A. Indefinite Centripetal Inflorescence.
I. Flowers solitary, axillary. _Vinca_, _Veronica hederifolia_.
II. Flowers in groups, pedicellate.
1. Elongated form (Raceme), _Hyacinth_, _Laburnum_, _Currant_.
(Corymb), _Ornithogalum_.
2. Contracted or shortened form (Umbel), _Cowslip_,
_Astrantia_.
III. Flowers in groups, sessile.
1. Elongated form (Spike), _Plantago_.
(Spikelet), _Grasses_.
(Amentum, Catkin), _Willow_, _Hazel_.
(Spadix) _Arum_, some _Palms_.
(Strobilus), _Hop_.
2. Contracted or shortened form (Capitulum), _Daisy_,
_Dandelion_, _Scabious_.
IV. Compound Indefinite Inflorescence.
a. Compound Spike, _Rye-grass_.
b. Compound Spadix, _Palms_.
c. Compound Raceme, _Astilbe_.
d. Compound Umbel, _Hemlock_ and most _Umbelliferae_.
e. Raceme of Capitula, _Petasites_.
f. Raceme of Umbels, _Ivy_.
B. Definite Centrifugal Inflorescence.
I. Flowers solitary, terminal. _Gentianella_, _Tulip_.
II. Flowers in Cymes.
1. Uniparous Cyme.
a. Helicoid Cyme (axes forming a spiral).
Elongated form, _Alstromeria_.
Contracted form, _Witsenia corymbosa_.
b. Scorpioid Cyme (axes unilateral, two rows).
Elongated form, _Forget-me-not_, _Symphytum_,
_Henbane_.
Contracted form, _Erodium_, _Alchemilla arvensis_.
2. Biparous Cyme (Dichotomous), including 3-5 chotomous
Cymes (Dichasium, Cymose Umbel, Anthela).
a. Elongated form, _Cerastium_, _Stellaria_.
b. Contracted form (Verticillaster), _Dead-nettle_,
_Pelargonium_.
3. Compound Definite Inflorescence. _Streptocarpus
polyanthus_, many _Calceolarias_.
C. Mixed Inflorescence.
Raceme of Scorpioid Cymes, _Horse-chestnut_.
Scorpioid Cyme of Capitula, _Vernonia scorpioides_.
Compound Umbel of Dichotomous Cymes, _Laurustinus_.
Capitulum of contracted Scorpioid Cymes (Glomerulus),
_Sea-pink_.
[Illustration:
FIG. 22.--Flower of _Sedum rubens_. s, Sepals; p, petals; a, stamens; c,
carpels.
FIG. 23.--Diagram of a completely symmetrical flower, consisting of four
whorls, each of five parts, s, Sepals; p, petals; a, stamens; c,
carpels.
FIG. 24.--Monochlamydeous (apetalous) flower of Goosefoot
(_Chenopodium_), consisting of a single perianth (calyx) of five parts,
enclosing five stamens, which are opposite the divisions of the
perianth, owing to the absence of the petals.
FIG. 25.--Stamen, consisting of a filament (stalk) f and an anther a,
containing the pollen p, which is discharged through slits in the two
lobes of the anther.
FIG. 26.--The pistil of Tobacco (_Nicotiana Tabacum_), consisting of the
ovary o, containing ovules, the style s, and the capitate stigma g. The
pistil is placed on the receptacle r, at the extremity of the peduncle.]
The flower.
