[Illustration: FIG. 9.--Head of flowers (capitulum) of _Scabiosa
atropurpurea_. The inflorescence is simple and indeterminate, and the
expansion of the flowers centripetal, those at the circumference opening
first.]
Inflorescence.
There are two distinct types of inflorescence--one in which the flowers
arise as lateral shoots from a primary axis, which goes on elongating,
and the lateral shoots never exceed in their development the length of
the primary axis beyond their point of origin. The flowers are thus
always _axillary_. Exceptions, such as in cruciferous plants, are due to
the non-appearance of the bracts. In the other type the primary axis
terminates in a single flower, but lateral axes are given off from the
axils of the bracts, which again repeat the primary axis; the
development of each lateral axis is stronger than that of the primary
axis beyond its point of origin. The flowers produced in this
inflorescence are thus _terminal_. The first kind of inflorescence is
_indeterminate_, _indefinite_ or _axillary_. Here the axis is either
elongated, producing flower-buds as it grows, the lower expanding first
(fig. 8), or it is shortened and depressed, and the outer flowers expand
first (fig. 9). The expansion of the flowers is thus _centripetal_, that
is, from base to apex, or from circumference to centre.
[Illustration: FIG. 10.--Plant of _Ranunculus bulbosus_, showing
determinate inflorescence.]
The second kind of inflorescence is _determinate_, _definite_ or
_terminal_. In this the axis is either elongated and ends in a solitary
flower, which thus terminates the axis, and if other flowers are
produced, they belong to secondary axes farther from the centre; or the
axis is shortened and flattened, producing a number of separate floral
axes, the central one expanding first, while the others are developed in
succession farther from the centre. The expansion of the flowers is in
this case _centrifugal_, that is, from apex to base, or from centre to
circumference. It is illustrated in fig. 10, _Ranunculus bulbosus_; a'
is the primary axis swollen at the base in a bulb-like manner b, and
with roots proceeding from it. From the leaves which are radical
proceeds the axis ending in a solitary terminal flower f'. About the
middle of this axis there is a leaf or bract, from which a secondary
floral axis a" is produced, ending in a single flower f", less advanced
than the flower f'. This secondary axis bears a leaf also, from which a
tertiary floral axis a"' is produced, bearing an unexpanded solitary
flower f"'. From this tertiary axis a fourth is in progress of
formation. Here f' is the termination of the primary axis, and this
flower expands first, while the other flowers are developed
centrifugally on separate axes.
A third series of inflorescences, termed _mixed_, may be recognized. In
them the primary axis has an arrangement belonging to the opposite type
from that of the branches, or vice versa. According to the mode and
degree of development of the lateral shoots and also of the bracts,
various forms of both inflorescences result.
Amongst indefinite forms the simplest occurs when a lateral shoot
produced in the axil of a large single foliage leaf of the plant ends in
a single flower, the axis of the plant elongating beyond, as in
_Veronica hederifolia_, _Vinca minor_ and _Lysimachia nemorum_. The
flower in this case is _solitary_, and the ordinary leaves become bracts
by producing flower-buds in place of leaf-buds; their number, like that
of the leaves of this main axis, is indefinite, varying with the vigour
of the plant. Usually, however, the floral axis, arising from a more or
less altered leaf or bract, instead of ending in a solitary flower, is
prolonged, and bears numerous bracteoles, from which smaller peduncles
are produced, and those again in their turn may be branched in a similar
way. Thus the flowers are arranged in groups, and frequently very
complicated forms of inflorescence result. When the primary peduncle or
floral axis, as in fig. 8, is elongated, and gives off pedicels, ending
in single flowers, a _raceme_ is produced, as in currant, hyacinth and
barberry. If the secondary floral axes give rise to tertiary ones, the
raceme is branching, and forms a _panicle_, as in _Yucca gloriosa_. If
in a raceme the lower flower-stalks are developed more strongly than the
upper, and thus all the flowers are nearly on a level, a _corymb_ is
formed, which may be simple, as in fig. 11, where the primary axis a'
gives off secondary axes a", a", which end in single flowers; or
branching, where the secondary axes again subdivide. If the pedicels are
very short or wanting, so that the flowers are sessile, a _spike_ is
produced, as in _Plantago_ and vervain (_Verbena officinalis_) (fig.
