Fertilisation, Seed and Fruit in Flowering Plants — Notes

Events leading to fertilisation. Compatible pollen germinates on the stigma; the pollen tube grows through the style, enters the ovule through the micropyle, and enters one of the synergids, guided by the filiform apparatus. It releases two male gametes into the cytoplasm of the synergid.

Syngamy. One male gamete fuses with the nucleus of the egg cell to form the diploid zygote (2n).

Triple fusion. The other male gamete fuses with the two polar nuclei in the central cell to form the triploid primary endosperm nucleus (PEN, 3n). Three haploid nuclei fuse, hence the name. The central cell becomes the primary endosperm cell (PEC), which develops into the endosperm; the zygote develops into the embryo.

Double fertilisation — two fusion events (syngamy and triple fusion) in one embryo sac — is a characteristic feature of angiosperms.

Significance:

• Syngamy restores the diploid number and combines characters of two parents, giving variation.
• Triple fusion produces the triploid endosperm, a nutritive tissue for the embryo.
• Food tissue forms only if fertilisation occurs, so reserves are not wasted on unfertilised ovules.
• After fertilisation the synergids and antipodals degenerate.

Endosperm. Endosperm development precedes embryo development. The PEC divides repeatedly to form a triploid endosperm tissue filled with reserve food, used to nourish the embryo.

• Nuclear endosperm (commonest): the PEN undergoes successive free-nuclear divisions; cell walls may later form from the periphery towards the centre. Coconut: tender coconut water is free-nuclear endosperm (thousands of nuclei) and the surrounding white kernel is cellular endosperm.
• Cellular endosperm: every nuclear division is followed by wall formation (e.g. Petunia, Datura).
• Helobial endosperm: the first division forms two unequal cells; the larger (micropylar) chamber develops free-nuclearly and the smaller (chalazal) one cellularly or not at all; common in monocots (e.g. Asphodelus).
• The endosperm may be completely consumed by the developing embryo (pea, groundnut, beans) or persist in the mature seed (castor, coconut, cereals).

Perisperm. In some seeds, remnants of the nucellus persist as a residual, persistent nucellus called perisperm (e.g. black pepper, beet). It is diploid and maternal.

Embryo (embryogeny). The embryo develops at the micropylar end. Most zygotes divide only after a certain amount of endosperm has formed — an adaptation that provides assured nutrition.

• Dicot: zygote → proembryo → globular → heart-shaped → mature embryo. The mature embryo has an embryonal axis and two cotyledons. The part above the cotyledons is the epicotyl, ending in the plumule (stem tip); the part below is the hypocotyl, ending in the radicle (root tip) covered by a root cap.
• Monocot (grass): only one cotyledon, the scutellum, situated towards one side (lateral) of the embryonal axis. At its lower end the axis has the radicle and root cap enclosed in an undifferentiated sheath, the coleorhiza. The shoot apex and a few leaf primordia are enclosed in a hollow foliar structure, the coleoptile. An epiblast (a remnant of a second cotyledon) is present in some grasses.

In angiosperms the seed is the final product of sexual reproduction, often described as the fertilised ovule. Seeds form inside fruits. A seed typically consists of seed coat(s), cotyledon(s) and an embryonal axis.

Dicot seed — bean. The seed coat has two layers, outer testa and inner tegmen. The hilum is a scar where the seed was attached to the fruit (funicle); the micropyle is a small pore above it. The embryo has two large, fleshy cotyledons and an embryonal axis with plumule and radicle. No endosperm remains.

Monocot seed — maize grain. The seed coat is membranous and generally fused with the fruit wall (pericarp). The bulky endosperm stores food and is separated from the embryo by an epithelial layer; its outer protein-rich layer is the aleurone layer. The embryo is small, in a groove at one end, with one large shield-shaped cotyledon, the scutellum, and a short axis with plumule enclosed in the coleoptile and radicle enclosed in the coleorhiza.

Albuminous (endospermic) seeds retain part of the endosperm (wheat, maize, barley, castor, coconut). Non-albuminous (exalbuminous) seeds have no residual endosperm, as it is consumed during embryo development (pea, groundnut, bean, gram).

As seeds mature, their water content falls to about 10–15 percent by mass; general metabolism slows and the embryo may enter dormancy. Under favourable conditions (moisture, oxygen, suitable temperature) seeds germinate.

Fruit. The ovary develops into the fruit; the ovary wall becomes the pericarp, which may be fleshy (guava, orange, mango) or dry (groundnut, mustard).

• True fruits develop only from the ovary (mango, coconut, tomato, pea).
• False fruits develop from the ovary together with other floral parts, especially the thalamus (apple, strawberry, cashew).
• Parthenocarpic fruits develop without fertilisation and are seedless (banana).

Mango (drupe). Develops from a monocarpellary superior ovary and is one-seeded. Pericarp: thin epicarp (skin), fleshy edible mesocarp, hard stony endocarp, enclosing the seed.

Coconut (drupe). Pericarp: smooth epicarp, thick fibrous mesocarp (coir), hard endocarp (shell). The seed has a white solid endosperm (kernel) and liquid endosperm (coconut water), and a small embryo below one of the three “eyes”. The fibrous mesocarp makes the fruit buoyant, aiding dispersal by water.

Significance of seeds and fruits:

• Pollination and fertilisation are independent of water, and seed formation is more dependable.
• Seeds have food reserves for the young seedling until it can photosynthesise.
• A hard seed coat protects the embryo; dormancy and long viability allow storage — a lupine seed (Lupinus arcticus) about 10,000 years old and a date palm (Phoenix dactylifera) seed about 2,000 years old have germinated.
• As products of sexual reproduction, seeds generate new genetic combinations (variation).
• Seeds are the basis of agriculture; fruits protect seeds and help their dispersal.
• Dispersal (by wind, water and animals) lets plants colonise new areas and reduces crowding and competition near the parent.

Apomixis is the formation of seeds without fertilisation, a form of asexual reproduction that mimics sexual reproduction (found in some species of Asteraceae and grasses). Ways in which it occurs:

• A diploid egg cell forms without reduction division and develops into the embryo without fertilisation.
• Some nucellar cells surrounding the embryo sac divide, protrude into the embryo sac and develop into embryos.

Importance: many crop varieties are hybrids; if farmers sow seeds collected from a hybrid, the progeny segregate and lose the hybrid characters, so new hybrid seed must be bought every year. If hybrids are made apomictic, there is no segregation, and farmers can keep using the seed. Scientists are trying to identify the genes for apomixis and transfer them into hybrid varieties.

Polyembryony is the occurrence of more than one embryo in a seed; in citrus and mango, nucellar cells grow into the embryo sac and form additional embryos.

Parthenocarpy is the development of a fruit without fertilisation; such fruits are seedless (banana). It can be induced by growth hormones, and such fruits are commercially valuable.