The flower consists of the floral axis bearing the sporophylls (stamens
and carpels), usually with certain protective envelopes. The axis is
usually very much contracted, no internodes being developed, and the
portion bearing the floral leaves, termed the _thalamus_ or _torus_,
frequently expands into a conical, flattened or hollowed expansion; at
other times, though rarely, the internodes are developed and it is
elongated. Upon this torus the parts of the flower are arranged in a
crowded manner, usually forming a series of verticils, the parts of
which alternate; but they are sometimes arranged spirally especially if
the floral axis be elongated. In a typical flower, as in fig. 22, we
recognize four distinct whorls of leaves: an outer whorl, the _calyx_ of
_sepals_; within it, another whorl, the parts alternating with those of
the outer whorl, the _corolla_ of _petals_; next a whorl of parts
alternating with the parts of the corolla, the _androecium_ of
_stamens_; and in the centre the _gynoecium_ of _carpels_. Fig. 23 is a
diagrammatic representation of the arrangement of the parts of such a
flower; it is known as a _floral diagram_. The flower is supposed to be
cut transversely, and the parts of each whorl are distinguished by a
different symbol. Of these whorls the two internal, forming the
sporophylls, constitute the _essential_ organs of reproduction; the two
outer whorls are the protective coverings or floral _envelopes_. The
sepals are generally of a greenish colour; their function is mainly
protective, shielding the more delicate internal organs before the
flower opens. The petals are usually showy, and normally alternate with
the sepals. Sometimes, as usually in monocotyledons, the calyx and
corolla are similar; in such cases the term _perianth_, or _perigone_,
is applied. Thus, in the tulip, crocus, lily, hyacinth, we speak of the
parts of the perianth, in place of calyx and corolla, although in these
plants there is an outer whorl (calyx), of three parts, and an inner
(corolla), of a similar number, alternating with them. When the parts of
the calyx are in appearance like petals they are said to be _petaloid_,
as in Liliaceae. In some cases the petals have the appearance of sepals,
then they are _sepaloid_, as in Juncaceae. In plants, as _Nymphaea
alba_, where a spiral arrangement of the floral leaves occurs, it is not
easy to say where the calyx ends and the corolla begins, as these two
whorls pass insensibly into each other. When both calyx and corolla are
present, the plants are _dichlamydeous_; when one only is present, the
flower is termed _monochlamydeous_ or _apetalous_, having no petals
(fig. 24). Sometimes both are absent, when the flower is _achlamydeous_,
or naked, as in willow. The outermost series of the essential organs,
collectively termed the _androecium_, is composed of the
microsporophylls known as the staminal leaves or _stamens_. In their
most differentiated form each consists of a stalk, the _filament_ (fig.
25, f), supporting at its summit the anther (a), consisting of the
pollen-sacs which contain the powdery _pollen_ (p), the microspores,
which is ultimately discharged therefrom. The _gynoecium_ or _pistil_ is
the central portion of the flower, terminating the floral axis. It
consists of one or more _carpels_ (megasporophylls), either separate
(fig. 22, c) or combined (fig. 24). The parts distinguished in the
pistil are the _ovary_ (fig. 26, o), which is the lower portion
enclosing the _ovules_ destined to become seeds, and the _stigma_ (g), a
portion of loose cellular tissue, the receptive surface on which the
pollen is deposited, which is either sessile on the apex of the ovary,
as in the poppy, or is separated from it by a prolonged portion called
the _style_ (s). The androecium and gynoecium are not present in all
flowers. When both are present the flower is hermaphrodite; and in
descriptive botany such a flower is indicated by the symbol
[female/male]. When only one of those organs is present the flower is
_unisexual_ or _diclinous_, and is either male (_staminate_), [male] or
female (_pistillate_), [female]. A flower then normally consists of the
four series of leaves--calyx, corolla, androecium and gynoecium--and
when these are all present the flower is _complete_. These are usually
densely crowded upon the thalamus, but in some instances, after apical
growth has ceased in the axis, an elongation of portions of the
receptacle by intercalary growth occurs, by which changes in the
position of the parts may be brought about. Thus in _Lychnis_ an
elongation of the axis betwixt the calyx and the corolla takes place,
and in this way they are separated by an interval. Again, in the
passion-flower (_Passiflora_) the stamens are separated from the corolla
by an elongated portion of the axis, which has consequently been termed
the _androphore_, and in _Passiflora_ also, fraxinella (fig. 27),
Capparidaceae, and some other plants, the ovary is raised upon a
distinct stalk termed the _gynophore_; it is thus separated from the
stamens, and is said to be _stipitate_. Usually the successive whorls of
the flower, disposed from below upwards or from without inwards upon the
floral axis, are of the same number of parts, or are a multiple of the
same number of parts, those of one whorl alternating with those of the
whorls next it.
