What is pollination? This is the transfer of pollen grains from the anther to the stigma. Types of pollination Self pollination is the transfer of pollen grains from the anther of one flower to the stigma of the same flower.
Cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of a different flower, of the same species.
Agents of pollination
- Agents of pollination include wind, insects, birds and mammals.
- Insect pollinators include bees, butterflies and mosquitoes.
Mechanisms that hinder self-pollination
- Stamens ripen early and release their pollen grains before the stigma, mature. This is called protandry e.g. in sunflower.
- The stigma matures earlier and dries before the anthers release the pollen grains.
- This is called protogyny and is common in grasses.
- Self-sterility or incompatibility
- Pollen grains are sterile to the stigma of the same flower, e.g. in maize flower.
- Shorter stamens than pistils.
Fertilization in Plants
- The pollen grain contains the generative nucleus and a tube nucleus.
- When the pollen grain lands on the stigma, it absorbs nutrient and germinates forming a pollen tube.
- This pollen tube grows through the style pushing its way between the cells.
- It gets nourishment from these cells.
- The tube nucleus occupies the position at the tip of the growing pollen tube.
- The generative nucleus follows behind the tube nucleus, and divides to form two male gamete nuclei.
- The pollen tube enters the ovule through the micropyle.
- When the pollen tube penetrates the ovule disintegrates and the pollen tube bursts open leaving a clear way for the male nuclei.
- One male nucleus fuses with the egg cell nucleus to form a diploid zygote which develops into an embryo.
- The other male gamete nucleus fuses with the polar nucleus to form a triploid nucleus which forms the primary endosperm.
- This is called double fertilisation.
After fertilization the following changes take place in a flower:
- The integuments develop into seed coat (testa).
- The zygote develops into an embryo.
- The triploid nucleus develops into an endosperm.
- The ovules become seeds.
- The ovary develops into a fruit.
- The ovary wall develops into pericarp.
- The style dries up and falls off leaving a scar.
- The corolla, calyx and stamens dry up and fall off.
- In some the calyx persists.
- Fruit development without fertilization is called parthenocarpy e.g. as in pineapples and bananas.
- Such fruits do not have seeds.
Classification of fruits
False fruits develops from other parts such as calyx, corolla and receptacle e.g. apple and pineapple which develops from an inflorescence.
True fruits develop from the ovary, e.g. bean fruit (pod).
True fruits can be divided into fleshy or succulent fruits e.g. berries and drupes and dry fruits.
The dry ones can be divided into Dehiscent which split open to release seeds and indehiscent which do not open.
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