leaves separated from them, leaving the placentas and ovules free
in the centre; or by supposing that the placentas are not _marginal_ but
_axile_ formations, produced by an elongation of the axis, and the
carpels verticillate leaves, united together around the axis. The first
of these views applies to Caryophyllaceae, the second to Primulaceae.
Occasionally, divisions take place in ovaries which are not formed by
the edges of contiguous carpels. These are called _spurious
dissepiments_. They are often horizontal, as in _Cathartocarpus
Fistula_, where they consist of transverse cellular prolongations from
the walls of the ovary, only developed after fertilization, and
therefore more properly noticed under fruit. At other times they are
vertical, as in _Datura_, where the ovary, in place of being two-celled,
becomes four-celled; in Cruciferae, where the prolongation of the
placentas forms a vertical partition; in _Astragalus_ and _Thespesia_,
where the dorsal suture is folded inwards; and in _Oxytropis_, where the
ventral suture is folded inwards.
The ovary is usually of a more or less spherical or curved form,
sometimes smooth and uniform on its surface, at other times hairy and
grooved. The grooves usually indicate the divisions between the carpels
and correspond to the dissepiments. The dorsal suture may be marked by a
slight projection or by a superficial groove. When the ovary is situated
on the centre of the receptacle, free from the other whorls, so that its
base is above the insertion of the stamens, it is termed _superior_, as
in _Lychnis_, _Primula_ (fig. 61) and Peony (fig. 64) (see also fig.
28). When the margin of the receptacle is prolonged upwards, carrying
with it the floral envelopes and staminal leaves, the basal portion of
the ovary being formed by the receptacle, and the carpellary leaves
alone closing in the apex, the ovary is _inferior_, as in pomegranate,
aralia (fig. 65), gooseberry and fuchsia (see fig. 30). In some plants,
as many Saxifragaceae, there are intermediate forms, in which the term
_half-inferior_ is applied to the ovary, whilst the floral whorls are
_half-superior_.
[Illustration:
FIG. 101.--Carpel of Lady's-mantle (_Alchemilla_) with lateral style s;
o, ovary, _st_, stigma. Enlarged.
FIG. 102.--Pistil of Primrose (_Primula_) composed of five carpels which
are completely united; o, ovary; s, style; st, stigma. Enlarged.
FIG. 103.--Gynoecium of the Flower-de-Luce (_Iris_), consisting of an
inferior ovary (o) and a style which divides into three petaloid
segments (s), each bearing a stigma (st).
FIG. 104.--Capsule of Poppy, opening by pores (p), under the radiating
peltate stigma (s).]
The style.
The _style_ proceeds from the summit of the carpel (fig. 102), and is
traversed by a narrow canal, in which there are some loose projecting
cells, a continuation of the placenta, constituting what is called
conducting tissue, which ends in the stigma. This is particularly
abundant when the pistil is ready for fertilization. In some cases,
owing to more rapid growth of the dorsal side of the ovary, the style
becomes _lateral_ (fig. 101); this may so increase that the style
appears to arise from near the base, as in the strawberry, or from the
base, as in _Chrysobalanus Icaco_, when it is called _basilar_. In all
these cases the style still indicates the organic apex of the ovary,
although it may not be the apparent apex. When in a compound pistil the
style of each carpel is thus displaced, it appears as if the ovary were
depressed in the centre, and the style rising from the depression in the
midst of the carpels seems to come from the torus. Such a style is
_gynobasic_, and is well seen in Boraginaceae. The form of the style is
usually cylindrical, more or less filiform and simple; sometimes it is
grooved on one side, at other times it is flat, thick, angular,
compressed and even petaloid, as in _Iris_ (fig. 103) and _Canna_. In
Goodeniaceae it ends in a cuplike expansion, enclosing the stigma. It
sometimes bears hairs, which aid in the application of the pollen to the
stigma, and are called _collecting hairs_, as in _Campanula_, and also
in _Aster_ and other Compositae. These hairs, during the upward growth
of the style, come into contact with the already ripened pollen, and
carry it up along with them, ready to be applied by insects to the
mature stigma of other flowers. In _Vicia_ and _Lobelia_ the hairs
frequently form a tuft below the stigma. The styles of a syncarpous
pistil are either separate or united; when separate, they alternate with
the septa; when united completely, the style is said to be _simple_
(fig. 102). The style of a single carpel, or of each carpel of a
compound pistil, may also be divided. Each division of the tricarpellary
ovary of _Jatropha Curcas_ has a _bifurcate_ or forked style, and the
ovary of _Emblica officinalis_ has three styles, each of which is twice
forked. The length of the style is determined by the relation which
should subsist between the position of the stigma and that of the
anthers, so as to allow the proper application of the pollen. The style
is deciduous or persists after fertilization.
