Fruit Type
9.1 Pollination as a tendency #
Pollination is transfer of pollen to a stigma, or to an ovule in gymnosperms. Fertilisation is fusion of gametes, later. A visitor is a pollinator only if it moves pollen and that pollen sires seeds. Nectar robbers take the reward through a hole and do not qualify. The test is an exclusion experiment: bag the flowers, return one class of visitor, and measure seed. That is a design, not a bench protocol.
Wind-pollinated flowers tend to be small, without nectar, with abundant smooth pollen and feathery stigmas, often opening before the leaves or above a canopy (grasses, oaks, many conifers). Water pollination is rare (Vallisneria, eelgrass) and is not the rain splash of a moss. Animal pollination is the rest. The classical syndromes are a statistical summary of frequent visitors, not identification rules. Comparative tests find the match weaker than a textbook table suggests. A flower often has more than one effective pollinator.
| Tendency | Flower, as usually described | Reward | Why it fails as a law |
|---|---|---|---|
| Bee | Day-opening, blue, yellow, or ultraviolet, a platform, nectar guides | Nectar and pollen | Bees visit many other forms |
| Butterfly | Day, a narrow tube, a platform | Nectar | Some visit open flowers |
| Moth | Night, pale, a long tube, heavy scent | Nectar | Dusk visitors break the night rule |
| Carrion fly | Dull, mottled, foul scent, sometimes a trap | Often none; deceit | Many flies take ordinary nectar |
| Bird | Red or orange, tubular, little scent, thin nectar | Nectar | Not all bird flowers are red |
| Bat | Night, large, pale, exposed, musky | Nectar and pollen | Mostly tropical; not the only night shift |
Geitonogamy is transfer between flowers of one individual: cross-flower, genetically self. It matters in large displays and in clones. Self-pollination is pollen from the same plant. Cross-pollination is pollen from another. The genetic consequence depends on whether that other plant is a clone, which external appearance does not show.
9.2 Mating systems and self-incompatibility #
Outcrossing is favoured when inbreeding depression is strong. That genetics is Evolutionary Biology. The devices are here. Dichogamy separates the sexes in time (protandry or protogyny). Herkogamy separates them in space. Heterostyly, pin and thrum in Primula, as Darwin described, ties anther height, stigma height, and compatibility in one supergene. Monoecy and dioecy separate them by development.
Self-incompatibility rejects self pollen even when it lands, and it is allele-specific.
Gametophytic self-incompatibility uses the pollen’s own haploid genotype. In Solanaceae, Rosaceae, and Plantaginaceae, a style S-RNase enters the tube and a pollen F-box protein destroys non-self RNases. A matching RNase survives and the tube dies in the style. An S1S2 pistil rejects S1 and S2 pollen.
Sporophytic self-incompatibility uses the diploid parent. The tapetum loads the pollen coat. In Brassicaceae a stigma receptor kinase recognises a matching coat ligand (SCR/SP11) and rejects the grain at the surface. Dominance between alleles is common, so the ratios do not match the gametophytic case. Both systems have many alleles, because a rare allele is rejected less often. That frequency dependence is Evolutionary Biology.
Not every failure of self seed is incompatibility. Early-acting inbreeding depression can abort embryos and mimic it. Incompatibility is allele-specific and acts at the stigma or in the tube. Inbreeding depression is genome-wide and acts later.
Selfing is reproductive assurance when mates or pollinators are scarce, as Darwin argued from flower form. Cleistogamous flowers, as in many violets, never open and self, often beside open flowers. The cost is inbreeding depression. Shifts from outcrossing to selfing, often by loss of self-incompatibility, have happened many times. They are not a ranking of plants.
9.3 Dispersal of fruits and seeds #
Syndromes here have the same status as pollination syndromes: a good guess at the usual agent, not a proof.
| Syndrome | Diaspore | Example |
|---|---|---|
| Wind, wing or hair | Samara, pappus, plume | Maple; dandelion; willow |
| Wind, dust | Minute seeds, orchids often needing a fungus to establish | Orchid |
| Water | Buoyant fibrous or air-filled coat | Coconut; water lily |
| External on animals | Hooks, barbs, mucilage | Burdock; mistletoe viscin |
| Internal in animals | Fleshy reward, resistant seeds, often a colour change | Cherry; mistletoe berry |
| Scatter-hoarding | Large edible seed, forgotten caches | Oak; hazel |
| Ballistic | Fruit splits and throws | Geranium; Impatiens |
| Ants | Seed with an elaiosome | Many woodland herbs; violets |
An elaiosome is an oil body on the seed. Ants eat it and discard the seed (myrmecochory). A parachute suggests wind and may also catch on fur. The effective agent is an empirical question. Dispersal in time is dormancy. A quiescent seed is waiting for water. A dormant seed refuses to germinate even when water, warmth, and air are supplied, because of the coat or because of the embryo. Abscisic acid and gibberellin are Plant Physiology. A soil seed bank is the living seeds in the ground. It can outlast the plants above it, and it is not a packet in a store.
9.4 Vegetative reproduction and apomixis #
Vegetative reproduction copies the sporophyte with no meiosis and no fertilisation: rhizomes, stolons, tubers, bulbs, corms, leaf plantlets of Kalanchoe, broken stems. The offspring are ramets of one genet, fed by the parent and uniform enough that a disease can sweep them. Commercial banana clones are the agricultural case. Some genets are far older than any one shoot. Quoted ages for particular clones are estimates and are not repeated as facts.
Apomixis is a maternal embryo in a seed, without fertilisation of the egg. In gametophytic apomixis an unreduced embryo sac forms by diplospory (the megaspore mother cell skips reduction) or apospory (a nucellus cell). The egg develops by parthenogenesis. Endosperm may still need fertilisation (pseudogamy, common in grasses) or form without it (many dandelions). In sporophytic apomixis, embryos bud from nucellus or integument beside a sexual embryo. Citrus nucellar seedlings are maternal clones used as rootstocks. Apomixis can hold heterozygosity and odd ploidy, as in triploid dandelions. It is not selfing, which still has meiosis and fusion, and not vegetative spread, which makes no seed. Not every Taraxacum is apomictic.
9.5 Key terms, summary, and self-check #
Key terms. Pollination; geitonogamy; protandry; heterostyly; gametophytic self-incompatibility; sporophytic self-incompatibility; cleistogamy; elaiosome; seed bank; ramet; genet; apomixis; pseudogamy.
Summary. Syndromes of flower and fruit describe frequent agents of pollen and seed movement and over-simplify. A visitor is a pollinator only if seed is sired. Gametophytic incompatibility kills a matching pollen tube in the style by the pollen’s own allele; sporophytic incompatibility rejects the grain at the stigma using coat proteins loaded by the tapetum. Selfing, including cleistogamy, is assurance at the price of inbreeding. Vegetative reproduction clones the sporophyte. Apomixis makes a maternal embryo inside a seed, with or without a fertilisation that serves only the endosperm.
Self-check.
1. Why is a floral visitor not automatically a pollinator?
2. In gametophytic self-incompatibility, whose genotype decides, and where does a matching tube fail?
3. What does the tapetum contribute to sporophytic self-incompatibility?
4. How do geitonogamy, selfing, and apomixis differ?
5. What is an elaiosome, and how does a soil seed bank differ from stored seed?
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