[Illustration: FIG. 27.--Calyx and pistil of Fraxinella (_Dictamnus
Fraxinella_). The pistil consists of several carpels, which are elevated
on a stalk or _gynophore_ prolonged from the receptacle.]
In the more primitive types of flowers the torus is more or less convex,
and the series of organs follow in regular succession, culminating in
the carpels, in the formation of which the growth of the axis is closed
(fig. 28). This arrangement is known as hypogynous, the other series
(calyx, corolla and stamens) being beneath (_hypo-_) the gynoecium. In
other cases, the apex of the growing point ceases to develop, and the
parts below form a cup around it, from the rim of which the outer
members of the flower are developed around (_peri-_) the carpels, which
are formed from the apex of the growing-point at the bottom of the cup.
This arrangement is known as _perigynous_ (fig. 29). In many cases this
is carried farther and a cavity is formed which is roofed over by the
carpels, so that the outer members of the flower spring from the edge of
the receptacle which is immediately above the ovary (epigynous), hence
the term epigyny (fig. 30).
[Illustration: FIGS. 28, 29 and 30.--Diagrams illustrating hypogyny,
perigyny and epigyny of the flower. a, Stamens; c, carpels; p, petals;
s, sepals.]
Symmetry of the flower.
When a flower consists of parts arranged in whorls it is said to be
_cyclic_, and if all the whorls have an equal number of parts and are
alternate it is _eucyclic_ (figs. 22, 23). In contrast to the cyclic
flowers are those, as in Magnoliaceae, where the parts are in spirals
(_acyclic_). Flowers which are cyclic at one portion and spiral at
another, as in many Ranunculaceae, are termed _hemicyclic_. In spiral
flowers the distinction into series is by no means easy, and usually
there is a gradual passage from sepaloid through petaloid to staminal
parts, as in the water-lily family, Nymphaeaceae (figs. 31, 32),
although in some plants there is no such distinction, the parts being
all petaloid, as in _Trollius_. Normally, the parts of successive whorls
alternate; but in some cases we find the parts of one whorl opposite or
_superposed_ to those of the next whorl. In some cases, as in the
vine-family Ampelidaceae, this seems to be the ordinary mode of
development, but the superposition of the stamens on the sepals in many
plants, as in the pink family, Caryophyllaceae, is due to the
suppression or abortion of the whorl of petals, and this idea is borne
out by the development, in some plants of the order, of the suppressed
whorl. As a rule, whenever we find the parts of one whorl superposed on
those of another we may suspect some abnormality.
[Illustration: From Strasburger's _Text-Book of Botany_, by permission
of Macmillan & Co., Ltd.
FIGS. 31 and 32.--White Water Lily. Fig. 31, flower; fig. 32, successive
stages, a-f, in the transition from petals to stamens. (After
Wossidlo.)]
A flower is said to be _symmetrical_ when each of its whorls consists of
an equal number of parts, or when the parts of any one whorl are
multiples of that preceding it. Thus, a symmetrical flower may have five
sepals, five petals, five stamens and five carpels, or the number of any
of these parts may be ten, twenty or some multiple of five. Fig. 23 is a
diagram of a symmetrical flower, with five parts in each whorl,
alternating with each other. Fig. 33 is a diagram of a symmetrical
flower of stone-crop, with five sepals, five alternating petals, ten
stamens and five carpels. Here the number of parts in the staminal
whorl is double that in the others, and in such a case the additional
five parts form a second row alternating with the others. In the
staminal whorl especially it is common to find additional rows. Fig. 34
shows a symmetrical flower, with five parts in the three outer rows, and
ten divisions in the inner. In this case it is the gynoecium which has
an additional number of parts. Fig. 35 shows a flower of heath, with
four divisions of the calyx and corolla, eight stamens in two rows, and
four divisions of the pistil. In fig. 36 there are three parts in each
whorl; and in fig. 37 there are three divisions of the calyx, corolla
and pistil, and six stamens in two rows. In all these cases the flower
is symmetrical. In Monocotyledons it is usual for the staminal whorl to
be double, it rarely having more than two rows, whilst amongst
dicotyledons there are often very numerous rows of stamens. The floral
envelopes are rarely multiplied. Flowers in which the number of parts in
each whorl is the same, are _isomerous_ (of equal number); when the
number in some of the whorls is different, the flower is _anisomerous_
(of unequal number). The pistillate whorl is very liable to changes. It
frequently happens that when it is fully formed, the number of its parts
is not in conformity with that of the other whorls. In such
circumstances, however, a flower has been called symmetrical, provided
the parts of the other whorls are normal,--the permanent state of the
pistil not being taken into account in determining symmetry. Thus fig.