The stigma.
The _stigma_ is the termination of the conducting tissue of the style,
and is usually in direct communication with the placenta. It consists of
loose cellular tissue, and secretes a viscid matter which detains the
pollen, and causes it to germinate. This secreting portion is, strictly
speaking, the true stigma, but the name is generally applied to all the
divisions of the style on which the stigmatic apparatus is situated. The
stigma alternates with the dissepiments of a syncarpous pistil, or, in
other words, corresponds with the back of the loculaments; but in some
cases it would appear that half the stigma of one carpel unites with
half that of the contiguous carpel, and thus the stigma is opposite the
dissepiments, that is, alternates with the loculaments, as in the poppy.
[Illustration: FIG. 105.--Flower of a grass with glumes removed, showing
three stamens and two feathery styles. p, Pale; l, lodicules. Enlarged.]
The divisions of the stigma mark the number of carpels which compose the
pistil. Thus in _Campanula_ a five-cleft stigma indicates five carpels;
in Bignoniaceae, Scrophulariaceae and Acanthaceae, the two-lobed or
bilamellar stigma indicates a bilocular ovary. Sometimes, however, as in
Gramineae, the stigma of a single carpel divides. Its position may be
terminal or lateral. In _Iris_ it is situated on a cleft on the back of
the petaloid divisions of the style (fig. 103). Some stigmas, as those
of _Mimulus_, present sensitive flattened laminae, which close when
touched. The stigma presents various forms. It may be globular, as in
_Mirabilis Jalapa_; orbicular, as in _Arbutus Andrachne_; umbrella-like,
as in _Sarracenia_, where, however, the proper stigmatic surface is
beneath the angles of the large expansion of the apex of the style;
ovoid, as in fuchsia; hemispherical; polyhedral; radiating, as in the
poppy (fig. 104), where the true stigmatic rays are attached to a sort
of _peltate_ or shield-like body, which may represent depressed or
flattened styles; _cucullate_, i.e. covered by a hood, in calabar bean.
The lobes of a stigma are flat and pointed as in _Mimulus_ and
_Bignonia_, fleshy and blunt, smooth or granular, or they are feathery,
as in many grasses (fig. 105) and other wind-pollinated flowers. In
Orchidaceae the stigma is situated on the anterior surface of the column
beneath the anther. In Asclepiadaceae the stigmas are united to the face
of the anthers, and along with them form a solid mass.
The ovule.
The ovule is attached to the placenta, and destined to become the seed.
Ovules are most usually produced on the margins of the carpellary
leaves, but are also formed over the whole surface of the leaf, as in
_Butomus_. In other instances they rise from the floral axis itself,
either terminal, as in Polygonaceae and Piperaceae, or lateral, as in
Primulaceae and Compositae. The ovule is usually contained in an ovary,
and all plants in which the ovule is so enclosed are termed
_angiospermous_; but in Coniferae and Cycadaceae it has no proper
ovarian covering, and is called naked, these orders being denominated
_gymnospermous_. In _Cycas_ the altered leaf, upon the margin of which
the ovule is produced, and the peltate scales, from which they are
pendulous in _Zamia_, are regarded by all botanists as carpellary
leaves. As for the Coniferae great discussion has arisen regarding the
morphology of parts in many genera. The carpellary leaves are sometimes
united in such a way as to leave an opening at the apex of the pistil,
so that the ovules are exposed, as in mignonette. In _Leontice
thalictroides_ (Blue Cohosh), species of _Ophiopogon_, _Peliosanthes_
and _Stateria_, the ovary ruptures immediately after flowering, and the
ovules are exposed; and in species of _Cuphea_ the placenta ultimately
bursts through the ovary and corolla, and becomes erect, bearing the
exposed ovules. The ovule is attached to the placenta either directly,
when it is _sessile_, or by means of a prolongation _funicle_ (fig. 110,
f). This cord sometimes becomes much elongated after fertilization. The
part by which the ovule is attached to the placenta or cord is its
_base_ or _hilum_, the opposite extremity being its _apex_. The latter
is frequently turned round in such a way as to approach the base. The
ovule is sometimes embedded in the placenta, as in _Hydnora_.
