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Biosphere Community

Plant Pages


Seed Dispersal

 

 
Dispersal by Wind
 
Spores of Mosses and Ferns
 
Tree Seed Spinners and Gliders
 
Silky Parachutes
   
Tossed from Cups
   
Shot from Pods
 
Fruits & Berries: Seasonal Animal Symbiosis
  Hitchhikers: Sticking with the Animals
  Dispersal by Ants: Eliasomes
    Dispersal by Mud and Water
  Dormancy: Dispersal Through Time

 

 

 

 

 


 

 

Background



Like all life, plants want to reproduce, or propagate. They can reproduce sexually and form spores or seeds. But after plants form spores or seeds, they have a problem. Plants are rooted in place. How can they disperse (distribute) their seeds? Minimally, plants have to get their seeds out of the parental shadow, a problem shared throughout the biosphere, shadow being a word whose meaning can enlarge. Plants have evolved many ways to spread seeds, and many of them require the help of animals, in seasonal symbiosis.

Adapting to Seasons

Plants, like all life, have to struggle with seasonal change: how to stay alive during winters, dry seasons, and flood. Seeds are ways to stay alive during stress conditions which often kill the parent plant.

Annuals: Plants that die after one growing season are called annuals. Their seeds are a stage of the plant's life cycle that persists through time.

Perennial plants, including trees and shrubs, have found different ways to survive seasonal changes. Woody perennials may drop leaves and pull sap and nutrients down into roots. Non-woody perennials, grasses for example, also store food in roots, die back to the soil surface, and re-sprout when conditions improve (weather warms; rains arrive; flood recedes).

In any natural community, populations of animal species do best if they are spread out, or well-dispersed. Many plants do best dispersed as well. Every habitat has limited resources. A dispersed population gives animals a way to provide food for a maximum sustainable number of their own kind. Available and suitable soil space is the requirement of plants, along with its species' sunlight requirements. Dispersal provides balance, a kind of homeostasis, in which competion with members of one's own species is minimized, and during mating, the greatest genetic variation is likely to be maintained.

The ancestors of green plants are freshwater green algae. They reproduce by 1) releasing swimming spores into water, and 2) releasing swimming male and female gametes.

When plants came to land, they explored other ways to disperse their reproductive parts. The land pioneers, mosses and liverworts (bryophytes), still rely on moisture for their gametes to swim through. Mist and fog, or simply high humidity and quiet air, are the occasion of such mating encounters. The result of fertilization is a new plant, a stalk that grows up from the parent moss and grows a capsule full of spores. The small lift above the parent gives the spores access to breezes that spread the spores. Mosses and liverworts use two fluids in sequence to reproduce, water to fertilize, then wind to disperse. Ferns also rely on wind to spread their spores.

Dispersal by Wind

 

Spores of Mosses and Ferns

sporangia of a tropical liverwort
sporangia of a temperate moss
spores of cinnamon fern, greatly magnified
sporangia on underside of deer fern frond

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Tree Seed Spinners and Gliders

Many trees rely on wind to disperse their seeds. These include the conifers,firs, pines and spruces, and such common deciduous trees as maples, aspens, elms, cottonwoods and ash. These trees usually package the seed with a wind foil that will at least propel the seed out of the parent's shadow. Some tree seeds spin, some glide, some sail from side to side.

ash seeds glowing light
seed cluster of weeping willow
clustered samaras of mountain maple: helicopters
seeds of ponderosa pine, out of the cone
elm seed sails rather than spins
fluffy aspen seeds will float with the breeze
or whole catkins flow with the stream

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Silky Parachutes

Many plants grow each seed a parachute to carry them as far as wind will allow.

long pod releases parachutes along a river
milkweed seeds have both vanes and parachutes
salsify seeds have fine parachutes
a single thistle seed waits for a breeze
cottongrass 'bolls' cover a sphagnum bog in autumn, awaiting wind
dandelion seedhead almost spent: remember?

