Inflorescence

• Flowers are the organs of sexual reproduction in
higher plants.
• They are, in fact, modified shoots, specialized for
sexual reproduction.
• They may occur either singly (solitary flowers) or in
bunches on a common axis.
• Solitary flowers may be axillary or terminal.
• Axillary flower is a modified axillary bud, and
terminal flower is a modified terminal bud.
• An axis, bearing a bunch of flowers is known as
inflorescence. It may be axillary or terminal in
position.

• The axial stalk or the main axis of the inflorescence is
called rachis or peduncle.
• The individual flowers of an inflorescence are attached
to the peduncle by stalks of their own, known as the
pedicels.
• In some cases, flowers are stalkless and are sessile
flowers, e.g., Achyranthes.
• In some plants like Lotus, and Onion, a long peduncle
arises from the underground stem and bears flowers.
• Such a peduncle is called scape.
• It may bear a single flower as in Lotus, or a cluster of
flowers as in Onion.
• In Agave, it is branched and the branches bear flowers.

• In most plants, pedicel arise from the axil of a very
small greenish, or brownish leaf-like structure on the
peduncle, called bract.
• Bracts are highly reduced and modified leaves and their
arrangement on the peduncle is the same as that of
leaves on a vegetative shoot.
• When the peduncle is branched, the branches arise
from the axils of similar bracts.
• Bracts differ from leaves in some respects, such as
colour, size and shape.
• Besides these bracts, there may be one or two still
further reduced bract-like structures on the pedicel,
known as the bracteoles.
• Flowers having bracts and bracteoles are described as
bracteate and brocteolate respectively.
• If these structures are absent, then the flowers are said
to be ebracteate and ebracteolate.

Kinds of bracts
• Usually, flowers arise from the axils of bracts. But, in
some cases, flowers may not be seen in the axils of
bracts.
• For example, in pineapple (Ananas comosus), there are
a number of bracts without flower buds in their axils,
seen at the top of the inflorescence.
• Such bracts are said to be empty or sterile.
• The normal function of the bracts is the protection of
the flower bud.
• But, in some flowers, the bract becomes brightly
coloured to attract insects.
• Such coloured bracts are seen in Bougainvillea and
Poinsettia.

• In most plants, bracts are very small in size, and their leafy
nature is not very clear.
• But, in some plants like Adhathoda and Justicia, they are
exactly like foliage leaves in appearance.
• Such bracts are called leafy bracts.
• Sometimes, a bract of large size surrounds the entire
inflorescence and protects the flowers when young.
• Such a bract is known as spathe, e.g.. Cocos nucifera.
• In grasses, bracts are known as glumes.
• In condensed inflorescence, as in sunflower, a number of
bracts form an outer covering of the whole inflorescence.
• This covering is called are called involucral, and the bracts are
called involucral bracts.
• At times, a number of bracteoles are found in a whorl on the
pedicel immediately below the calyx (e.g. Hibiscus, cotton).
• This whorl is called epicalyx.

Classification of inflorescence
• Based on the nature of the peduncle and the
arrangement of flowers three main types of
inflorescence can be recognized, namely
• Racemose
• Cymose
• Special.

• Also called indefinite, or indeterminate inflorescence.
• In it, the main axis does not end in a flower and is
capable of unlimited (indefinite or indeterminate)
growth.
• Flowers are arranged on either side of it in acropetal
succession.
• Sometimes, the main axis may give rise to a number of
branches in acropetal succession and the flowers are
arranged on these branches, in a similar manner.
• At times, the main axis may become very short and the
flowers form a cluster with the older flowers at the
periphery and the youngest one at centre.
• Racemose inflorescence is of several kinds.
• The commonest ones among them are raceme,
panicle, corymb, spike, spadix, umbel, capitulum and
catkin.

I. Raceme
• This is the simplest type of racemose inflorescence.
• It has a main peduncle on which a number of
flowers are borne in acropetal succession.
• All the flowers are pedicellate,
• e.g, Glyricidia, Crotalaria.

II. Panicle
• Basically this is a branched raceme.
• The main peduncle gives rise to a number of
secondary peduncles in acropetal succession and
flowers arise on the secondar peduncles in
acropetal succession.
• In yet other instances, secondary peduncles may
branch and the flowers arise on the tertiary
peduncles
• e.g., mango, teak.

III. Corymb
• This is a type of raceme in which the peduncle is very
short and it bears acropetally arranged pedicellate
flowers.
• The pedicels of the flowers have different lengths.
• The oldest (basal) flowers have longest pedicels, and
the youngest (terminal) ones have shortest pedicels.
• As a result, all the flowers of the inflorescence stand
more or less at the same level, e.g.. Caesalpinia,
Gynandropsis.
• In many cases, the main axis of the corymb elongates
after flowering so that the corymb becomes a raceme.