12). If the spike bears unisexual flowers, as in willow or hazel (fig.
13), it is an _amentum_ or _catkin_, hence such trees are called
_amentiferous_; at other times it becomes succulent, bearing numerous
flowers, surrounded by a sheathing bract or spathe, and then it
constitutes a _spadix_, which may be simple, as in _Arum maculatum_
(fig. 14), or branching as in palms. A spike bearing female flowers
only, and covered with scales, is a _strobilus_, as in the hop. In
grasses there are usually numerous sessile flowers arranged in small
spikes, called _locustae_ or _spikelets_, which are either set closely
along a central axis, or produced on secondary axes formed by the
branching of the central one; to the latter form the term panicle is
applied.
[Illustration:
FIG. 11.--Corymb of _Cerasus Mahaleb_, terminating an abortive branch,
at the base of which are modified leaves in the form of scales, e. a',
Primary axis; a", secondary axes bearing flowers; b, bract in the axils
of which the secondary axes arise.
FIG. 12.--Spike of Vervain (_Verbena officinalis_), showing sessile
flowers on a common rachis. The flowers at the lower part of the spike
have passed into fruit, those towards the middle are in full bloom, and
those at the top are only in bud.
FIG. 13.--Amentum or catkin of Hazel (_Corylus Avellana_), consisting of
an axis or rachis covered with bracts in the form of scales, each of
which covers a male flower, the stamens of which are seen projecting
beyond the scale. The catkin falls off in a mass, separating from the
branch by an articulation.]
[Illustration: (From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.)
FIG. 14.--Spadix of _Arum maculatum_. (After Wossidlo.) a, Female
flowers; b, male flowers; c, hairs representing sterile flowers.]
[Illustration: FIG. 15.--Compound umbel of Common Dill (_Anethum
graveolens_), having a primary umbel a, and secondary umbels b, without
either involucre or involucel.]
If the primary axis, in place of being elongated, is contracted, it
gives rise to other forms of indefinite inflorescence. When the axis is
so shortened that the secondary axes arise from a common point, and
spread out as _radii_ of nearly equal length, each ending in a single
flower or dividing again in a similar radiating manner, an _umbel_ is
produced, as in fig. 15. From the primary floral axis a the secondary
axes come off in a radiating or umbrella-like manner, and end in small
umbels b, which are called _partial umbels_ or _umbellules_. This
inflorescence is seen in hemlock and other allied plants, which are
hence called umbelliferous. If there are numerous flowers on a
flattened, convex or slightly concave receptacle, having either very
short pedicels or none, a _capitulum_ (head) is formed, as in
dandelion, daisy and other composite plants (fig. 2), also in scabious
(fig. 9) and teazel. In the American button-bush the heads are globular,
in some species of teazel elliptical, while in scabious and in composite
plants, as sunflower, dandelion, thistle, centaury and marigold, they
are somewhat hemispherical, with a flattened, slightly hollowed, or
convex disk. If the margins of such a receptacle be developed upwards,
the centre not developing, a concave receptacle is formed, which may
partially or completely enclose a number of flowers that are generally
unisexual. This gives rise to the peculiar inflorescence of _Dorstenia_,
or to that of the fig (fig. 6), where the flowers are placed on the
inner surface of the hollow receptacle, and are provided with
bracteoles. This inflorescence has been called a _hypanthodium_.
Lastly, we have what are called _compound indefinite_ inflorescences. In
these forms the lateral shoots, developed centripetally upon the primary
axis, bear numerous bracteoles, from which floral shoots arise which may
have a centripetal arrangement similar to that on the mother shoot, or
it may be different. Thus we may have a group of racemes, arranged in a
racemose manner on a common axis, forming a raceme of racemes or
compound raceme, as in _Astilbe_. In the same way we may have compound
umbels, as in hemlock and most Umbelliferae (fig. 15), a compound spike,
as in rye-grass, a compound spadix, as in some palms, and a compound
capitulum, as in the hen-and-chickens daisy. Again, there may be a
raceme of capitula, that is, a group of capitula disposed in a racemose
manner, as in _Petasites_, a raceme of umbels, as in ivy, and so on, all
the forms of inflorescence being indefinite in disposition. In
_Eryngium_ the shortening of the pedicels changes an umbel into a
capitulum.