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Tossed from Cups

Many plants rely on puffs of wind to bend and shake their cup-like pods to toss seeds a few feet:

the tippy pod of campion holds several seeds
black seeds line the four capsules of wild columbine seed pods
tiny pods of catmint, magnified
the 'birdmouth' pods of water horehound cast seeds far and wide
unknown papery pods in winter
tiny obedient plant pods line up on flower spikes
seed pod open mouthed
unknown 'candelabra' cup pods
tumbleweed plants break from roots and roll across the land dropping hundreds of tiny seeds
mullein pods pop open on top of their five foot swaying stalks

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Shot From Pods

Some plants have invented ways to catapult their seeds by drying their cases until something touches them to pop them open and spray their contents.

gorse pods burst with an audible pop
witch hazel pod throws seeds several feet
montbretia lily pod twisted open
wild iris pods twist open to spill seeds
jewelweed pod ready to explode seeds
jewelweed pod post-seed scatter

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Fruits & Berries: Seasonal Animal Symbiosis

Flowering plants are smart. They live in communities with all these animals that can move around easily while plants remain anchored with roots, so plants have gradually coevolved ways to get animals to cooperate with them in dispersing their seeds. Many of these arrangements benefit both symbionts.

Fruits and Berries provide prime examples of this cooperative arrangement. The plant encloses its seeds in an energy-rich package, such as an apple, a cherry, or a raspberry. The seeds inside are usually bitter, hard, and nearly indigestible, so they leave the animal surrounded with valuable fertilizer. Many fruit seeds must be etched by stomach acids before they can germinate.

Plants in northern climates synchronize fruit ripening with the annual fall bird migration, which is not accidental. Some six billion birds pass from the north into Central and South America, and they all require hearty foods to fuel their flight. This annual dispersal symbiosis is a marvel to behold. In contrast, in tropical climates, plants flower and fruit year round, with lulls in dry seasons.

California fan palm fruits are dispersed by coyotes
wild plum is dispersed by many animals, most after falling to the ground
Wild cherries are often bird dispersed, but bears and raccoons eat their share
small crab apples are favorites of birds such as cedar waxwings and robins
wild grapes and red osier dogwood berries are both enjoyed by thicket-foraging birds such as catbirds
many animals volunteer to disperse blackberry seeds; each tiny ball contains a seed.
dwarf cornel or bunchberry is favored by small animals of the north woods
juniper berries are favorite fall food
for cedar waxwings, robins, and many other birds.
red elderberries stand out so they will be dispersed by birds
sumac berries are sour favorites of such dispersers as robins
Mexican elderberries, like many blue berries, are attractive to birds
berries of spikenard disappear quickly when migrant birds pass through
Virginia creeper vine berries are another travel food of birds
wild strawberries are dispersed by countless fans
sparrows and juncos flutter against seedheads of woodland sunflower to shake seeds loose to snow, where they are highly visible.

winter birds bounce on goldenrod seedheads to shake seeds onto snow. Most are eaten, but enough are planted in snowmelt.

Nuts Among dispersal fruits are nuts. Most are eaten by insects and by mammals; raccoons and bears eat all they can get to prepare for winter sleep. But dispersal does happen because squirrels cache (store) acorns in the soil. Jays sometimes cache acorns as well. Many of our oaks were no doubt planted by animals who didn't find it over the winter. Clark's Nutcracker, a mountain bird of the West, caches thousands of pine seeds in autumn, and inevitably, some germinate instead of becoming food.

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Hitchhikers: Sticking with the Animals

For many forbs and herbs, seed dispersal is accomplished by recruiting animal help, by sticking to fur, paws, feathers, boots and clothing. You can call this a kind of symbiosis, but it's more a seasonal, mostly harmless, parasitism, or, in the right frame of mind, call it trickery. The ways seeds can stick, cling (and even burrow) are amazing and effective. History proclaims that Archimedes discovered and implemented the concept of the screw, but needle-and-thread grass did it a million years before.

needle-and-thread grass drills into fur, paws,
even boot leather to disperse seeds. Eventually,
actual screw threads drill the seed into soil.
Burdock seed hooks are incredibly effective. Supposedly, they inspired the inventor of Velcro
Biden seeds, beggar ticks or pitchforks,
are covered with tiny barbs
that can drive furry creatures to despair
bur marigold is innocent in flower,
but its seeds are beggar ticks.
sharp-barbed seeds of sweet cicely are
well-traveled autumn through spring
seeds of tick trefoil are covered in microscopic hooks that cling with painless efficiency
detail of the hooked seed
of Enchanters' Nightshade, named for Circe
tricky stickers wait above the snow for a rabbit or pheasant to pass beneath.
Many grasses have catchy seedheads. This invasive California grass can puncture.
This California beachgrass seedhead betrays
those who brush up against it.
many small anonymous plants rely on
small furry animals to disperse their seed
fine prickers cover the surface of Desmodium stick tights that treat stockings like long-lost lovers

