Sexual reproduction in plants

The flower

The flower is the sexual reproducing organ of the flowering plants. It is divided into 3 parts

1. Perianth

These surround the male and female parts of the flower. It consists of whorls. The outer whorl is made of sepals collectively known as the calyx. The inner whorl is made of petals – collectively known as corolla. Corolla is brightly coloured and scented to attract insect pollinators.

Diagram of a typical flower

2. Androecium (microsporophyte)

This is the male sex organ in flowers. It produces the pollen grain. Each androecium consists of a filament (a stick portion) and an anther (knoblike enlarged structure) that produces pollen grain.

Development of pollen grains

The anther contains pollen sacs that contain spore mother cells. Each spore mother cell undergoes meiosis to form pollen grains as shown below.

Immediately after meiosis, the young pollen grains are seen as tetrad.  Each grain develops a thick cuticle.

3. Gynoecium (megasporophyte)

Consists of a stigma, style and ovary. The Stigma has a receptor surface and found at the tip of the style. Pollen grains adhere to the surface onto sticky, sugary substance secreted by the stigma. The ovary is found at the base of the style and contains ovules.

Formation of female gametes

The formation of female gametes takes place in the ovary. The ovary is hollow and contains one or more ovules. The ovule starts as a small bulge of tissue called nucellus on the inside of the ovary wall. Two folds of tissue called integuments grow up and over the nucellus leaving a small pore, the micropyle, at the end.

The ovule is megasporangium. Inside it a single cell (embryo sac mother cell) undergoes meiotic cell division to form a row of four haploid cells (megaspore).

(i) Three of these cells usually disintegrate. The remaining one expands and its nucleus undergoes three successive mitotic cell division to form an immature embryo sac containing eight nuclei which become arranged in 3:2:3 pattern.

(ii) Three remain at the micropylar end where they become separated from each other by cell walls and form one egg cell and two similar helpers or synergid cells.

(iii) The three at the other end become antipodal cells.

(iv) The remaining two nuclei occupy a central position and do not become surrounded by cell walls. They are called polar nuclei. The mature sac is surrounded by the ovule and the ovary.

Diagram of an ovule

Functions of parts ovary

• Stalk/funiculus allows passage of food and water to the growing ovary
• Ovary wall protects the ovule
• Egg cell develop into seed
• Embryo sac protects the embryo
• Integument protect the embryo and develop into seed coat
• Micropyle allow entry of pollen nuclei.

Pollination

This is the transfer of pollen grains from an anther to the stigma.

Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower or another flower of the same plant.

Cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of another flower on a different plant of the same species.

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Development after pollination

(i) As soon as a mature pollen grain fall on a receptive stigma. The pollen grain then absorbs the sugary fluid and increase in size and volume.

(ii) The exine burst open and the entine grows into a long narrow tube called the pollen tube. The pollen tube nucleus occupies the position at the tip and controls its growth.

(iii) The generative nucleus again divides mitotically into 2 male nuclei. On reaching the ovary the pollen tube enters, usually through the micropyle to the embryo.

(iv) One male nucleus fuses with an egg cell to form a diploid zygote.

(v) The second male nucleus fuses with both polar nuclei to form a triploid nucleus which give rise to endosperm.

(vi) The endosperm in cereals is where food reserves are stored. In seeds of other plants (dicotyledonous plants) endosperm is absorbed by the developing cotyledon which then provides the main food reserve.

Plants are therefore said to undergo double fertilisation because two male nuclei fuse within the ovum; one with the egg cell to form a zygote while another with the polar two polar cells to form an endosperm.

After fertilization

1. The zygote divides mitotically, growing and developing into the embryo. The embryo consists of a radicle (young root) plumule (young shoot) and either on cotyledon or two cotyledons (seed leaves). The embryo is attached to the wall of the expanding embryo sac by a suspensor which acts as passage of food to the embryo.
2. The primary endosperm nucleus (triploid) divides into a mass of nuclei which are separated from one another by thin cell walls. It becomes food storage for the seed.
3. The ovule develops into the seed. The integuments of the ovule become the seed coats. The outer integument is called the Testa while the inner is called tegmen. Both of these layers are tough and protective.
4. The ovary develops into a fruit.

Parthenogenesis

This is the development of a new offspring from an unfertilised egg. Haploid parthenogenesis, the egg is produced by meiosis whereas in diploid parthenogenesis the egg is produced by mitosis; e.g. production of wingless aphids.

