Plant Structure and Function

This is Unit Eleven of Biology, Part Four: reproduction. It closes the unit. A pollen grain leaving an anther does not contain sperm. It carries a generative cell and a tube cell, and the generative cell only divides into two sperm as the pollen tube grows toward the ovule. The sperm are made in transit. On the other side, meiosis makes four megaspores and three of them degenerate. One survives, divides by mitosis, and becomes an eight-nucleate embryo sac — including two synergid cells whose job is to chemically guide the arriving pollen tube in. The central correction here concerns self-pollination. It reduces genetic diversity, which makes it look like cloning — but it runs the full sexual sequence. Meiosis makes gametes with new allele combinations, fertilisation makes a distinct zygote, and the offspring are never genetic copies of the parent. Then double fertilisation in detail, and the ploidy that follows from it. One sperm plus the egg gives a diploid embryo. The other sperm plus two polar nuclei gives triploid endosperm — three sets, which is why a corn kernel's starch is genetically unlike the embryo it feeds. By the end you will be able to do five things. One: say what pollen carries when shed. Two: tell complete from perfect. Three: say why selfing is sexual. Four: say why endosperm is triploid. Five: say what each strategy trades.

Two entries, one for each side. Both carry a count worth noticing: pollen leaves without sperm, and three of the four megaspores meiosis produces never become anything at all.

The male side.

Bio 2601 Male Gametophyte Development: Microsporogenesis and the Pollen Grain

Inside an anther, mother cells divide by meiosis into microspores that develop into pollen grains, each holding a generative cell and a tube cell. On germination the tube cell forms the pollen tube while the generative cell divides into two sperm. The shed grain contains no sperm — those form only as the tube grows toward the ovule.

The female side.

Bio 2603 Female Gametophyte Development: Megasporogenesis and the Embryo Sac

Inside an ovule, one megaspore divides by mitosis into an eight-nucleate embryo sac: an egg cell, two synergids, three antipodal cells, and a central cell with two polar nuclei — the synergids secreting signals that guide an arriving pollen tube through the micropyle. Of the four megaspores meiosis produced, only one survives; the rest degenerate.

Three entries. Two vocabularies that overlap and are not the same — complete against perfect, and monoecious against dioecious, which measures something else entirely — plus the trait set matched to a pollinator.

All four whorls.

Bio 2605 Complete versus Incomplete Flowers

A complete flower carries all four whorls; an incomplete flower lacks one — a lily is complete, a willow's flower lacks petals and sepals entirely. Perfect is a narrower term, describing a flower that has both the pollen-bearing and ovule-bearing whorls, whether or not it also has the other two.

Across a plant.

Bio 2606 Monoecious versus Dioecious Species

Monoecious species bear pollen-producing and ovule-producing flowers on one plant; dioecious species bear them on separate plants — corn's tassels and ears on one plant, papaya's on two. This describes sex distribution across a plant's flowers, not flower completeness; a monoecious species can have complete or incomplete flowers.

Matched traits.

Bio 2607 Pollination Mechanisms and Pollinator Syndromes

A pollinator syndrome is a matched set of flower traits — colour, scent, shape, reward — suited to a specific pollinator: bat-pollinated flowers opening at night, pale and scented, where wind-pollinated ones stay small and scentless and make abundant light pollen. A syndrome predicts the typical pollinator statistically; unmatched visitors still turn up.

Four entries. Two routes pollen can take, a genetic mechanism that blocks one of them, and a twinned pair correcting what selfing actually is — which is the misconception this part exists to fix.

Same plant, or another.

Bio 2609 Self-Pollination versus Cross-Pollination

Self-pollination deposits pollen on the same flower or plant's stigma; cross-pollination deposits it on a different plant, producing greater genetic diversity in the offspring — a pea flower with stamen and carpel close together can self without any outside agent. Self-pollination is not a lesser form of reproduction; it remains sexual, with meiosis and fertilisation.

Blocking its own.

Bio 2611 Self-Incompatibility Mechanisms

Self-incompatibility genes block a plant's own pollen from germinating through its stigma and style, forcing cross-pollination — the style secreting an enzyme that degrades a pollen tube's R N A once it recognises matching self markers. This is genetic and acts after pollen lands, distinct from structural barriers such as whorls maturing at different times.

The collapse.

Bio 2613 "Self-Pollination and Asexual Reproduction Are the Same Thing"

Treating self-pollination as asexual reproduction overstates a real similarity: both reduce genetic diversity, but self-pollination still involves meiosis and fertilisation. A self-pollinated pea's offspring carry a new combination of the parent's own alleles rather than an identical genetic copy of it.

What it skips.

Bio 2615 Self-Pollination Still Requires Meiosis and Fertilization

Self-pollination runs the whole sexual sequence within one plant — meiosis forming gametes with new allele combinations, then fertilisation forming a distinct zygote. True asexual reproduction produces offspring genetically identical to the parent, because no meiosis and no gamete fusion occur at all.

Three entries. What each of the two fusion products becomes, where a seed's food ends up depending on the group, and what actually ends dormancy.

Two products.

