Dispersal Of Fruits And Seeds

Recall that one property of populations is dispersion, the spatial distribution of individuals. If individual plants are too close together, they must compete with each other for available water, nutrients, and sunlight. One reason for the success of the seed plants is the development of structures that are adapted for dispersing offspring—fruits and seeds.

Fruits and seeds are dispersed by animals, wind, water, forcible discharge, and gravity. You may have walked through a field and unwittingly collected burrs, or stickers, on your shoes or socks. These burrs are fruits, and you helped disperse them. The smell, bright color, or flavor of many fruits attract animals. When animals eat such fruits, the seeds are transported to new locations and often pass unharmed through the animals' digestive systems.

Fruits and seeds dispersed by wind or water are adapted for those methods of dispersal. Orchids have tiny, dustlike seeds that can easily be carried by a slight breeze. Figure 30-10 shows an example of seeds adapted for wind dispersal. Many plants that grow near water produce fruits and seeds that contain air chambers, which allow them to float. Coconuts, for example, may float thousands of kilometers on ocean currents.

The most dramatic method of seed dispersal occurs in plants that forcibly discharge their seeds from their fruits. The tropical sandbox tree, which has fruits that hurl seeds up to 100 m (328 ft), appears to hold the distance record.

Although gymnosperms do not produce fruits, their cones may help protect seeds and aid in seed dispersal. Female pine cones close after pollination and open again only when the seeds are mature. Pine seeds may be dispersed when gravity causes cones to drop and roll away from the parent tree. Pine seeds have "wings" that aid in wind dispersal.

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