[Illustration:
FIGS. 106 and 107.--Successive stages in the development of an ovule. n,
Nucellus; i, inner; o, outer integument in section; m, micropyle.
FIG. 108.--Orthotropous ovule of _Polygonum_ in section, showing the
embryo-sac s, in the nucellus n, the different ovular coverings, the
base of the nucellus or chalaza ch, and the apex of the ovule with its
micropyle m.
FIG. 109.--Vertical section of the ovule of the Austrian Pine (_Pinus
austriaca_), showing the nucellus a, consisting of delicate cellular
tissue containing deep in its substance an embryo-sac b. The micropyle m
is very wide.]
The ovule appears at first as a small cellular projection from the
placenta. The cells multiply until they assume a more or less enlarged
ovate form constituting what has been called the _nucellus_ (fig. 106,
n), or central cellular mass of the ovule. This nucellus may remain
naked, and alone form the ovule, as in some orders of parasitic plants
such as Balanophoraceae, Santalaceae, &c.; but in most plants it becomes
surrounded by certain coverings or integuments during its development.
These appear first in the form of cellular rings at the base of the
nucellus, which gradually spread over its surface (figs. 106, 107). In
some cases only one covering is formed, especially amongst gamopetalous
dicotyledons, as in Compositae, Campanulaceae, also in walnut, &c. But
usually besides the single covering another is developed subsequently
(fig. 106, o), which gradually extends over that first formed, and
ultimately covers it completely, except at the apex. There are thus two
integuments to the nucellus, an outer and an inner. The integuments do
not completely invest the apex of the nucellus, but an opening termed
the _micropyle_ is left. The micropyle indicates the organic apex of the
ovule. A single cell of the nucellus enlarges greatly to form the
_embryo-sac_ or megaspore (fig. 108, s). This embryo-sac increases in
size, gradually supplanting the cellular tissue of the nucellus until it
is surrounded only by a thin layer of it; or it may actually extend at
the apex beyond it, as in _Phaseolus_ and _Alsine media_; or it may pass
into the micropyle, as in _Santalum_. In Gymnosperms it usually remains
deep in the nucellus and surrounded by a thick mass of cellular tissue
(fig. 109). For an account of the further development of the megaspore,
and the formation of the egg-cell, from which after fertilization is
formed the embryo, see GYMNOSPERMS and ANGIOSPERMS.
[Illustration:
FIG. 110.--Campylotropous ovule of wall-flower (_Cheiranthus_), showing
the funicle f, which attaches the ovule to the placenta; p, the outer,
s, the inner coat, n, the nucellus, ch, the chalaza. The ovule is curved
upon itself, so that the micropyle is near the funicle.
FIG. 111.--Anatropous ovule of Dandelion (_Taraxacum_), n, nucellus,
which is inverted, so that the chalaza ch, is removed from the base or
hilum h, while the micropyle f is near the base. The connexion between
the base of the ovule and the base of the nucellus is kept up by means
of the raphe r.]
The point where the integuments are united to the base of the nucellus
is called the _chalaza_ (figs. 111, 112). This is often coloured, is of
a denser texture than the surrounding tissue, and is traversed by
fibro-vascular bundles, which pass from the placenta to nourish the
ovule.