Bio 2617 Double Fertilization Detail: Endosperm and Seed Coat Development

Beyond the two-sperm fertilisation, the polar-nuclei fusion product becomes triploid endosperm nourishing the seed, while the ovule's integuments become the seed coat — a corn kernel's starchy interior and its tough covering. Endosperm is triploid, from one sperm fusing with two polar nuclei, a different ploidy from the diploid embryo it feeds.

Where reserves sit.

Bio 2619 Seed Structure and Monocot/Dicot Storage Strategy

A seed holds an embryo, a seed coat and stored reserves — monocots keeping reserves in endosperm absorbed through a single cotyledon, dicots moving them into two cotyledons, as a peanut's two halves show. Monocot seeds are always endospermic; dicots split into endospermic and non-endospermic by where the reserves end up.

Waiting for a signal.

Bio 2621 Seed Dormancy and Germination Cues

Seed dormancy is suspended growth ending only when species-specific cues — cold, light, fire, or a softened coat — signal favourable conditions, as a temperate seed waits out winter and germinates in spring. Dormancy runs from months to decades depending on species, not a fixed universal period.

Three entries. Fruits classified by what tissue they came from, and two routes to new plants that skip fertilisation — one the plant does itself, one a horticulturist does to it.

Four origins.

Bio 2623 Fruit Types Revisited: Simple, Aggregate, Multiple, Accessory

Fruits also classify by origin: simple from one ovary, aggregate from multiple carpels in one flower, multiple from a whole cluster, and accessory from non-ovary tissue — a raspberry aggregate, a pineapple multiple. An accessory fruit like a strawberry develops mainly from tissue outside the ovary; the true fruit is only a small part of what is eaten.

On its own.

Bio 2625 Asexual/Vegetative Reproduction: Natural Methods

Plants propagate asexually via budding stems, adventitious roots, runners, and apomixis — seed formation without fertilisation — all producing offspring identical to the parent, as a strawberry's stolon roots at each node. Apomictic seeds look like ordinary sexual seeds but carry no new genetic combination, since no fertilisation occurred.

By hand.

Bio 2627 Asexual/Vegetative Reproduction: Artificial Methods

Horticulturists propagate by grafting a scion onto a rootstock, by rooting cuttings, by layering a bent stem, and by micropropagation in sterile culture — grafting joining a fruit-producing scion to disease-resistant stock. A grafted tree's scion and stock stay genetically distinct: two individuals joined without merging their genomes.

Two entries closing the argument. The trade between reproducing fast and reproducing varied, and the life-span vocabulary — with a caution about reading a record as a limit.

Speed against variety.

Bio 2629 Evolutionary Tradeoffs: Sexual versus Asexual Reproduction

Asexual reproduction matures faster and suits stable environments, since offspring match the parent exactly; sexual reproduction trades that speed for diversity that helps offspring survive changing conditions. A population reproducing only asexually spreads fast but shares one vulnerability to any new stress. Neither strategy is superior, and many species use both.

How long, how often.

Bio 2631 Plant Life Spans and Flowering Frequency

Plants classify by life span as annual, biennial or perennial, and by flowering frequency as monocarpic, flowering once, or polycarpic, flowering repeatedly — bamboo growing vegetatively for years, then flowering once and dying. A documented life span is a recorded observation rather than a fixed ceiling for the species.

One capstone running a flower from a bee's visit to a seed on the ground. Its warning is that this is a sequence of stages, each able to fail, rather than one automatic consequence of pollen landing.

The capstone. Pollen to dispersal.

Bio 2633 Tracing a Flower From Pollination to Seed Dispersal, Worked Capstone

A bee deposits pollen on a stigma. The grain germinates, its tube cell growing a tube down the style, guided at the end by synergid signals. By arrival the generative cell has divided into two sperm. Double fertilisation follows: one sperm and the egg make a diploid zygote, the other and two polar nuclei make triploid endosperm, while the integuments become a seed coat. The zygote becomes an embryo, endosperm accumulates reserves, and the ovary wall becomes fruit whose type determines how the seed travels.

Five things to carry out of Part Four, and out of Unit Eleven. A shed pollen grain carries two cells and no sperm. The tube cell will grow the pollen tube; the generative cell divides into two sperm only once that tube is growing toward the ovule. On the other side, meiosis makes four megaspores and three degenerate — one survives to become the eight-nucleate embryo sac. Complete and perfect are different words. Complete means all four whorls are present. Perfect means both reproductive whorls are present, which is narrower. Monoecious and dioecious measure a different axis again — whether one plant carries both flower sexes or they sit on separate plants. Self-pollination is sexual reproduction. It lowers genetic diversity, which makes it resemble cloning, but meiosis still happens and gametes still fuse, so the offspring carry a new combination of the parent's alleles rather than being copies. True asexual reproduction has neither step. Endosperm is triploid because it comes from one sperm fusing with two polar nuclei — three sets of chromosomes. The embryo beside it is diploid, from one sperm and the egg. A corn kernel's starch is genetically unlike the embryo it nourishes. And the two reproductive strategies trade different things. Asexual reproduction is fast and exact, which suits a stable environment and leaves a population sharing one vulnerability. Sexual reproduction is slower and produces variety that helps when conditions change. Neither is superior, and many plants do both. Unit Twelve turns to animal form and function.