IV. Spike
• This resembles a raceme except for the sessile nature
of flowers.
• e.g., Achyranthes, Celosia.
• In some cases, the main axis of the spike is branched,
and each branches resembles a spike.
• Such an inflorescence is called compound spike,
• e.g., Aerva.
• The small spikes found in grasses are called spikelets.

v. Spadix
• This is the racemose inflorescence in which the
peduncle is thick, fleshy and coloured.
• Flowers are small, sessile and unisexual and they sink
into the depressions on the axis called spodices.
• The whole inflorescence is covered by a leathery bract
called spathe.
• Generally, female flowers are basal and male flowers
terminal in position.
• Spadix inflorescence is characteristic of monocots.
• e.g. Colocasia, Cocos, Areca, Musa, Amorphophallus.
• In Cocos and Areca the inflorescence is branched and
the spathe is boat-like.

VI. Umbel
• This is the recemose inflorescence in which the main peduncle is short and bears at
its tip a whorl of bracts.
• From the axil of each bract, arises a pedicellate flower.
• flowers are arranged in centripetal succession
• (older flowers towards the towards the margin and younger ones towards the
centre).
• As in corymb, here also all the flowers are in the same level.
• But in corymb, the pedicels are different lengths and start from different length of
the peduncle.
• In umbel, all the pedicles are of equal lengths and all of them start from the apex of
the peduncle.
• The recemose nature of the inflorescence is evident from the centripetal
arrangement of flowers.
• The main peduncle may be simple, or it may bear a cluster of flowers at the apex as
in onion.
• In compound umbel, the main peduncle may give rise to secondary peduncles, and
Pedicel pedicels start their tips in an umbellate manner as in coriander and carrot.
• The whorl of bracts below the cluster of pedicels forms an involucre.

VII. Capitulum or head
• This racemose inflorescece is peculiar in that the main peduncle gets
flattened out at the apex into a slightly convex platform-like structure,
called receptacle.
• On the surface of the receptacle numerous small and sessile flowers are
arranged in centripetal succession.
• At the base of the receptacle, there are numerous green and triangular
bracts, which form an involucre.
• The individuals flowers on a head inflorescence are called florets.
• The florets arranged along the periphery of the receptacle are called ray
florets and those arranged along the centre are called disc florets.
• Ray florets are usually zygomorphic and female, whereas disc florets are
actinomorphic and bisexual.
• In some plants, such as Mimosa, Gomphrena, etc., the receptacle is not
flat, but is a swollen and spherical structure and the flowers are arranged all
over it.
• The whole inflorescence looks like a small badminton ball, and is known as a
globose head.

VIII. Catkin or amentum
• Catkin is a pendulous type of inflorescence,
similar to spike, with sessile and unisexual
flowers.
• The axis is thin and weak.
• e.g.. Acalypha, Morus, Artocarpus.

• Cymose inflorescence corresponds to the sympodial
branching of the vegetative shoot.
• Here, the peduncle has only limited (determinate) growth,
and it terminates in a flower.
• This terminal flower blocks the further growth of the
peduncle.
• Subsequent flowers arise from the axils of the bracteoles of
the first formed flowers.
• In racemose inflorescence, the bracteoles of the flowers do
not bear flowers.
• But here, the bracteoles of the first formed flower become
the bracts of the second set of flowers.
• Peduncle may have lateral branches which also end in
flowers.
• In most cases, flowers are arranged in basipetal succession so
that basal flowers are younger and terminal ones are older.

• There are five main kinds of cymose inflorescence,
namely
• Solitary cyme
• Simple cyme
• Dichasial or biparous cyme
• Monochasial or uniparous cyme
• Polychasial or multiparous cyme.

I. Solitary cyme
• This is the simplest type of cymose inflorescence in
which there is only a single flower.
• The flower may be terminal on the primary axis, or
axillary in position.
• The former is called terminal solitary cyme
• (e.g., Gossypium),
• and the latter is called axillary solitary cyme
• (e.g., Hibiscus, Datura).

II. Simple cyme or cymule
• This is a cluster of three pedicellate flowers, one terminal and
the others lateral and axillary (e.g., Jasminum).
• In simple cyme, the main axis ends in a flower.
• This flower has two bracteoles and from the axil of each
bracteole, a flower develops.
• Thus, the middle flower is the oldest one and it will open first,
while the lateral ones are younger.
• The development of the flowers is centrifugal, i.e., from the
centre to periphery.
• The arrangement is just the opposite of what is seen in
racemose cluster, where the older flowers are at the
periphery and the younger ones in the centre of the cluster.

