Seed Dispersal

How are seeds dispersed?

What are fleshy fruits for, and why have they been favored by natural selection? They are not there to nourish the seedling—the endosperm does that. So why should these plants have seeds that are wrapped in an additional layer of nutrient-packed tissue? It seems pointless, but in evolutionary terms, it makes all the sense in the world.

Think of the blackberries that grow wild in the forests of North America. Each seed is enclosed in a sweet, juicy fruit, making it a tasty treat for all kinds of animals. What good is such sweetness if all it does is get the seed eaten? Well, believe it or not, that's exactly the point.

FIGURE 24–10 Mechanisms of Seed Dispersal A Bohemian waxwing feasts on mountain ash berries (left) and will later disperse the seeds in its feces. Parachute-like dandelion fruits catch the wind, carrying the tiny attached seeds far and wide (middle). The buoyant coconut fruit can disperse its seed over great distances of water (right).

Dispersal by Animals The seeds of many plants, especially those encased in sweet, fleshy fruits, are often eaten by animals. The seeds are covered with tough coatings, allowing them to pass through an animal's digestive system unharmed. The seeds then sprout in the feces eliminated from the animal. These fruits provide nutrition for the animal and also help the plant disperse its seeds—often to areas where there is less competition with the parent plants. Seeds contained in fleshy, nutritious fruits are usually dispersed by animals. Three mechanisms of seed dispersal are shown in Figure 24–10.

Animals also disperse many dry fruits, but not necessarily by eating them. Dry fruits sometimes have burs or hooks that catch in an animal's fur, enabling them to be carried many miles from the parent plant.

Dispersal by Wind and Water Animals are not the only means by which plants can scatter their seeds. Seeds are also adapted for dispersal by wind and water. Seeds dispersed by wind or water are typically contained in lightweight fruits that allow them to be carried in the air or in buoyant fruits that allow them to float on the surface of the water. A dandelion seed, for example, is attached to a dry fruit that has a parachute- like structure. This adaptation allows the seed to glide considerable distances away from the parent plant. Some seeds, like the coconut, are dispersed by water. Coconut fruits are buoyant enough to float in seawater for many weeks, enabling the seeds to reach and colonize even remote islands.

Table of Contents