38 shows a pentamerous symmetrical flower, with dimerous pistil.
Symmetry, then, in botanical language, has reference to a certain
definite numerical relation of parts. A flower in which the parts are
arranged in twos is called _dimerous_; when the parts of the whorls are
three, four or five, the flower is _trimerous_, _tetramerous_ or
_pentamerous_, respectively. The symmetry which is most commonly met
with is trimerous and pentamerous--the former occurring generally among
monocotyledons, the latter among dicotyledons. Dimerous and tetramerous
symmetry occur also among dicotyledons.
[Illustration:
FIG. 33.--Diagrammatic section of a symmetrical pentamerous flower of
Stone-crop (_Sedum_), consisting of five sepals (s), five petals (p)
alternating with the sepals, ten stamens (a) in two rows, and five
carpels (c) containing ovules. The dark lines (d) on the outside of the
carpels are glands.
FIG. 34.--Diagram of the flower of Flax (_Linum_), consisting of five
sepals (s), five petals (p), five stamens (a), and five carpels (c),
each of which is partially divided into two. The dots represent a whorl
of stamens which has disappeared. It is pentamerous, complete,
symmetrical and regular.
FIG. 35.--Diagram of the flower of Heath (_Erica_), a regular
tetramerous flower.
FIG. 36.--Diagram of the trimerous symmetrical flower of Iris.
FIG. 37.--Diagram of the symmetrical trimerous flower of Fritillary
(_Fritillaria_).
FIG. 38.--Diagram of the flower of Saxifrage (_Saxifraga
tridactylites_). The calyx and corolla consist of five parts, the
stamens are ten in two rows, while the pistil has only two parts
developed.]
The various parts of the flower have a certain definite relation to the
axis. Thus, in axillary tetramerous flowers (fig. 35), one sepal is next
the axis, and is called _superior_ or _posterior_; another is next the
bract, and is _inferior_ or _anterior_, and the other two are _lateral_;
and certain terms are used to indicate that position. A plane passing
through the anterior and posterior sepal and through the floral axis is
termed the _median plane_ of the flower; a plane cutting it at right
angles, and passing through the lateral sepals, is the _lateral plane_;
whilst the planes which bisect the angles formed by the lateral and
median planes are the _diagonal planes_, and in these flowers the petals
which alternate with the sepals are cut by the diagonal planes.
[Illustration: FIG. 39.--Diagram of flower of Sweet-pea (_Lathyrus_),
showing five sepals (s), two superior, one inferior, and two lateral;
five petals (p), one superior, two inferior, and two lateral; ten
stamens in two rows (a); and one carpel (c).
FIG. 40.--Flower of Pea (_Pisum sativum_), showing a papilionaceous
corolla, with one petal superior (st) called the standard (vexillum),
two inferior (car) called the keel (carina), and two lateral (a) called
wings (alae). The calyx is marked c.]
In a pentamerous flower one sepal may be superior, as in the calyx of
Rosaceae and Labiatae; or it may be inferior, as in the calyx of
Leguminosae (fig. 39)--the reverse, by the law of alternation, being the
case with the petals. Thus, in the blossom of the pea (figs. 39, 40),
the odd petal (vexillum) st is superior, while the odd sepal is
inferior. In the order Scrophulariaceae one of the two carpels is
posterior and the other anterior, whilst in Convolvulaceae the carpels
are arranged laterally. Sometimes the twisting of a part makes a change
in the position of other parts, as in Orchids, where the twisting of the
ovary changes the position of the labellum.
