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Vectors of Propagule Transport

The great number of different means of transport for propagules requires a structured overview of the vectors and mechanisms. Three large groups are distinguished:

• autochory, where the plant itself carries out the dispersal of its propagules,

• allochory, where the plant exploits different means of transport (vectors),

• atelochory, where dispersal is inhibited

Autochory

Plants have evolved many mechanisms which ensure distribution in different ways. In the simplest form of autochory, barochory, propagules are transported via gravity without special aids, a method that may only result in distribution over a wider area on steep slopes. Usually, the highest density of seeds is reached near the mother plant (seed shade). In contrast, with blastochory, propagules only find suitable growing sites by growth processes, e.g. via scions (vegetative shoots) or pedicels as in Cymbalaria muralis. In herpochory propagules move themselves short distances, e.g. the awns of Trifolium stellatum have hygroscopic characteristics, and twist with changes in humidity, dispersing the propagules (Fig. 4.2.1 B).

Special forms of autochory are summarised under the term ballochory, where propagules are propelled by a single impulse from the

Table 4.2.1. Distribution Syndrome. Collected from Müller-Schneider (1977), Lindacher (1995), Frey and Lösch (1998)

Autochory (self dispersal) Barochory (transport via gravity)

Blastochory (dispersal via runners) Herpochory (transport via active creeping) Ballochory (self seeder)

- Zooballochory (impetus provided by animals)

- Anemoballochory (impetus provided by wind)

- Hydroballochory (impetus provided by water)

- Autoballochory (propulsion mechanisms based on sap pressure or drying)

Allochory (dispersal via a vector) Anemochory (transport via wind)

- Chamaechory (transport close to the soil because of large size and soil adherence to animals, etc.)

- Meteochory (transport in air for small seeds)

- Boleochory (transport started by wind, further dispersal assured by other mechanisms)

Hydrochory (transport in water)

- Nautochory (transport by movement in the sea)

- Bythisochory (drifting in flowing water)

- Ombrochory (transport via rain drops)

Zoochory (dispersal via animals), further differentiation by animal types, e.g.:

- Ornithochory, myrmecochory, etc.

- Epichory (transport by attachment of propagules to animals)

- Endochory (transport of propagules during passage through the gut)

- Stomatochory (transport in the mouth)

- Dysochory (transport of accidentally ingested propagules)

Hemerochory (dispersal by man)

- Ethelochory (deliberate dispersal)

- Speirochory (dispersal due to accidental dispersal with seeds)

- Agochory (unintentional dispersal)

Atelochory (transport and dispersal are not impeded)

mother plant. This may be triggered by animals, wind or even a raindrop. In autoballochory even such impulses are not required and the pressure is produced via differences in turgour (Ecbal-Hum elaterium) or drying fruits or seed cases (Bauhinia purpurea, broom species). Seeds can be thrown over considerable distances, often several metres.

Allochory Anemochory

Wind is a very important vector for allochoric seed transport. Even propagules blown by wind across the ground surface possess morphological adaptations. Such chamaechory is frequent in dry regions where there are rarely impediments to such transport. More important, however, is transport by air currents (meteochory). Small, light propagules which do not settle easily are transported in vertical - as well as horizontal -air streams across large distances, e.g. seeds of orchids or spores of cryptograms (so-called dust-fliers). Balloon fliers (Astragalus spinosus) possess special morphological adaptations, as do seeds or fruits with parachutes (Taraxacum officinale) and wings (Acer and Fraxinus species). Wind also triggers seed transport in plants which scatter seeds, e.g. the scatter mechanisms of Papaver species (boleochory).

Hydrochory

Transport in, and with the help of, water is important for many plant species. Some propagules are able to float because of special tissues or large intercellular spaces, have a low specific weight and an external cover which is difficult to wet (Nymphea and Nuphar species). Coconuts, as well as seeds and seedlings of mangrove species, are even able to stay in salt water for long periods without damage. Seed dispersion in salt water and stagnant water is called nauto-chory; that in flowing water is called bythiso-chory. In flowing water seeds are transported by floating, and often drift over large distances. Small distances are often achieved by the impact of raindrops (ombrochory, e.g. Anastatica hiero-chuntica in dry regions). It is important that seeds in water do not germinate prematurely because of continuously moist conditions.

