Integrative Biology 335:
Systematics of Plants

Lecture 28: Introduction to Dispersal

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Announcements: Today's lecture on dispersal will be given by Dr. Ken Robertson of the Illinois Natural History Survey.

IMPORTANT! Click here to download a pdf file of some notes for today's lecture. Bring these notes to class with you.

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Remember, there are two critical stages in the life cycle of a flowering plant:

1. The transfer of pollen from anther to stigma. As we will saw in the lecture on "Breeding Systems", most flowering plants have different types of mechanisms to promote the transfer of pollen from an anther in one flower to a stigma in a different flower, and hopefully this flower will be on a different plant.

2. The dispersal of seeds away from the parent plant.

Plants often solicit the aid of animals, as well as abiotic forces such as wind, to accomplish both of these.

• insects — insects much less important for dispersal than pollination, but ants often involved in dispersal
• birds, mammals, reptiles, and even fish — much more important for dispersal than pollination
• wind — important in both pollination and dispersal
• water — minor importance in pollination, somewhat greater in dispersal
• self dispersal — just like some plants carry out self pollination, some plants have mechanisms for self dispersal.

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Why is dispersal important?

• Avoid competition with parent and siblings
• Colonize new habitats
• Avoid pathogens and predators
• Minimize inbreeding

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Units of Dispersal — Different plant parts become modified for dispersal.

• Seeds are the units of dispersal, and the seeds are released from the fruits ready for dispersal.
• Fruits, or parts of fruit, for dispersal.
• Other structures sometimes modified for dispersal — ovary wall, style, sepals, hypanthium, receptacle, bracts, even whole plant.
• The term DIASPORE is used for the unit of dispersal, no matter what it is morphologically,

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Methods of dispersal are often tied to certain habitats

• Wind — prairie/grasslands, mountains, forest trees, weedy areas
• External attachment to animals — forest plants relatively low to ground
• Ingestion by animals — forest plants
• Water — plants that grow in wetlands and along streams
• Ballistic — various; some parasitic plants, some forest plants, some weedy plants

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OUTLINE Of DISPERSAL MECHANISMS

Wind Dispersal

• Dust-like seeds
  • orchid seeds
• Tiny seeds
  • Indian paintbrush
• Winged Diaspores
  • downy gentian seeds
  • Capsules releasing large winded seeds in Campsis radicans, trumpet creeper
  • Maple samaras
• Plumed diaspores
  • Milkweed seeds
  • dandelion fruits
• Woolly diaspores
  • Anemone achenes
  • cattail achenes
• Balloons
  • Staphylea trifoliata, bladder pod fruits
  • Cardiospermum fruits
• Roller plants
  • plants of Cycloloma atriplicifolium, winged pigweed
  • tumbleweed blowing across a wintery road

Water Dispersal

• Splash cups
  • capsules of lousewort, Pedicularis canadensis
• Sea currents
  • map of sea currents
  • coconut trees on tropical beaches
  • tropical "drift seeds" found on beaches in England
• Streams
  • Nelumbo flowers
  • Nelumbo fruits
  • Nelumbo fruits

Animal Dispersal — Passive

• Simple adhesion
  • sticky, mucilaginous coating around seeds when wet
  • mistletoe fruits enclosing seeds covered with sticky viscin
• Awns, hooks, & barbs
  • fruits attached to pants leg
  • Sock Removal Difficulty Unit (SRDU)
  • Bidens, beggar's tick fruits
  • Geum, white avens fruits
  • Arctium minus, burdock fruits

Animal Dispersal — Active

• Carrying & caching
  • acorns of oak trees
  • bird caching acorns in bark of trees
• Ants (elaiosomes)
  • seeds of some Papaveraceae, poppy family, have elaisomes
• Ingestion
  • plants with fleshy fruits, in this case all members of the Rosaceae, the rose family
  • bird eating fleshy fruits
  • hornbills eating figs
  • durian fruits smell horrible but taste terrific & tigers like them
  • drupes of Prunus padus
  • fruits of a tropical Licania plant floating on water
  • Tambaqui fish eating the fruits

Mechanical Dispersal

• Ballistic fruits
  • capsules of Hamamelis with fruits open and seeds waiting to be ejected
  • dwarf mistletoe fruits
  • other ballistic fruits
  • fruits of the squirting cucumber, Echbalium
• Shaker fruits
  • denticidal capsules of shooting star Dodecatheon meadia
  • poricidal capusles of papaver, Papaver
• Hygroscopic
  • the fruits of stork's bill, Erodium cicutarium, curl and uncurl with changes of humidity, planting themselves

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