Lecture 10: flowers, pollination and fertilization
Flower
Flower parts arranged in whorls on a flower stalk (Peduncle)
Peduncle tip enlarges forming a receptacle to hold the reproductive structures- stamens & carpels (Pistil)
Peduncle may terminate in a single flower or a cluster of flowers (Inflorescence)
Flower inflorescence
Racemose/ indefinite
Cymose/ definite
Main axis of inflorescence continues to grow and give off flowers laterally
The oldest flower is borne at the newest flower near the growing tip
Development of terminal flower stops the upward growth of the floral axis
Floral axis stops elongating once the first flower opens and so called a determinate inflorescence
Flowers that are borne as a group on a common stalk
Sepals
Petals
Stamens
Pistil
Other parts
Peduncle: The stalk of a flower
Receptacle: The part of a flower where the parts of the flower are attached
Anther: The part of the stamen where pollen is produced
Stigma: The part of the pistil where the pollen germinates
Ovary: The enlarged basal portion of the pistil where ovules are produced.
Outer parts of the flower that encloses a developing bud
Outermost whorl, leaf-like and green
Cover and protect the bud
Collective term for all the sepals is calyx
The parts of a flower that are often conspicuously colored
Broad, flat and thin shaped (vary tremendously)
Frequently brightly ccoloured
Collective term for all petals is corolla
The pollen producing part of a flower, usually with a slender filament supporting the anther
Has two parts: Anthers & Filament
Anther carries pollen, generally yellow
Filaments hold up the anther
The ovule producing part of the flower. Ovary often supports a long style, topped by a stigma. The mature ovary is a fruit, and the mature ovule is a seed
Has 3 parts: Stigma, style and ovary
The stigma has a sticky top surface on top of the pistil to trap and hold pollen
The style is tube like structure that holds up the stigma and leads down to the ovary that contains the ovules
Type of flowers
Complete flower: Has all four main parts
Incomplete flower: Flower lacks 1 or more parts
Perfect flower: Has both stamens and carpels
Imperfect color: Has either stamens or carpels
Hypogyny: ovaries occupies highest position on the thalamus (Superior ovary)
Perigyny: Thalamus grows upwards forming a cup around the ovary (Inferior ovary)
Floral diagram and formulae
For e.g.
The floral formula for Lamium album is
.|. K(5) C(5) A4 G(2)
.|. indicates a zygomorphic flower (having one plane of symmetry in which it may be cut to give matching sides)
K = calyx, C = corolla, A = androecium, and G = gynoecium.
Numbers refer to the number of members in a whorl
5 calyx, and brackets that they are united together
If members of two separate whorls are joined, this is indicated by bridging lines (or square brackets)
A bar above or below the gynoecium number indicates an inferior or superior ovary respectively.
Pollination
Transfer of pollen from anther to stigma
Self-pollination (autogamy)
Transfer of pollen grains within a flower or to another flower on the same plant
Both the anthers and the stigma of a bisexual flower mature at the same time (Homogamy)
If flowers never open and the pollen grain pollinate only the stigma of the same flower (Clestogamy)
Cross-pollination (Allogamy)
Transfer of pollen grains from one flower to another flower on another plant
Pollination agents
Insects
Animals
Wind
Water
Flowers attracts insects by color
Sepals of Mussaenda modified to colored leaf
Bracts of bougainvilleas highly colored and attractive
Spathes in banana are brightly colored
Head/ capitulum of sunflower consisting of many small florets
Nectar
Produced by a special gland (Nectary) or in sac or tube-like structure (Spur)
Found at the base of one of the floral whorls
Scent
Nocturnal flowers emit sweet scent
Or Stinking smell to attracts flies
Pollen grains sticky or spiny
Stigma sticky
Pollen grains provide food
Shelter for insects from rain and sun
Small and inconspicuous flowers
Do not emit scent or nectar
Produce large amounts of pollen grains that are light, dry and with wings
Stigmas comparatively large & protruding, sometimes branches and often feathery
For e.g. Vallisneria
Minute male flowers get detached from the male plant and float on water
Female flowers on a long stalks brought to the level of water
On contact, the anthers burst and pollen grains are distributed on the stigma
After pollination, the stalk of the female flower becomes closely coiled and the fruit develops under water
Birds, squirrels, bats and snails, etc.
Fertilization
Androecium/ Stamens
The male reproductive organs
Pollen grains are a single cell covered by 2 coats
Exine: a tough, cutinized layer with patterns or with spines or sometimes smooth. (Coat 1)
Intine: a thin, cellulose layer which grows out to form the pollen tube (Coat 2)
Pollen grain (2 nuclei)
Vegetative/ tube nucleus
Generative nucleus: divides to give 2 male gametes
Gynoecium/ Pistils
The female reproductive part of flower
Composed of one or more carpels (Pistil) bearing ovules
Ovules develop into seeds
Three main part: Stigma, style and ovary
Each ovule contains one female gamete (an egg) and two polar nuclei
Double fertilization
After pollination, stimulated by sugary substance on the stigma, intine of pollen grain will grow into a pollen tube through weak spots in the exine
Pollen tube penetrates stigma & carries the tube nucleus and generative nucleus that divides to from 2 male gametes
Travels through the style and into the ovary
Pollen tube grows towards the micropyle and enters it, penetrates the nucellus and comes in contacts with the embryo-sac
One male gamete fuses with egg to form a zygote
The other fuses with the nucleus formed by 2 polar nuclei to form the endosperm
Double Fertilization completed
The ovules will become the seed and the ovaries the fruit
Epigyny: Thalamus completely enclosing the ovary carrying its rim the sepals, petals and stamens (Half- inferior)