The simplest form of the definite type of the inflorescence is seen in
_Anemone nemorosa_ and in gentianella (_Gentiana acaulis_), where the
axis terminates in a single flower, no other flowers being produced upon
the plant. This is a _solitary terminal_ inflorescence. If other flowers
were produced, they would arise as lateral shoots from the bracts below
the first-formed flower. The general name of _cyme_ is applied to the
arrangement of a group of flowers in a definite inflorescence. A
_cymose_ inflorescence is an inflorescence where the primary floral axis
before terminating in a flower gives off one or more lateral unifloral
axes which repeat the process--the development being only limited by the
vigour of the plant. The floral axes are thus centrifugally developed.
The cyme, according to its development, has been characterized as
_biparous_ or _uniparous_. In fig. 16 the biparous cyme is represented
in the flowering branch of _Cerastium_. Here the primary axis t ends in
a flower, which has passed into the state of fruit. At its base two
leaves are produced, in each of which arise secondary axes t' t', ending
in single flowers, and at the base of these axes a pair of opposite
leaves is produced, giving rise to tertiary axes t" t", ending in single
flowers, and so on. The term _dichasium_ has also been applied to this
form of cyme.
In the natural order Carophyllaceae (pink family) the dichasial form of
inflorescence is very general. In some members of the order, as
_Dianthus barbatus_, _D. carthusianorum_, &c., in which the peduncles
are short, and the flowers closely approximated, with a centrifugal
expansion, the inflorescence has the form of a contracted dichasium, and
receives the name of _fascicle_. When the axes become very much
shortened, the arrangement is more complicated in appearance, and the
nature of the inflorescence can only be recognized by the order of
opening of the flowers. In Labiate plants, as the dead-nettle
(_Lamium_), the flowers are produced in the axil of each of the foliage
leaves of the plant, and they appear as if arranged in a simple whorl of
flowers. But on examination it is found that there is a central flower
expanding first, and from its axis two secondary axes spring bearing
solitary flowers; the expansion is thus centrifugal. The inflorescence
is therefore a contracted dichasium, the flowers being sessile, or
nearly so, and the clusters are called _verticillasters_ (fig. 17).
Sometimes, especially towards the summit of a dichasium, owing to the
exhaustion of the growing power of the plant, only one of the bracts
gives origin to a new axis, the other remaining empty; thus the
inflorescence becomes unilateral, and further development is arrested.
In addition to the dichasial form there are others where more than two
lateral axes are produced from the primary floral axis, each of which in
turn produces numerous axes. To this form the terms _trichasial_ and
_polychasial cyme_ have been applied; but these are now usually
designated _cymose umbels_. They are well seen in some species of
_Euphorbia_. Another term, _anthela_, has been used to distinguish such
forms as occur in several species of _Luzula_ and _Juncus_, where
numerous lateral axes arising from the primary axis grow very strongly
and develop in an irregular manner.
[Illustration: (From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.)
FIG. 16.--Cymose inflorescence (dichasium) of _Cerastium collinum_;
t-t"", successive axes. (After Duchartre.)]
[Illustration: FIG. 17.--Flowering stalk of the White Dead-nettle
(_Lamium album_). The bracts are like the ordinary leaves of the plant,
and produce clusters of flowers in their axil. The clusters are called
verticillasters, and consist of flowers which are produced in a
centrifugal manner.]
In the uniparous cyme a number of floral axes are successively developed
one from the other, but the axis of each successive generation, instead
of producing a pair of bracts, produces only one. The basal portion of
the consecutive axes may become much thickened and arranged more or less
in a straight line, and thus collectively form an apparent or false
axis or _sympodium_, and the inflorescence thus simulates a raceme. In
the true raceme, however, we find only a single axis, producing in
succession a series of bracts, from which the floral peduncles arise as
lateral shoots, and thus each flower is on the same side of the floral
axis as the bract in the axil of which it is developed; but in the
uniparous cyme the flower of each of these axes, the basal portions of
which unite to form the false axis, is situated on the opposite side of
the axis to the bract from which it apparently arises (fig. 18). The
bract is not, however, the one from which the axis terminating in the
flower arises, but is a bract produced upon it, and gives origin in its
axil to a new axis, the basal portion of which, constituting the next
part of the false axis, occupies the angle between this bract and its
parent axis--the bract from which the axis really does arise being
situated lower down upon the same side of the axis with itself. The
uniparous cyme presents two forms, the _scorpioid_ or _cicinal_ and the
_helicoid_ or _bostrychoid_.
