Flowering Plant Life Cycle
The flowering plant life cycle begins at the point in spring where the plant produces flowers, describes the reproductive steps that take place to produce a fruit and seed and then follows the seed germination stage which results in the growth of a new plant. Note that this page describes the life cycle of a flowering plant, an Angiosperm. ‘Angio’ comes from the Greek for “vessel”. So an Angiosperm has its seeds in a vessel, in this case an ovary. It differs from that of a Conifer, a Gymnosperm. Conifers are classed as Gymnosperms because they have “naked seeds” which are not surrounded by an ovary. Gymno comes from the Greek for “naked”.To learn more about the difference between Angiosperms and Gymnosperms click HERE. To find out more about the Conifer life cycle click HERE
The mature plant – The flowering plant life cycle described here begins with a fully grown Wild Cherry tree Prunus avium. Two to four white flowers emerge from each bud in April. Mature trees are covered in white flowers for several weeks and are very attractive to bees seeking pollen and nectar. As the bee pushes into the flower to access nectar it collects pollen on its body, some of which is brushed off when it enters the next flower. Not all Cherry trees are mature at the same time and so pollen transfer by bees can result in cross-pollination. To learn more about bees click HERE. To learn more about the Wild Cherry click HERE. Note that the Sweet Cherry is a variety of the Wild Cherry. It has a larger, juicier and sweeter fruit than the Wild Cherry.
The flower – Each flower has a cup-shaped corolla of five white petals and a central stigma located on the top of a long style which leads to a single ovary. Around the style are 20 stamens, each consisting of a long filament topped with a pollen-producing anther. Nectar is located at the base of the style around the ovary. When a cherry flower first opens, the stamens are usually erect and shedding pollen, while the stigma is still immature and not receptive to pollen. As the flower matures, the stamens eventually become bent or fall off, the style grows and the stigma becomes receptive and ready to receive pollen. This time lag between the maturity of the male and female reproductive structures helps to prevent self-pollination and encourages cross-pollination with other cherry trees. Note that in this photo the style is fully grown, the stigma is receptive and the anthers are no longer releasing pollen.
The Anthers – Each anther has two pollen sacs which split longitudinally to release pollen grains. Pollen grains adhere to the body of the bee as it attempts to access the nectar at the base of the ovary. Each grain contains a pollen tube cell and two haploid sex cells (each has one set of male chromosomes).
The pollen tube. On reaching the stigma of another flower the pollen grain germinates and, if the stigma is receptive, starts to grow a pollen tube.The pollen tube grows down the style and, guided by signals from the ovule, passes through the funiculus into the ovule. The tube then goes round the integuments before reaching a gap called the micropyle and entering the embryo sac.
Double fertilisation. The two haploid sperm cells pass down the tube and enter the embryo-sac. One male sex cell fuses with the egg cell to become an embryo. The other fuses with a cell that is formed from the fusion of the two polar nuclei. The resultant triple haploid cell becomes the endosperm that provides nutrients for the growing embryo. This process, known as ‘double fertilisation’, involves 2 sperm fertilising 2 egg cells. The process must be repeated for each ovule in the ovary.
The Fruit – After fertilisation, the ovary becomes a fruit and the ovules become the seeds within it. The Cherry has a single ovule and produces a stone fruit called a drupe which has a hard stone at its centre, containing a single seed. Note that this is the fruit of a Sweet Cherry.
The stone – is surrounded by a fleshy layer and an outer skin. The three layers develop from the ovary wall. The exocarp is the outer skin, the mesocarp is the middle fleshy layer and the endocarp is the innermost layer which is hardened to form the stone and surrounds the seed. The endocarp protects the seed until the seed germinates when it splits open. The seed contains a small embryo surrounded by a large, starchy nucellus and endosperm, which provide nutrients for the developing seedling.
The seed – The endosperm grows and largely replaces the nucellus. The endosperm is rich in carbohydrates, proteins, and fats, and it also contains small amounts of vitamins and minerals. As the embryo grows the endosperm supplies it with energy and nutrients. The embryo consists of a shoot tip or plumule which will become the shoot and leaves and a root tip or radicle which will become the root system. The seed also contains one or two embryo leaves called cotyledons. Around the embryo is the endosperm which grows in the early stage of seed development but then is replaced by the embryo as it grows.
Germination – The seed absorbs water and swells causing the seed coat to split. The radicle growth tip is activated and the radicle emerges first and extends downwards into the soil and branches to form the root system. The plumule growth point is then activated and this pushes a shoot upwards above the soil and then pulls the cotyledons out of the soil exposing them to light.
Growth from the seed – What happens next is determined by the plant’s genetics. Specifically whether the plant is herbaceous or woody.
A herbaceous plant is a type of plant that has soft, green stems and does not develop woody tissue as it matures. Herbaceous plants are generally characterised by their green, flexible, and non-woody above-ground parts.
Trees are woody plants adapted to grow taller as rapidly as possible as they compete for access to sunlight. The main stem grows at the fastest rate through primary growth at the stem tip. Secondary growth, which involves the development of tissues such as xylem and phloem, is a characteristic feature of woody plants. It gives the tree strength and provides tubes which allow water and nutrients to move between the leaves and soil. To learn more about secondary growth click HERE.