III. Dichasial (biparous) cyme
• This corresponds to the dichotomous branching of vegetative
shoots.
• In this case, the main axis ends in a flower. This flower has
opposite bracteoles.
• A flower arises from the axil of each bracteole.
• Each of these flowers again has two bracteoles, which become the
bracts of the third set of flowers.
• This results in the formation of a regular cluster developing in a
symmetrical manner.
• The cluster can easily be divided into small groups of three flowers
each.
• In each group the centrifugal development can be seen clearly.
• Dichasial cyme is found in Clerodendron, Ixora, Dianthus, etc.

IV. Monochasial (uniparous) cyme
• This is the cymose inflorescence in which the peduncle ends in a single
flower.
• It differs from dichasial cyme in that the main peduncle produces only a
single secondary peduncle, instead of two.
• The secondary peduncle, in turn, produces a single tertiary peduncle.
• Of the two bracteoles of each flower only one is developed, while the other
one is suppressed.
• When this suppression occurs only on one side, lateral branches and
flowers appear on one side only.
• This type of is called monochasial helicoid cyme (e.g., Hamelia).
• If branches and flowers develop alternately on both the sides of the
pedencle, the inforescence is called monochasial scorpiod cyme
(e.g., Heliotropium).
• In helicoid cyme, all flowers are on the same side of the axis while in
scorpioid cyme the flowers alternate on the axis.

Monochasial scorpioid cyme -
Heliotropium inflorescence

V. Polychasial (multiparous) cyme
• This is the cyme in which more than two secondary
and tertiary peduncles are produced (e.g.,
Calotropis, Mollugo.)

Differences between racemose and
cymose inflorescences
Racemose inflorescence Cymose inflorescence
1. Peduncle has unlimited
growth
Peduncle has only limited
growth
2. Peduncle does not end in
a flower
Peduncle ends in a flower.
3. Flowers develop in
acropetal or centrifugal
succession.
Flowers develop in basipetal
or centripetal succession

C. Special types of
inflorescence

• These are strictly neither racemose nor cymose.
• At the same time, they are modified forms of the
cymose type. They include
• Hypanthodium
• Verticillaster
• Cyathium
• Thyrsus.

I. Hypanthodium
• This is the condense type of cymose inflorescence, more
precisely a modifi-cation of the capitulum, characteristic of
Ficus.
• Here, the peduncle forms a hollow, fleshy and flask-shaped
receptacle.
• The receptacele has a terminal opening, called ostiole, for the
entry of insect pollinators.
• The ostiole is guarded by numerous incurved hairs.
• Many small, sessile and unisexual flowers are arranged along
the inner walls of the receptacle.
• The flowers include male female, and sterile flowers.
• Male flowers are seen near the ostiole, sterile flowers in the
middle and the female flowers towards the base.

II. Verticillaster
• This is a compound inflorescence, consisting of two axillary
opposite cymose inflorescences developing one on either side
and meeting around the stem.
• It is typically found in plants with opposite leaves, and is
characteristic of the family Labiatae (e.g., Leucas).
• In this case, two clusters of sessile and opposite cymose
inflorescences develop from the axis of opposite leaves as a
dichasium.
• The subsequent development is of a monochasial scorpioid
nature.
• During this, they bend round the stem and node and meet
together.
• So, at each node, there is a cluster of flowers completely
surrounding the stem.

III. Cyathium
• This is a highly condensed type of cymose inflorescence.
• It look a single flower, with an involucral cup, formed by the
fusion of bracts.
• Peduncle is very much reduced.
• Terminally, it bears a single, highly reduced and central female
flower, surrounded by five scorpioid cymes of reduced male
flowers.
• Female flower is long-stalked and naked, and male flowers
are short - stalked and naked and each of them is reduced to
a stamen.
• Cyathium is often described as "cyme of cymes".
• It is the characteric inflorescence of the genera Euphorbia,
• e.g.. Acalypha, Poinsettia, etc.

Cyathium inflorescence-Euphorbia
HABIT LONGITUDINAL SECTION

IV. Thyrsus
• This is a mixed inflorescence, composed of a number of
simple cymes arranged in a racemose manner.
• There is a main peduncle giving rise to secondary
inflorescences on the axils of bracts in acropetal
succession.
• Each secondary peduncle has flower two bracteoles
• From the axils of each bracteole a single flower arises.
• Thus each secondary peduncle forms a simple cyme of
three flowers of which the central one is the oldest,
• e.g.,Ocimum.

Inflorescence

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