[Illustration:
FIG. 18.--Helicoid cyme of a species of _Alstroemeria_. a1, a2, a3, a4,
&c., separate axes successively developed in the axils of the
corresponding bracts b2, b3, b4, &c., and ending in a flower f2, f3, f4,
&c. The whole appears to form a simple raceme of which the axes form the
internodes.
FIG. 19.--Scorpioidal or cicinal cyme of Forget-me-not (_Myosotis
palustris_).
FIG. 20.--Diagram of definite floral axes a, b, c, d, e, &c.
FIG. 21.--Flowering stalk of Ragwort (_Senecio_). The flowers are in
heads (capitula), and open from the circumference inwards in an
indefinite centripetal manner. The heads of flowers, on the other hand,
taken collectively, expand centrifugally--the central one a first.]
In the scorpioid cyme the flowers are arranged alternately in a double
row along one side of the false axis (fig. 19), the bracts when
developed forming a second double row on the opposite side; the whole
inflorescence usually curves on itself like a scorpion's tail, hence its
name. In fig. 20 is shown a diagrammatic sketch of this arrangement. The
false axis, a b c d, is formed by successive generations of unifloral
axes, the flowers being arranged along one side alternately and in a
double row; had the bracts been developed they would have formed a
similar double row on the opposite side of the false axis; the whole
inflorescence is represented as curved on itself. The inflorescence in
the family Boraginaceae are usually regarded as true scorpioid cymes.
In the helicoid cyme there is also a false axis formed by the basal
portion of the separate axes, but the flowers are not placed in a double
row, but in a single row, and form a spiral or helix round the false
axis. In _Alstroemeria_, as represented in fig. 18, the axis a1 ends in
a flower (cut off in the figure) and bears a leaf. From the axil of this
leaf, that is, between it and the primary axis a1 arises a secondary
axis a2, ending in a flower f2, and producing a leaf about the middle.
From the axil of this leaf a tertiary floral axis a3, ending in a flower
f3, takes origin. In this case the axes are not arranged in two rows
along one side of the false axis, but are placed at regular intervals,
so as to form an elongated spiral round it.
Compound definite inflorescences are by no means common, but in
_Streptocarpus polyanthus_ and in several calceolarias we probably have
examples. Here there are _scorpioid cymes of pairs of flowers_, each
pair consisting of an older and a younger flower.
Mixed inflorescence.
Forms of inflorescence occur, in which both the definite and indefinite
types are represented--_mixed_ inflorescences. Thus in Composite plants,
such as hawk weeds (_Hieracia_) and ragworts (_Senecio_, fig. 21), the
_heads_ of flowers, taken as a whole, are developed centrifugally, the
terminal head first, while the _florets_, or small flowers on the
receptacle, open centripetally, those at the circumference first. So
also in Labiatae, such as dead-nettle (_Lamium_), the different whorls
of inflorescence are developed centripetally, while the florets of the
verticillaster are centrifugal. This mixed character presents
difficulties in such cases as Labiatae, where the leaves, in place of
retaining their ordinary form, become bracts, and thus might lead to the
supposition of the whole series of flowers being one inflorescence. In
such cases the cymes are described as spiked, racemose, or panicled,
according to circumstances. In _Saxifraga umbrosa_ (London-pride) and in
the horse-chestnut we meet with a raceme of scorpioid cymes; in
sea-pink, a capitulum of contracted scorpioid cymes (often called a
glomerulus); in laurustinus, a compound umbel of dichasial cymes; a
scorpioid cyme of capitula in _Vernonia scorpioides_. The so-called
catkins of the birch are, in reality, spikes of contracted dichasial
cymes. In the bell-flower (_Campanula_) there is a racemose uniparous
cyme. In the privet (_Ligustrum vulgare_) there are numerous racemes of
dichasia arranged in a racemose manner along an axis; the whole
inflorescence thus has an appearance not unlike a bunch of grapes, and
has been called a _thyrsus_.