Introduction

Plants undergo both asexual and sexual reproduction. We will focus on sexual reproduction in angiosperms (flowering plants).

Generalised angiosperm life cycle

The seed germinates to produce a seedling which grows into a mature plant which produces flowers containing male and/or female reproductive parts.

Flowers produce stamens (male) and/or carpels (female) (carpel also called psitil)
Stamens produce pollen, carpels produce ovules
After pollination and fertilisation, seeds are produced in a fruit
Seeds are dispersed but may remain dormant

There is a huge diversity of flower structure. Flowers may be borne singly or grouped together in an inflorescence. Each flower is formed on a receptacle. Sepals form the calyx and petals form the corolla. Stamens form the androecium (male) and carpels (pistil) form the gynoecium (female).

Double fertilisation

Angiosperms are unique as they undergo double fertilisation during sexual reproduction.

Pollen is produced in pollen sacs in the anthers. Each mature pollen grain contains two cells, a tube cell and a generative cell.

Eggs are produced in ovules within the ovary. Each ovule contains an egg cell and two polar nuclei (plus synergids and antipodal cells).

When a compatible pollen grain lands on the stigma, the tube cell grows a pollen tube down through the style and the generative cell divides to produce two sperm cells. One sperm cell unites with the egg cell in the ovary, the other sperm cell unites with the two polar nuclei to form a triploid (3n) cell. This is double fertilisation. The triploid cell forms the endosperm, which provides nourishment to the seed when it is developing. The carpel becomes the fruit which packages the seed and provides a nutritious snack for the seed disperser.

The following video helps you to visualise the phenomenon of double fertilisation and angiosperm reproduction.

Pollination and seed dispersal

Pollen may be transferred in a number of ways; wind, water, insects, birds and mammals.

Plants attract animal pollinators in a number of ways, including brightly coloured petals with UV guide marks, a reward for the pollinator (pollen, plus nectar; a sugary substance), scent (a daily rhythm of scent production in tune with the behaviour of the specific pollinator) and mimicry (produce flower that resembles an arthropod) e.g. bee orchid, spider orchid etc.

bee orchid.JPG

After successful pollination and fertilisation, a seed is produced. In angiosperms, the seed is packaged into a fruit. There are a variety of fruits produced including dry fruits (e.g. nuts, achenes) and fleshy fruits (e.g. berrries, pomes, drupes), and these are dispersed in different ways, including by wind (e.g. dandelion clocks, sycamore samaras), water (coconut), mechanically (explosive fruits e.g. Himalayan Balsam, squirting cucmber) and by animals both passively (fruits that cling onto animal fur) and actively (fruit consumed by animals, seed regurgitated/excreted).