tree-logo.gif (7741 bytes) Plant Taxonomy (BIOL308)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;


I. Pollination.
    Transfer of the pollen from the anther to the stigma. The pollen produced by the stamens of a flower can pollinate a stigma of: (a) the same flower (self-pollination); or (b) another flower on the same plant (also self-pollination); or (c) a flower on a different plant (cross-pollination). Cross-pollination is more "desirable" because it provides for the genetic recombination of alleles necessary for maintaining variation in a population.

A. Most plants have some method to prevent self-pollination. These include:

  1. unisexual plants (i.e., monoecious, dioecious, polygamous). This mechanism functions to separate individuals in space (spatial separation);
  2. asynchronous maturation of flowers - i.e, stamens shed pollen before stigma becomes receptive (protoandry), or the stigma is receptive before the stamens shed pollen (protogyny). This mechanism separates individuals in time (temporal separation);
  3. self-incompatibility mechanisms (homomorphic - no visible difference in flowers, pollen simply won't germinate on an incompatible stigma. This involves a variety of alleles designated ('S'); heteromorphic - physical difference in flowers. For example, recall our discussion of water hyacinth. This plant exhibits heterostyly – some individuals of a species have pin flowers (long style and short stamens) while other individuals have thrum flowers (long stamens, short style); and
  4. chasmogamous flowers (flowers open up, typical fashion) vs. cleistogamous (flowers remain closed, these are selfers). Violets (Viola sp.) and Hog peanut (Amphicarpaea bracteata) produce both cleistogamous and chasmogamous flowers

B. Comparison of self-compatible (autogamous) vs. self-incompatible (out crossing or allogamous) species

Generalized Comparison of Self-Compatible and Self-Incompatible Species

Character Self-Compatible (Autogamous) Self-Incompatible (Allogamous)
Cross/Self Pollination? both; cross & self cross
Flowers few/many? few many
Pedicels long/short? short long
Petals/sepals large/small? small large
Nectaries absent/present? absent present
Nectar guides absent/present? absent present
Scented/unscented? no odor odor
Ovules per flower? (few/many) few many
Ovules maturing to seed? (few/many) many few
Weedy/not weedy weedy not weedy
Distribution wide/restricted? wide restricted

II. Floral evolution and the need for pollen transfer.
    Since plants are non-motile, they rely on outside agents to get the pollen from the androecium to the gynoecium. The agent, or vector, used to transfer pollen can be abiotic (non-living, like air or water) or biotic (living, such as an animal). Common vectors for pollinating flowers include wind, water, beetles, bees, butterflies and moths, birds, and bats. Even lizards, snakes and some mammals (such as mice and giraffes) pollinate some species.

    A flower must be adapted for its pollinator. In fact, floral morphology is directly related to pollinator type. The specialized adaptations of a flower for a pollinator type are collectively known as pollination syndromes or systems. Plants differ in the degree of their specialization for a particular pollinator. Some are specialists, others generalists. For example, many orchids are pollinated by only a single species of bee. Other flowers are not as specialized and may be pollinated by a variety of bees or perhaps beetles. In other cases, insects may visit flowers without actually transferring pollen, veritable thieves.

III. Pollination Systems. The following lists the characteristics for a typical flower pollinated by each of the indicated vectors.

A. Wind Pollination (anemophily).
    Common in the Grass family (Poaceae), Beech family (Fagaceae), Sedge family (Cyperaceae), and Walnut family (Juglandaceae). Occurs almost entirely in temperate plants, absent from plants in tropical areas. This makes sense since tropical forests are usually dense with few individuals of any one species widely scattered and the high humidity reduces the buoyancy of the pollen.

    Wind-pollinated flowers are generally unisexual; produced before leaves or exposed to the outside of the leaf mass. The perianth is insignificant, small or absent; attractants absent; anthers and stigmas exposed, often feathery; pollen grains small, smooth, dry and produced in large quantities; flowers often aggregated into catkins; ovules often reduced to one - because the chance of more than one pollen grain reaching the ovary is slim. This pollination system is derived from insect pollinated systems. It is an inefficient system, only works if another individual is growing nearby, which is one reason why there are few wind-pollinated tropical species. Many wind pollinated species often flower early in the season before trees leaf out.

B. Beetle Pollination (cantharophily).
    Common in primitive plants such as the Magnolia family (Magnoliaceae) and Calycanthaceae. Beetles may have been first pollinators - the evidence is indirect; beetles were present in Cretaceous when angiosperms first appeared (although lizard pollination may have been even earlier).