In Grapes of Wrath, author John Steinbeck describes hitchiking seeds as they seem to humans: "The concrete highway was edged with a mat of tangled, dry grass, and the grass heads were heavy with oat beards to catch on a dog's coat, and foxtails to tangle in a horse's fetlocks, and clover burrs to fasten in sheep's wool; sleeping life waiting to be spread and dispersed, every seed armed with an appliance of dispersal, balls of tiny thorns, and all waiting for animals and for the wind, for a man's trouser cuff or the hem of a woman's skirt, all passive but armed..."

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Dispersal by Ants: Elaiosomes

Ants are the most common animal on earth, and at the same time uncommonly surprising. No creature is embedded so deeply in the fine web of relationships among species as is the ant, the master of symbiosis. Plants and ants of many species in every habitat are intertwined in life. Ants defend plants; plants feed ants. For details of several ant symbioses go here.

Plants have learned that the key to ant cooperation is their stomachs. So some plants use ants to disperse their seeds and give those seeds a head start on germination. Such plants attach a treat to their seeds, called an eliasome. These treats are rich in nutrients. Ants take the whole seed+eliasome back to the nest, where the eliasome is often fed to ant larvae, and the seed is then deposited in the colony trash pile, which is rich in nutrients, a kind of compost bin. So the seed sits protected in a dark, relatively humid nutrient collection some inches below the soil surface. A good place to grow.

an ant carries a seed to its colony using the eliasome as a handy handle
the hard-to-miss eliasome of bloodroot seeds
an ant drags a seed with a treat
back home to share
Some seeds of the trout lily have shucked their seed coats. The Eliasome is the curl at the end.
eliasomes on seedsof the wood poppy
open seed pod of the wood poppy
White trout lily planted by ants
Trillium grandiflorum planted by ants
California bush poppy seed with eliasome
California bush poppy in flower

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Dispersal by Mud & Water

Zostera marina, Eelgrass in ocean shallows, is a flowering plant that returned to the sea, so it produces seeds, which must somehow disperse. Nature seems to have an elegant solution for every problem. When an eelgrass seed is ripe, it ascends to the surface attached to a purpose-made bubble of gas and floats with that bubble as long as it lasts.

Happenstance dispersals occur frequently. Every time a duck flies from pond to pond with a bit of mud stuck to it, there is very likely a seed, or several seeds, in that mud. Many birds and mammals hunt on mudflats and carry mud from place to place. One researcher reported 500 seeds that germinated in a single scoop of swamp mud.

Floating in Water is one of the most ancient ways of dispersal. Trees such as willow and poplars grow heavily on riverbanks. They routinely drop floating catkins into rivers and populate riverbanks and wetlands downstream.

Ocean islands owe their flora and fauna both to floating seeds such as seabeans and coconuts, and logs with animal passengers. No doubt birds played a part here as well.

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Dispersal Through Time: Dormancy

Seed Dormancy is a powerful process to ensure plant survival. Dormancy is the green plant way of harnessing time to its reproductive needs. A dormant seed, alive but metabolically inert, will not germinate until some event triggers growth.

Seedcoat Dormancy: The seedcoat of some plant seeds is so hard no water or gas can enter, so it remains dormant until the seedcoat is scarred or etched. Winter freeze and thaw cycles may be scarify the seedcoat enough, or microbial attacks my do the trick. Fleshy fruits intended for animal consumption to aid dispersal have hardcoated seeds that must pass through digestive acids to etch the seedcoat enough to break dormancy.

Some plants have a seedcoat that is permeable to water but impermeable to oxygen at warmer temperatures. Periods of cold are required before the seed can acquire enough oxygen to begin respiration (the process of burning stored food that begins germination). This is clearly a climate adaptation that ensures that seeds don't germinate just before a winter that will kill them. Gardeners often cool some seeds for a month or two to ensure strong spring germination.

