Biology Today - February 2018

Floral Symmetry

• The arrangement of floral organs around the axis of a flower is known as floral symmetry. The shoot on which the flower
  is borne is called mother axis. The side of the mother axis is posterior while the side of the bract is anterior. In terminal
  flowers, a distinction into anterior and posterior sides is absent. Various types of flower are summarised as follows.
  Zygomorphic flower
  It can be divided into two equal halves
  by only one vertical plane, HJ, pea,
  2FLPXP.

Actinomorphic flower

It can be divided by any vertical plane

into two equal and similar halves, HJ

mustard, &DWKDUDQWKXVURVHXV.

Asymmetrical flower

It cannot be divided into

two equal halves by any

vertical plane, HJ&DQQD.

On the basis

of floral

symmetry

Heteromerous

Number in all the whorls

is neither the same nor

any multiple.

Isomerous

Whorls have equal number of parts or number in one whorl is multiple

of the number of other whorl.

It can be bimerous (floral parts in 2 or multiple of 2), trimerous (floral

parts in 3 or multiple of 3), tetramerous (floral parts in 4 or multiple of

4) or pentamerous (floral parts in 5 or multiple of 5).

On the basis

of number of

floral parts

Floral Phyllotaxy

• Floral organs are borne on the thalamus either in spirals (acyclic or spiral, HJ., 0DJQROLD1\PSKDHD), whorls (cyclic,
  HJ., 3HWXQLD) or with some organs in spirals and other organs in whorls (spirocyclic or hemicyclic, HJ., 5DQXQFXOXV).

BRACT

• Bract is a small leaf like structure present on the peduncle which bears a flower in its axil. The floral buds are usually protected
  by the bracts. Flower with a bract is described as bracteate and the flower without a bract is known as ebracteate.

Involucre

A large number of bracts

form compact one or more

whorls around a flower or an

inflorescence, HJ., Sunflower.

Epicalyx

One or more whorls of

bracteoles are found at

the base of calyx, HJ

China rose, strawberry.

Petaloid

Petals-like large,

coloured bracts,

HJ%RXJDLQYLOOHD

(XSKRUELD

Spathe

A large modified

bract which

encloses spadix

inflorescence

totally or partially,

HJ$ORFDVLD

0XVDetc

Types

of bract

Foliaceous

Leaf like,

expanded

green

bract, HJ

$GKDWRGD

*URVVDQGUD

Glumes

The bracts found

on the rachilla

of spikelets. They

may be sterile

glumes or fertile

glumes, HJ

2U\]DVDWLYD.

Scaly

A membranous

small bract

present at

the base of

involucral

bract, HJ

Sunflower.^

CALYX

• Calyx is the outermost whorl made up of sepals. It may be polysepalous (with free sepals) or gamosepalous (with fused sepals).

Deciduous

Sepals fall off along with petals just

after fertilisation, HJ mustard.

Persistent

Sepals remain attached with the ripe fruit,

HJ3LVXP2FLPXP, Family Solanaceae.

Caducous

Sepals fall off just after opening the

bud, HJ $UJHPRQH3DSDYHUetc.

On the

basis of

duration