Instant Notes: Plant Biology

has a more or less swollen top just underneath the flower called the receptacle

and on this there are four different organs borne in whorls: (from outside in)

sepals, petals, stamens and carpels. Sepalsnormally surround the flower in

bud, providing a protective function. The petalsare usually the main organs

attracting insects to flowers and many are brightly colored, though they may be

photosynthetic as well. There may be an indefinite number of petals and/or

sepals, but many plants have a fixed number, often three or five.

Thefertile partsof the flower are inside the petals (Fig. 1). They consist of the

pollen-bearing male parts,stamens, in one or more whorls and the female

carpelsin the center. Stamens consist of a stalk, the filament, and an anther

enclosing the pollen grains. In some flowers there is an indefinitely large

number of stamens, whereas in others the number corresponds to the number of

petals or twice the number if there are two whorls. The carpels are usually fewer

in number and consist of a basal ovarywhich contains the ovules, and above

this a stalk, the style, which is topped by a stigma, the surface onto which

pollen grains will land for fertilization. In most plants the stamens and carpels

are of similar length and position in the open flower, but in many one sex

matures before the other (Topic H2).

One or more of the whorls may not be present. Either sepals, petals or both

may be absent, or the two whorls not fully differentiated. About 15% of

the world’s plant species have unisexualflowers and these do not have one of

the whorls of fertile organs, though many retain small infertile remnants of the

other parts. If male flowers and female flowers occur on the same plant, the

plant is known as monoecious; if they occur on separate plants it is known as

dioecious. Wind-pollinated flowers are often very small without sepals or

petals, many also being unisexual such as on catkin-bearing trees. These flowers

may just have the fertile organs of one sex with bracts around them.

The sepals are the most leaf-like floral organ; they are often green and photosyn-

thetic and, in structure, resemble a simple leaf. In some plants they resemble the

petals. Petals resemble leaves in their overall organization but have a special-

ized epidermis and sub-epidermal layer. The epidermal cells frequently have

small projections that absorb the incident light and scatter the light reflected

from the mesophyll layer underneath. This layer has many air spaces between

its cells and reflects all the light. The pigmentis usually in the vacuolesof the

Sepal and petal
structure

42 Section D – Reproductive anatomy

Petals

Stamen

Carpels

Sepals

Receptacle

Carpels

Stamen

Petals

Sepals

1mm

(a) (b)

(c)

Fig. 1. A flower of Arabidopsis thaliana: (a) side view, (b) top view, (c) diagrammatic overview of floral parts.