Zoochory

The most important and ecologically most complex forms of allochoric distribution have devel oped in zoochory. Close interrelations between some plant and animal species point towards a long co-evolutionary development. This applies particularly to endochory, where propagules are taken up by the animal and pass through the intestines. During this time (retention time) they are transported by the animal. Most such seeds have a relatively hard shell and do not get damaged during passage through the gut, nor is it necessary for germination. However, after excretion, seeds are provided with very good starting conditions for germination in the nutrient-rich excrement. Some of the propagules are taken up randomly, particularly by large herbivorous mammals.

In addition to endochory, there are many other ways in which seeds are distributed by animals: Birds and bats, as well as ants, are specifically attracted; plants invest in nutrient-rich fruits and attract attention by striking colours or strong smells. Ants are particularly well known for their distribution processes (myrmecochory, however, this is a form of epichory, see below) and so are birds (ornithochory). The former carry seeds possessing lipid-rich elaiosomes (e.g. Corydalis cava) but the seeds remain untouched. This form of distribution is also called synchory. Birds react first and foremost to colours and are able to distinguish ripe seeds from unripe seeds.

In dysochory propagules are eaten and sometimes damaged. Such propagules, which are often starch-rich, are collected and accumulated as reserves (e.g. seeds of Pinus cembra collected by a bird, Nucifraga caryocatactes), but some propagules still have a chance of surviving because not all hiding places are found again and some seeds are lost during transport. Some propagules are taken, together with the fruit pulp, into the mouth and spit out again (stomatochory) by many species of monkeys.

Propagules and their fruit pulp or endosperm are not always used as food. These means of attraction are often not developed at all, but still seeds or whole fruiting bodies are transported by animals. Propagules often possess glue-like excretions, glandular hair, barbs with awns and other outgrowths formed from the pericarp. Therefore, they are easily attached to fur and feathers and are transported until they fall off or are stripped off by animals during grooming and preening. In this form of transport, epichory, animals distributing the seeds are not rewarded by food or energy and the plant invests less. In endochory close interaction exists be-

Table 4.2.2 Relationship between propagules and dispersal by vertebrates. (Howe and Westley 1986, with additions)

Animal/animal group Propagule colour Propagule smell Propagule form Use to animals

Mammals in herds Birds in flocks

Brown

Green, brown

Little smell Without

Frugivor mammals in Yellow, green, white, Aromatic trees Bats

Ground living and frugivorous mammals

Frugivorous birds (obligate)

Frugivorous birds (facultative)

Furry or feathery orange, brown

Green, white, light Aromatic, musty yellow

Green, brown Without

Black, blue, red, green Without

Black, blue, red, white Without

Insignificant Without

Thick husked nuts, do not burst open

Seeds without wings and small nuts

Seeds often with arils, whole fruits, burst open

Diverse, often pendent fruit

Hard, over 50 mm long fruits, do not burst open

Big seeds with arils, whole seeds often burst open

Small seeds with arils, berries and stone fruits

Seeds

Seeds

Arils, pulp rich in proteins and sugars

Lipid- and starch-rich fruits

Lipid-rich fruits

Lipid- and protein-rich fruit flesh

Mostly carbohydrate-rich fruit

Sticky and barbed hooks None tween plant and animals and distribution is regulated, sometimes even targeted. Epichory, in contrast, is often random. Some of the close links between characteristics of propagules and the animals distributing them are summarised in Table 4.2.2 (Howe and Westley 1986).

Hemerochory

Man plays an increasingly important role in the recent history of plant distribution. In this particular allochoric form, hemerochory or anthro-pochory, any distances may be covered and all geographical and ecological barriers overcome. In ethelochory, plants are purposefully moved to different regions, e.g. to provide food or ornament. If distribution occurs unintentionally along with other propagules (e.g. weed seeds in seeds of cereal crops), this is called speirochory or in the case of random distribution agochory.

Atelochory

The most important methods of distribution, autochory and allochory, are contrasted with atelochory (also called achory). This is a special form of distribution, as it is prevented. The consequence of this evolutionary development is that reproduction takes place at the site where the mother plant grows, which is favourable to the species. Examples are Arachis hypogaea or Trifolium subterraneum. After pollination pedicel and ovary penetrate into the ground.

Heterospory is widely used by therophytes in arid regions as the chance for survival are particularly good at sites where the mother plant is able to form fruits. Only some of the propagules are distributed to "conquer" new growing sites (Evenari et al. 1982).

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