Biology Today — January 2018

Racemose

• An indeterminate type of inflorescence which shows indefinite growth and bears a number of flowers due to the presence
  of active growing point. The arrangement of flowers on peduncle is either acropetal (i.e., younger towards the apex and
  older towards the base) or centripetal (i.e., younger towards the centre and older towards the periphery).


• There can be simple racemose or compound racemose.

Simple racemose

(Peduncle is unbranched)

Capitulum or Racemose head

The flattened peduncle called as

receptacle bears numerous small

sessile flowers called as florets

arranged in a centripetal fashion.

The inflorescence is surrounded

by one or more whorls of bracts

called involucre. Florets can be ray

florets (unisexual, ligulate, sessile,

strap shaped, petaloid, zygomorphic

flowers arranged on the periphery of

receptacle) or disc florets (tubular,

bisexual, sessile, actinomorphic

flowers occupying central position

on the receptacle). E.g., Zinnia,

sunflower, etc.

Fig.:Vertical section of

capitulum of sunflower

Spikelets

Spikelets are small

and few flowered

spikes which are

surrounded at

the base by two

scales or glumes,

e.g., members of

Family Gramineae

(= Poaceae).

Spike

Sessile flowers arranged in

acropetal succession, e.g.,

Callistemon, Achyranthes.

Fig.: Spike of Achyranthes

Umbel

All the pedicellate flowers arise

from a single point in a centripetal

fashion. The peduncle is very much

reduced, e.g., Hydrocotyle, Prunus.

Fig.: Umbel of Prunus

Catkin

Pendulous unisexual spike which

bears naked pistillate or staminate

flowers, e.g., Morus alba, Populus,

etc.

Fig.: Catkin of mulberry

Spadix

Spike with fleshy peduncle and

having both male and female flowers.

It is surrounded by a large green or

coloured bract called spathe. The two

types of flowers are separated by

downwardly directed sterile hair or

neuter flowers, e.g., Colocasia, Arum.

Spathe

Sterile portion

of spadix

Male flowers

Neuter flowers

Female flowers

Fig.: Spadix of Colocasia

Corymbose raceme

The young flowers appear to be

arranged like a corymb but in mature

state, the longer pedicels of the lower

flowers do not bring them to the level

of upper ones, e.g., mustard.

Fig.: Corymbose raceme

of mustard

Corymb

Older flowers have longer pedicel

and younger flowers have shorter

pedicel. As a result, all the

flowers are brought to the same

Long pedicel Flower level, e.g., Iberis.

Peduncle

Fig.: Corymb of Iberis

Typical raceme

Pedicellate bisexual flowers

arranged in acropetal succession,

e.g., larkspur, Lupinus, etc.

Fig.: Typical raceme

of larkspur

Flower

Bract

Feathery stigma

Gynoecium

Androecium

Second empty

glume

Lodicule

Superior palea

Rachilla (axis)

Inferior palea or

lemma

First empty glume

Fig.: Spikelets in Family Gramineae