When the different members of each whorl are like in size and shape, the
flower is said to be _regular_; while differences in the size and shape
of the parts of a whorl make the flower _irregular_, as in the
papilionaceous flower, represented in fig. 39. When a flower can be
divided by a single plane into two exactly similar parts; then it is
said to be _zygomorphic_. Such flowers as Papilionaceae, Labiatae, are
examples. In contrast with this are _polysymmetrical_ or actinomorphic
flowers, which have a radial symmetry and can be divided by several
planes into several exactly similar portions; such are all regular,
symmetrical flowers. When the parts of any whorl are not equal to or
some multiple of the others, then the flower is _asymmetrical_. This
want of symmetry may be brought about in various ways. Alteration in the
symmetrical arrangement as well as in the completeness and regularity of
flowers has been traced to _suppression_ or the _non-development_ of
parts, _degeneration_ or imperfect formation, _cohesion_ or union of
parts of the same whorl, _adhesion_ or union of the parts of different
whorls, _multiplication_ of parts, and _deduplication_ (sometimes called
_chorisis_) or splitting of parts.
By _suppression_ or non-appearance of a part at the place where it ought
to appear if the structure was normal, the symmetry or completeness of
the flower is disturbed. This suppression when confined to the parts of
certain verticils makes the flower asymmetrical. Thus, in many
Caryophyllaceae, as _Polycarpon_ and _Holosteum_, while the calyx and
corolla are pentamerous, there are only three or four stamens and three
carpels; in _Impatiens Noli-me-tangere_ the calyx is composed of three
parts, while the other verticils have five; in labiate flowers there are
five parts of the calyx and corolla, and only four stamens; and in
_Tropaeolum pentaphyllum_ there are five sepals, two petals, eight
stamens and three carpels. In all these cases the want of symmetry is
traced to the suppression of certain parts. In the last-mentioned plant
the normal number is five, hence it is said that there are three petals
suppressed, as shown by the position of the two remaining ones; there
are two rows of stamens, in each of which one is wanting; and there are
two carpels suppressed. In many instances the parts which are afterwards
suppressed can be seen in the early stages of growth, and occasionally
some vestiges of them remain in the fully developed flower. By the
suppression of the verticil of the stamens, or of the carpels, flowers
become _unisexual_ or _diclinous_, and by the suppression of one or both
of the floral envelopes, monochlamydeous and achlamydeous flowers are
produced. The suppression of parts of the flower may be carried so far
that at last a flower consists of only one part of one whorl. In the
Euphorbiaceae we have an excellent example of the gradual suppression of
parts, where from an apetalous, trimerous, staminal flower we pass to
one where one of the stamens is suppressed, and then to forms where two
of them are wanting. We next have flowers in which the calyx is
suppressed, and its place occupied by one, two or three bracts (so that
the flower is, properly speaking, achlamydeous), and only one or two
stamens are produced. And finally, we find flowers consisting of a
single stamen with a bract. There is thus traced a _degradation_, as it
is called, from a flower with three stamens and three divisions of the
calyx, to one with a single bract and a single stamen.
_Degeneration_, or the transformation of parts, often gives rise either
to an apparent want of symmetry or to irregularity in form. In unisexual
flowers it is not uncommon to find vestiges of the undeveloped stamens
in the form of filiform bodies or scales. In double flowers
transformations of the stamens and pistils take place, so that they
appear as petals. In _Canna_, what are called petals are in reality
metamorphosed stamens. In the capitula of Compositae we sometimes find
the florets converted into green leaves. The limb of the calyx may
appear as a rim, as in some Umbelliferae; or as pappus, in Compositae
and _Valeriana_. In _Scrophularia_ the fifth stamen appears as a
scale-like body; in other Scrophulariaceae, as in _Pentstemon_, it
assumes the form of a filament, with hairs at its apex in place of an
anther.
_Cohesion_, or the union of parts of the same whorl, and _adhesion_, or
the growing together of parts of different whorls, are causes of change
both as regards form and symmetry. Thus in _Cucurbita_ the stamens are
originally five in number, but subsequently some cohere, so that three
stamens only are seen in the mature flower. Adhesion is well seen in the
_gynostemium_ of orchids, where the stamens and stigmas adhere. In
Capparidaceae the calyx and petals occupy their usual position, but the
axis is prolonged in the form of a gynophore, to which the stamens are
united.