Parthenocarpy

This is the development of a fruit without fertilization such fruits cart be artificially produced for commercial purpose by spraying with auxins.

Cross and self-fertilization

1. Self-fertilization is the union of gametes from the same individual flower.

Advantage:

1. it increases the chances of fertilization and formation of new organism.
2. only one parent is required, and that beneficial qualities are more likely to be passed on to the offspring since all offspring are genetically identical to the parent.

Disadvantage:

1. it reduces genetic variability, so the organism will be less adapted to changes in the environment.
2. It may transfer diseases to the offspring

• Cross-fertilization: is the union of gametes from the different individual or flower of the same species. This brings in genetic mixing and genetic variability which increase the hybrid vigour.

Advantages of sexual reproduction

• Genetic mixing
• Seeds can go through adverse conditions in a dormant stage.
• Allow genetic improvement.

Means employed by plants to limit self-fertilization in plants

• Dichogamy: anthers mature and stigma becomes receptive at different times
• Protandry: anther mature before the stigma
• Protogyny: stigma mature before the anther
• Self-incompatibility: the pollen grain fails to develop on the stigma of the same flower.
• Special floral structure: most hermaphrodite flowers have structural features that favour cross-pollination; e.g. stigma may be above the anthers thus removing the possibility of pollen falling on the stigma of the same flower. Other have nectar and good scent to attract a pollinator.
• Inflorescence: having many flowers in close proximity on the same stalk favours cross-pollination.
• Some plants have monoecious flowers, i.e. separate male and female flowers on the same plant. e.g. maize and coconut.
• Some plants are dioecious, separate male and female flower of different plants. Despite the advantage of cross-fertilization, dioecious plants are not many because only half of the plants are able to produce seeds and there is waste of pollen grains in wind dispersal.

Adaptations promoting self-fertilization

1. Bisexual, hermaphrodite flowers e.g. marigold.
2. Anther and stigma ripen at the same time. E.g. tomato.
3. Flowers remain enclosed until fertilization has taken place. E.g. garden pea
4. The flowers are buried in ground e.g. G. nuts.

Advantages and disadvantages of reproduction by seed

Advantages

1. The plant is independent of water for sexual reproduction and therefore better adapted for land environment.
2. The seed protects the embryo
3. The seed contains food for embryo either in cotyledon or in the endosperm
4. The seed is adapted for dispersal
5. The seed remains dormant and survives the adverse condition
6. The seed as a product of sexual reproduction has advantages genetic variation

Disadvantage

1. Seeds are relatively large structure because of extensive food reserves which make dispersal more difficult than spores
2. Seeds are often eaten by animals for their food reserves.
3. There is the reliance on external agent such as wind, insects and water for pollination which is a risk
4. There is large wastage of seed because the chances of survival of a given seed are limited
5. The food supply in a seed is limited as compared to vegetative reproduction
6. Two individuals are required in dioecious species making the process risky than reproduction in which only one parent is involved.

Differences between sexual reproduction in flowering plants and animal

Fruits and seed dispersal

This is the scattering of seed and fruits from the parent.

Why dispersal?

1. To avoid overcrowding
2. To increase the distribution of plants so that they can colonise better places
3. To preserve species by spreading them and preventing them from extermination by natural hazard e.g. fire.

Dispersal agent

1. Wind

Fruits and seed dispersal by wind has the following features.

• They are small and light
• They have, flattened wing-like structures e.g. Tecoma or a parachute of fine hair e.g. tridax to increase their surface area and air resistance.
• Animal

Fruits and seed dispersal by animals have the following features

• May have sticky hairs e.g. Desmodium
• May have hooks to stick on fur
• Some fruits have attractive colour, scent and sweet mesocarp when ripe, e.g.
• May have small indigestible seed which is deposited in faeces, e.g. passion fruit.
• Some plants have seed enclosed in woody endosperm that cannot be chewed, e.g. mango
• Water

Fruits dispersed by water

• Have floating devices, e.g. the seeds of the water lily have aril, small float, that have in air. The seed can float on water until the aril decays, then it sinks to the bottom and germinates
• Explosive mechanism of dispersal

e.g. balsam, bean

Seed Dormancy

Is the state in which a seed that is viable will not germinate even if the conditions that are necessary for germination are provided?

Dormant seed are usually dry, their metabolic activity is much reduced and they respire anaerobically.

