Page 162 - Biology_F5

Comparative studies of natural groups of organisms

Structure of the flower Examples include the flowers of Aloe and
A flower is a modified leaf of a plant which Tulipa genera. They may be separate or
is highly specialised for reproduction fused into a single outer sheath collectively
(Figure 3.32). It can be bisexual or called calyx.
unisexual, depending on a plant species.
Bisexual and unisexual flowers are (c) Petals
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called complete and incomplete flowers, These are normally located inside and
respectively. Flowers have the following
major parts: above the sepals. They are brightly
coloured and they expand to form the most
(a) Receptacle conspicuous part of the flower. As such,

This is a swollen tip of a pedicel on which they serve to attract pollinators, such as
all floral parts are borne or attached. It insects and birds, which are essential for
has limited growth, which ceases as the accomplishing the reproductive process.
last floral part is formed. The receptacle The petals may be separate or united in a
varies in shapes between members of one single tube or united in several groups. The
species and another. The floral parts are collective term for petals is corolla. In some
arranged in whorls around the receptacle, flowers, both calyx and corolla are united
as rings of calyx, corolla, androecium and to give a structure termed as “Perianth.”
gynoecium. When all these whorls are

present, the flower is termed as a perfect (d) Androecium
or complete flower. Examples include This is a collective term for the male part
flowers of cherries, roses, and orchids. In of the flower, and includes anther and
contrast, when one of the four whorls is filament. Anthers are usually bright orange
missing, the flower is termed as incomplete or yellow in colour. The anther and filament
flower. Examples include; flowers of together are termed stamen. The transverse
sweet corn and most grasses. The floral section of the anther shows that it is made
parts are arranged in the following order up of 2 to 4 pollen sacs containing pollen
from the base to the apex of receptacle; grains or microspores. The whole stamen
the first whorl is for the calyx, the second can be regarded as microsporophyll bearing
whorl is for corolla, the third whorl is for microsporangia. The fertile microsporangia
androecium, and the fourth whorl is for are borne as pollen sacs at the distal end
gynoecium. of the filament. When mature, the pollen
sac ruptures to release pollen grains
(b) Sepals (microspores or male gametophyte), which
These are the lowermost or outermost develop into male gametes.
structure, which are usually green, leaf-
like in structure, protecting the developing (e) Gynoecium
flower whilst the flower is still a bud. As the This refers to the female reproductive parts
inner parts of the flower grow and expand, of the flower, which consists of the stigma,
sepals are pushed outwards. In some style, and ovary. These three parts are
instances, they may become expanded collectively called pistil or carpel. The shape
and brightly more coloured, like the petals. of gynoecium varies in different species, as

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