_Multiplication_, or an increase of the number of parts, gives rise to
changes. We have already alluded to the interposition of new members in
a whorl. This takes place chiefly in the staminal whorl, but usually the
additional parts produced form a symmetrical whorl with the others. In
some instances, however, this is not the case. Thus in the
horse-chestnut there is an interposition of two stamens, and thus seven
stamens are formed in the flower, which is asymmetrical.
Parts of the flower are often increased by a process of _deduplication_,
or _chorisis_, i.e. the splitting of a part so that two or more parts
are formed out of what was originally one. Thus in Cruciferous plants
the staminal whorl consists of four long stamens and two short ones
(_tetradynamous_). The symmetry in the flower is evidently dimerous, and
the abnormality in the androecium, where the four long stamens are
opposite the posterior sepals, takes place by a splitting, at a very
early stage of development, of a single outgrowth into two. Many cases
of what was considered chorisis are in reality due to the development of
stipules from the staminal leaf. Thus in _Dicentra_ and _Corydalis_
there are six stamens in two bundles; the central one of each bundle
alone is perfect, the lateral ones have each only half an anther, and
are really stipules formed from the staminal leaf. Branching of stamens
also produces apparent want of symmetry; thus, in the so-called
polyadelphous stamens of Hypericaceae there are really only five stamens
which give off numerous branches, but the basal portion remaining short,
the branches have the appearance of separate stamens, and the flower
thus seems asymmetrical.
_Cultivation_ has a great effect in causing changes in the various parts
of plants. Many alterations in form, size, number and adhesion of parts
are due to the art of the horticulturist. The changes in the colour and
forms of flowers thus produced are endless. In the dahlia the florets
are rendered quilled, and are made to assume many glowing colours. In
pelargonium the flowers have been rendered larger and more showy; and
such is also the case with the _Ranunculus_, the auricula and the
carnation. Some flowers, with spurred petals in their usual state, as
columbine, are changed so that the spurs disappear; and others, as
_Linaria_, in which one petal only is usually spurred, are altered so as
to have all the petals spurred, and to present what are called
_pelorian_ varieties.
[Illustration:
FIG. 41.--Tetramerous monochlamydeous male flower of the Nettle
(_Urtica_).
FIG. 42.--Diagram to illustrate valvular or valvate aestivation, in
which the parts are placed in a circle, without overlapping or folding.
FIG. 43.--Diagram to illustrate induplicative or induplicate
aestivation, in which the parts of the verticil are slightly turned
inwards at the edges.]
As a convenient method of expressing the arrangement of the parts of the
flower, _floral formulae_ have been devised. Several modes of expression
are employed. The following is a very simple mode which has been
proposed:--The several whorls are represented by the letters S (sepals),
P (petals), St (stamens), C (carpels), and a figure marked after each
indicates the number of parts in that whorl. Thus the formula S5P5St5C5
means that the flower is perfect, and has pentamerous symmetry, the
whorls being isomerous. Such a flower as that of Sedum (fig. 33) would
be represented by the formula S5P5St_(5+5)C5, where St_(5+5) indicates
that the staminal whorl consists of two rows of five parts each. A
flower such as the male flower of the nettle (fig. 41) would be
expressed S4P0St4C0. When no other mark is appended the whorls are
supposed to be alternate; but if it is desired to mark the position of
the whorls special symbols are employed. Thus, to express the
superposition of one whorl upon another, a line is drawn between them,
e.g. the symbol S5P5 | St5C5 is the formula of the flower of
Primulaceae.
[Illustration:
FIG. 44.--Diagram to illustrate reduplicative or reduplicate
aestivation, in which the parts of the whorl are slightly turned
outwards at the edges.
FIG. 45.--Diagram to illustrate contorted or twisted aestivation, in
which the parts of the whorl are overlapped by each other in turn, and
are twisted on their axis.
