RHS Level 2 Certificate

Review of cells, pollination and fertilization and fruits

Learning outcomes

• Identify and describe the different parts of the plant cell

• Describe the processes of wind and insect pollination

• Define pollination • Define fertilization• Describe the process of fertilization• Classify various types of fruit

Plant Cells

• Cell Wall – made of cellulose, holes allow plasmodesmata (cytoplasm) out to connect the cells to each other chemically

• Cell membrane – surrounds the living content of the cell; inside the cell wall.

• Cytoplasm – the living content of the cell• Nucleus – contains chromosomes that are strands of DNA• Vacuole – sac containing dilute sugars and nutrients and

waste products.• Chloroplast – contains chlorophyll, site of photosynthesis• Mitochondria – site of respiration, releases energy from

carbohydrate

Cell diagram

Structure of a typical insect pollinated flower

Structure of a typical wind pollinated flower

Pollination

• The transfer of pollen from the anther (where it is made) to the stigma.

• Cross pollination: pollen transferred to a different individual of same species

• Self pollination: pollen transferred to the stigmas of the same flower (hermaphrodite flower) or to a female flower on the same plant (monoecious)

Pollination

• Agents have to be used to transfer pollen

• Animals: insects, birds, bats mice etc….

• Wind• Water currents

Wind Pollination

• Flowers have no need to attract insect

• Stigma and stamen usually small and on long stalks to catch the wind

• Huge quantities of tiny light coloured pollen grains produced

Wind Pollination

• They are the usual cause of hay fever!

• Wind pollen is usually small and light

• Often contains air sacs• Grasses and many

trees are wind pollinated

Insect Pollination

• Insect pollen is often sticky!• Nectar rewards the insect• Pollen often contains

carbohydrates as food for the insect

• Flowers have ultra-violet sensitive pigments that direct the insects to the stamen and pistil

Fertilization takes place within the flower.

• Male gametophytes, or pollen grains, are produced in the anthers. – Haploid male gametes

produced in anthers

by meiosis– each gamete divides by

mitosis to form twoidentical haploid cells

– two cells form asingle pollen grain

•pollen grain

– Haploid female gametes produced in ovule by meiosis – one gamete develops into female gametophyte – female gametophyte contains a number of cells – one cell has two nuclei, or polar nuclei (one set of chromosomes

each – a double haploid)– one haploid cell will develop into an egg

• One female gametophyte can form in each ovule of a flower’s ovary.

• Pollination occurs when a pollen grain lands on a stigma.

– The pollen must be compatible to fertilize the ovum – that means that the pollen must ‘recognise’ the chemical signature of the stigma

– If the pollen is compatible it is stimulated to divide by the chemicals produced by the stigma. If not compatible it will die.

– one cell from pollen grain forms pollen tube– other cell forms two sperm that travel down tube

• Flowering plants go through the process of double fertilization.

•female•gametophyte

•ovule

•egg

•sperm

•polar nuclei

Pollen tube

•endosperm

•seed coat

•embryo

– one sperm fertilizes the egg and forms a diploid zygote which has DNA from both parents

– other sperm unites with polar nuclei, forming endosperm

– endosperm provides food supply for embryo

• Flowering plants go through the process of double fertilization.

• Each ovule becomes a seed.• The surrounding ovary grows into a fruit.

Fruits

• The part of the reproductive structure surrounding the seeds, e.g. pods and nuts as well as the fruits that we eat.

• Develops from the ovary; if the structure around the seeds develops from other parts of the flower it is a ‘false fruit’

• Parthenocarpy is the formation of fruit without fertilization

• Classification of fruit – a number of systems and no botanical agreement.

Classification of Fruit – simple scheme

• Simple fruit – formed from one ovary, may contain one or many seeds. E.g. Prunus x domestica

• Aggregate fruit – formed from the fusion of many ovaries in one flower. E.g. Raspberry Rubus ideaus

• Compound fruit – Formed from the ovaries of several grouped flowers e.g. Pineapple Ananus comosus

• Dehiscent – splits open to release seeds e.g. Lathyrus odoratus pod

• Indehiescent – does not split open e.g. Quercus robur acorn• Berry – a simple fruit with no hard layer of endocarp around

the seeds e.g. Lycopersicon esculentum• Drupe – a simple fruit with a stony layer of endocarp around

the seed e.g. Prunus x domestica