Biology Today — January 2018

• Capitulums are of two types viz., homogamous and heterogamous.
  
  
  • Homogamous head – In homogamous heads, all florets are alike in structure and function. They are bisexual and
    either tubular as in Vernonia and Ageratum or ligulate as in Cichorium and Taraxacum.
  
  
  • Heterogamous head – The head/capitulum consists of different types of florets, e.g., Helianthus. In heterogamous
    heads, ray florets are towards the periphery and disc florets are at the centre of the inflorescence. The disc florets
    are actinomorphic and bisexual while the ray florets are ligulate and generally pistillate or neutral. The inflorescence
    becomes conspicuous and attractive due to the brightly coloured ray florets.
  
  
  
  


• Head inflorescence (or capitulum) is considered to be the highly evolved type of inflorescence because of following reasons:


• There is economy of biological materials.


• There is enhancement of attraction due to aggregation of florets.


• Maximum protection of flowers is ensured.


• A single visit of the insect can pollinate many flowers.

Compound racemose

• An indefinite or indeterminate inflorescence in which
  the peduncle is branched repeatedly once or twice
  in a racemose fashion.


• It can be panicle, (e.g., Delonix, Cassia fistula),
  compound corymb, (e.g., Brassica oleracea v a r. botrytis
  (cauliflower), compound umbel, (e.g., Foeniculum
  vulgare, Coriandrum sativum etc.), compound spike,
  (e.g., Amaranthus, wheat), compound spadix, (e.g.,
  Cocos, date palm) or compound capitulum, (e.g.,
  Echinops).

Biparous or Dichasial

cyme

The main axis terminates

in a flower. A pair of lateral

branches arise below the

terminal flower which too,

terminate into a flower. The

process is repeated several

times, e.g., Spergula, Stellaria,

Dianthus, jasmine, etc. Fig.: Dichasial cyme of Dianthus

Multiparous or

Polychasial cyme

More than two lateral

branches continue the

growth of inflorescence

when the parent axis

ends in a flower, e.g.,

Hamelia, Calotropis.

Fig.: Polychasial cyme of

Hamelia

Fig.: Cymose head of Acacia

Bipinnate

leaf

Subsessile

flowers

Cymose head

Stem

Stipular spine

Cymose head

Sessile or

subsessile

flowers are borne

centrifugally

around a

receptacle,

e.g., Albizzia,

Anthocephalus

cadamba, Acacia.

Scapigerous head

The leafless flowering axis known as scape bears clusters of

flowers that form a head which is covered by spaths, e.g.,

Fig.: Scapigerous Allium cepa.

Helicoid cyme

All the flowers are

borne on the same side

forming a sort of helix,

e.g., Drosera, Begonia,

Myosotis. Fig.: Helicoid cyme

of Myosotis

Scorpioid cyme

Flowers are

alternately borne

on both the sides,

e.g., Tecoma,

Ranunculus,

Heliotropium. Fig.: of Scorpioid cyme Freesia

Uniparous or monochasial cyme

The terminal bud of main axis ends in

flower. A single lateral branch pushes

it to one side but also itself ends in a

flower. It is of two types:

Cymose

It is a determinate inflorescence where main axis (peduncle) terminates into a flower and further growth takes place by lateral branches. The

arrangement of flowers is basipetal (i.e., younger towards the base and older towards the apex) or centrifugal, (i.e., younger towards the

periphery and older towards the centre). It is of following types:

Fig.: Compound spadix of coconut Fig.: Compound umbel

of Chaerophyllum