FIG. 46.--Diagram to illustrate the quincuncial aestivation, in which
the parts of the flower are arranged in a spiral cycle, so that 1 and 2
are wholly external, 4 and 5 are internal, and 3 is partly external and
partly overlapped by 1.]
The manner in which the parts are arranged in the flower-bud with
respect to each other before opening is the _aestivation_ or
_praefloration_. The latter terms are applied to the flower-bud in the
same way as vernation is to the leaf-bud, and distinctive names have
been given to the different arrangements exhibited, both by the leaves
individually and in their relations to each other. As regards each leaf
of the flower, it is either spread out, as the sepals in the bud of the
lime-tree, or folded upon itself (conduplicate), as in the petals of
some species of _Lysimachia_, or slightly folded inwards or outwards at
the edges, as in the calyx of some species of clematis and of some
herbaceous plants, or rolled up at the edges (involute or revolute), or
folded transversely, becoming _crumpled_ or _corrugated_, as in the
poppy. When the parts of a whorl are placed in an exact circle, and are
applied to each other by their edges only, without overlapping or being
folded, thus resembling the valves of a seed-vessel, the aestivation is
_valvate_ (fig. 42). The edges of each of the parts may be turned either
inwards or outwards; in the former case the aestivation is _induplicate_
(fig. 43), in the latter case _reduplicate_ (fig. 44). When the parts of
a single whorl are placed in a circle, each of them exhibiting a torsion
of its axis, so that by one of its sides it overlaps its neighbour,
whilst its side is overlapped in like manner by that standing next to
it, the aestivation is _twisted_ or _contorted_ (fig. 45). This
arrangement is characteristic of the flower-buds of Malvaceae and
Apocynaceae, and it is also seen in Convolvulaceae and Caryophyllaceae.
When the flower expands, the traces of twisting often disappear, but
sometimes, as in Apocynaceae, they remain. Those forms of aestivation
are such as occur in cyclic flowers, and they are included under
_circular_ aestivation. But in spiral flowers we have a different
arrangement; thus the leaves of the calyx of _Camellia japonica_ cover
each other partially like tiles on a house. This aestivation is
_imbricate_. At other times, as in the petals of _Camellia_, the parts
envelop each other completely, so as to become _convolute_. This is also
seen in a transverse section of the calyx of _Magnolia grandiflora_,
where each of the three leaves embraces that within it. When the parts
of a whorl are five, as occurs in many dicotyledons, and the imbrication
is such that there are two parts external, two internal, and a fifth
which partially covers one of the internal parts by its margin, and is
in its turn partially covered by one of the external parts, the
aestivation is _quincuncial_ (fig. 46). This quincunx is common in the
corolla of Rosaceae. In fig. 47 a section is given of the bud of
_Antirrhinum majus_, showing the imbricate spiral arrangement. In this
case it will be seen that the part marked 5 has, by a slight change in
position, become overlapped by 1. This variety of imbricate aestivation
has been termed _cochlear_. In flowers such as those of the pea (fig.
40), one of the parts, the vexillum, is often large and folded over the
others, giving rise to _vexillary_ aestivation (fig. 48), or the carina
may perform a similar office, and then the aestivation is _carinal_, as
in the Judas-tree (_Cercis Siliquastrum_). The parts of the several
verticils often differ in their mode of aestivation. Thus, in Malvaceae
the corolla is contorted and the calyx valvate, or reduplicate; in St
John's-wort the calyx is imbricate, and the corolla contorted. In
Convolvulaceae, while the corolla is twisted, and has its parts arranged
in a circle, the calyx is imbricate, and exhibits a spiral arrangement.
In _Guazuma_ the calyx is valvate, and the corolla induplicate. The
circular aestivation is generally associated with a regular calyx and
corolla, while the spiral aestivations are connected with irregular as
well as with regular forms.
[Illustration:
FIG. 47.--Diagram to illustrate imbricated aestivation, in which the
parts are arranged in a spiral cycle, following the order indicated by
the figures 1, 2, 3, 4, 5.
FIG. 48.--Diagram of a papilionaceous flower, showing vexillary
aestivation.