    Beetle pollinated flowers are either solitary (as in Magnolia) or aggregated into inflorescences (Cornus); few visual attractants; no special or definite shape; little depth effect; no nectar guides; actinomorphic, relative flat or sometimes bowl-shaped; dull in color, greenish or whitish; and strong or fruity smelling odors. Beetles visit the flowers primarily for food (eating pollen and/or floral structures). Thus, they have numerous floral parts (especially carpels and stamens) because they get eaten. And, the ovules tend to be protected. This is not a particularly efficient system, so the flowers typically have a large # of floral parts.

C. Bee Pollination (melittophily).
    Typically found in the Mint family (Lamiaceae), Snapdragon (Scrophulariaceae ); Pea family (Fabaceae); and Orchid family (Orchidaceae). Bee pollinated flowers are typically zygomorphic with great depth effect; mechanically strong, with adequate facilities for landing and a surface that gives a good foothold; intricate and semi-closed, especially those visited by larger bees; brightly colored corolla, generally yellow or blue, but never pure red; and nectar guides. The nectar is hidden, with nectaries characteristically situated at the base of the corolla tube. Nectar is produced in moderate quantities. These flowers typically have a fresh odor, but generally not strong. The sexual organs are usually concealed and the flowers typically have few stamens and usually many ovules per ovary.

    Bees are probably the best adapted species for pollinating. There are at least 20,000 species and probably all visit flowers for feeding. Bees are quick to learn color, odor, outlines. In general, bee flowers have a distinctive pattern by which bees can recognize them. Because bees see from 300 - 650 nm (in contrast to humans - 400-700 nm) flowers often have distinctive patterns that are visible only in UV light.

D. Butterfly and Diurnal Moths.
    Butterflies were not present (abundantly) until the Tertiary, so this represents a later evolutionary advance. Butterfly pollinated flowers: diurnal anthesis and generally do not close at night; odor weak, generally fresh and agreeable; flowers vividly colored, including pure red; corolla lobes or rim not much dissected; flowers erect, actinomorphic, rim generally flat, but often narrow; stamens often epipetalous; nectar is well hidden in narrow tubes or spurs; nectar ample; simple nectar guides or mechanical tongue guides present.

E. Nocturnal Moths.
    These flowers typically have: nocturnal anthesis, are often closed during the day; strong, heavy-sweet odor at night; mostly white or faintly colored; deeply dissected corolla lobes or fringed petals; flowers horizontal or pendant, rim absent or bent back; zygomorphy may be present and is caused by the lower rim bending back; nectar deeply hidden in a long tube or spur; tubes narrower than in bird flowers; more nectar than in butterfly or bee flowers; nectar guides generally absent; moths guided by contour of flower.

F. Fly Pollination (myophily).
    Flies are rather unspecific pollinators. There is lots of variation in flowers pollinated by flies. And, these flowers are not very spectacular. The flower are generally: actinomorphic, simple with no depth effect; colors generally light, but dull; nectar guides often present; odor generally imperceptible, except in sapromyophily (carrion flies); nectar is open or easily obtainable; flower sexual organs well exposed.

G. Carrion Fly (sapromyophily).
    These flowers deceive pollinators into visiting by producing attractants. The flowers are typically: actinomorphic; but usually with great depth; tube of corolla often lined with filiform appendages; colors usually dull, dark, brown-purple-greenish; no nectar guides or other leading lines, but checkered with dark spots and often with transparent windows. Odor resembling that of decaying protein, rotten meat. Generally no nectar or other primary attractant. The sexual organs are generally hidden in the interior of the flower.

H. Bird Pollination (ornithophily).
    These flowers exhibit: diurnal anthesis; vivid colors, often scarlet; lid or margin of corolla absent or curved back; flower tubular and often hanging; zygomorphy usually absent; flower wall hard; filaments stiff or united; ovary generally protected, nectaries basal; odors absent (birds have a poor sense of smell); nectar very abundant; floral tubes or spurs deep; tubes wider than in butterfly flowers; nectar guide absent or plain.

    Hummingbirds are typical bird pollinators. Most red flowers are pollinated by hummers. For example, in Europe where there are no native hummers, there are no native red flowers

I. Bat Pollination (chiropterophily).
    These have nocturnal anthesis, often opening for only one night; flowers sometimes whitish or cream, but often drab colored (greenish, purplish, rarely pink); strong odor at night, stale smell reminiscent of fermentation; flowers large-mouthed and very strong mechanically; inflorescences either solitary or strong inflorescences of many small flowers; flowers pendulous or attached to tree trunk (cauliflorous) like cacao pods. Nectar is produced in exceedingly large quantity; pollen produced in large quantity; anthers large or numerous.

IV. Polyclave Key to Pollination Systems
    see the notes on "taxonomic keys"

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Last updated:  08/20/2007 / � Copyright by SG Saupe