Light Dormancy: Many small seeds cannot germinate until they receive light. Whenever soil is disturbed, many dormant seeds are brought to the surface where they germinate enthusiastically, some after waiting many years to break dormancy. Small seeded species often break dormancy this way, much to a gardener's dismay. Chickweed seems to have an inexhaustible supply of buried seed awaiting the innocent gardener. Soils everywhere are seed banks of dormant seeds waiting for the environmental trigger that will jumpstart growth.

Physiological or Internal Dormancy: Many plant seeds are dormant because of internal chemistry. Hormones required to grow may be inhibited for a timed period. When the inhibitor is used up, germination happens. This kind of internal dormancy is adaptive to seasonal change, winters and floods and dry seasons. One kind of timed dormancy is called "after ripening."

In general, seed dormancy aids a plant's survival by delaying some germination so that 'overall germination' is dispersed over time. This means that if the environmental conditions are not right during the normal germination period, a percentage of seeds will germinate at some later time when the conditions become right.

Slow Seed Release: Some plants spread seed release over several months. American Basswood does this, as does Paper Birch. This is a sort of parent-mediated dispersal through time.

Queen Anne's lace (right) holds its seeds in a tight basket shape until November or so, when it relaxes the cage and sprinkles its seeds.

 


Fire:
Some plants, forbs, grasses, shrubs and trees, produce seeds which will not germinate unless exposed to range fires or forest fires, that is, until the seeds reach a temperature in the soil raised to a trigger point by a fire passing through. Several conifers carry tightly closed cones gray with age, filled with viable but dormant seeds. Dormant jack pine cones 'wake up' and begin to open as soon as fire has passed through, whether the parent tree has survived or not. After a fire, ash contains some chemicals which help some seeds break dormancy, but the most important change for the success of newly germinating seeds is bare mineral soil coupled with the potash and calcium released by wood ash.

pine cones sleep, waiting patiently for fire
Fireweed blooms next to a burned log. It thrives on mineral soil and nutrients released by ash, with the high light levels in new forest openings.

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Explore Plants

Plants: What's New? Including Symbiosis with Fungi and Bacteria
Plants: Major Kinds (Images)
Plant Defense including Symbiosis with Wasps
Plant Pollination including Symbiosis with
Insects, Mammals and Birds
Seed Dispersal Including 'Compelled Symbiosis' with Birds, Insects and Mammals

Explore the Biosphere

 
Biosphere: Introduction
 
Biosphere as Place: Introduction
 
Biosphere as Ocean: Life Zones
 
Biosphere as Ocean Floor: Benthic Biomes One
 
Biosphere as Ocean Floor: Benthic Biomes Two
 
Biosphere on Land: Terrestrial Biomes
 
Biosphere on Land: Anthropogenic Biomes
 
Biosphere as Process: Introduction
 
Biosphere Process: Floating Continents, Tectonic Plates
 
Biosphere Process: Photosynthesis
 
Biosphere Process: Life Helps Make Earth's Crust
 
Biosphere Process:
Rock Cycle--Marriage of Water and Rock
 
Biosphere Process: Marriage of Wind and Water
   
Biosphere Process: Gas Exchange
 
Biosphere as An Expression of Spirit
 
The Ecological Function of Art
 
The Earth Goddess
 
The Tree of Life
 
The Green Man
 
Earth Art
 
Biosphere as Community
 
Biosphere Microcosm: Bacteria and Archaea
The Procaryote Domain
 
Biosphere Microcosm: Germs
 
Biosphere Community: The Eucaryote Domain
 
Biosphere Community: Protists 1: Algae
 
  Biosphere Community: Protists 2: Protozoa
 
Biosphere Community: Plants: What's New?
 
Biosphere Community: Kinds of Plants--Major Groups
 
Biosphere Community: Plant Defense
 
Biosphere Community: Plant Pollination
   
Biosphere Community: Plant Seed Dispersal
 
Biosphere Community: Kingdom Animals
 
Biosphere Community: Kingdom Fungi
 
Biosphere Community: Six Great Extinctions
 
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