Importance of seed dormancy

• Seed are able to withstand adverse external conditions such as very cold or very dry weather.
• It allows seed and fruits to disperse

Causes of seed dormancy

The main factors that causes the seed dormancy are:

1. Seed coats impermeable to water: The seed of certain family have very hard seed coats which are impermeable to water. This dormancy remains until the testa layer decay by soil microorganisms. The impermeable seed coats are found in the family Leguminosae, Malvaceae, Convolvulaceae.
2. Seed coat impermeable to oxygen:  This type of dormancy is because of the impermeability of the seed coats to oxygen. But later seeds become more permeable to oxygen so that it germinates afterwards. This type of dormancy is found in the family Compositae.
3. Mechanically resistant seed coat: In certain seeds of weeds have hard seed coats that prevent the expansion of embryo.
4. Immaturity of the embryo: In the seeds of plants like the Orchids, Ginkgo etc. The immaturity of the embryo is due to the failure of the embryo to develop when the seeds are shed.
5. Due to the effect of germination inhibitors: The inhibition caused due to the presence of the inhibitor substances in the seed coat, endosperm, embryo or any structure. Some of the important germination inhibitors are; Coumarin, Phythalids, Ferulic acid, Abscisic acid, Dehydracetic acid and parasorbic acid.
6. Low temperature: In certain plants the seeds remain dormant after harvest because they require low temperature for germination. The seeds germinate in the spring season.
7. Light sensitive seeds: In certain seed the germination is affected by the light so the absence of light results in the seed dormancy. These seeds which are sensitive to sunlight are termed as the photoblastic seeds, whereas in some other seeds the light inhibits the seed germination so they are negatively photoblastic.

Various methods have been used by seed scientist and technologists to break the dormancy of seed.

Simple and widely used methods are

A. Scarification:

Any treatment i.e. physical or chemical that weakness the seed coat, is known as scarification.

Scarification method is applied, when dormancy is imposed by hard seen coat e. g. in legumes- Cajanus cajan, (tur), gram etc.

In this method, there are various way to break hard seed coat such as:

1. Seeds are either rubbed on sandpaper manually. At the time of rubbing care should be taken that not to damage the axis of the seed e.g. Green gram & subabool.
2. When seed coat is too hard i.e. of woody nature, the seed coat has to be removed completely by breaking it. E.g. Rubber (Havea app) seed India teak wood seed.
3. Soaking treatment: Soaking hard seed coat in a concentrated or diluted solution of sulphuric acid for 1 to 60 minutes, it removes seed coat impermeability. E. g. cotton seeds, India teak wood seeds etc.

B. Temperature Treatments:

1. When the dormancy is due to embryo factor i.e. the seed is incubating at low temp. (0- 5o C) over a substratum for 3 to 10 days placing it at optimum temp. Required for germination. E.g. mustard. – (Brassica campestrits)
2. Some seeds required a brief period of incubation (from a few hours to one to five days) at 40 to 50⁰ C before germinating at required temp. ( in this method care should be taken that moisture content of the seed is not more than 15% e.g. paddy (Oryza Sativa)
3. Hot water treatment is also an effective method of breaking hard- seed ness in legumes. In this method, the seeds are soaked in water at 80oC temp. For 1 – 5 minutes (depending upon the type of seed) before putting for germination.

C. Light Treatments:

Same seeds do not germinate in dark thus it provides continuous or periodic exposure of light is essential e. g. Lettuce (Lactuca Sativa) required red light (660nm) or white light is essential for germination to occur.

D. Treatments with growth regulators & other Chemicals:

Endogenous dormancy may be due to the presence of germination inhibitors. Application of low level of growth regulators (i.e. Gibberellins, Cytokinins and Ethylene etc) may break the seed dormancy.

Most widely used growth regulators are gibberellins and kinetics e.g. seeds of sorghum crop presoaking seed treatment with GA3 at the conc. Of 100 ppm have been used for breaking seed dormancy

Among other chemicals potassium nitrate (0.2%) and thio – urea (0.5 to 3%) are widely used for breaking seed dormancy in oat (Avena Sativa), barley (Hordeum vulgare), tomato (Lycopersicon spp).

(For prepare 100 ppm solution of GA3, weigh 100 mg of GA3 & dissolve in a few drops of alcohol and make up the final volume (1000 ml) by adding distilled water).

(50 ppm kinetin 5 mg dissolved in few drops of alkaline made with sodium hydroxide and makes the final volume 100ml it gives to final conc. Of 50 ppm)

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Compiled by Dr. Bbosa Science