18.12: Fruits are a way for plants to disperse their seeds.

Every fruit you eat used to be a flower: tangerines, peaches, raspberries, cherries, pumpkin, and squash, among others. Fruits are produced by plants to aid in dispersing the seeds that will grow into their new offspring. By encasing seeds in a juicy, nutritious structure—the fruit—plants create a package that entices hungry animals. Animals eat the fruit, gaining valuable nutrients and energy. Then, at a later point—and in a location far from the parent plant—the animals eliminate the seeds, which have passed unharmed through their digestive system. At the new location, often surrounded by a bit of the animal’s feces that serves as fertilizer, the seed can grow.

There is a huge variety of fruits beyond those adapted for consumption by animals. And although they all serve the function of dispersing the seeds, not all require the assistance of animals. Just as with dispersal of pollen, plants can also use wind or water to transport their seeds. FIGURE 18-23 presents a small sampling of the rich variety of fruits and seeds produced by plants. As is apparent, the term “fruit” applies to a much broader range of structures than is typically associated with it. And the makeup of each fruit clearly reflects the method by which it is dispersed—wind, water, or animals.

Wind-Dispersed Fruits and Seeds Several characteristics of fruits and seeds make them well adapted for wind dispersal.

• 1. Hairy. Bushy hairs allow the fruits of dandelions and milkweed to float in the air.
• 2. Winged. Usually released from tall trees such as elms and maples, winged fruits, and in some cases winged seeds that are expelled from fruits prior to dispersal, float away from the tree as they slowly descend.
• 3. Tiny, dust-like. Many orchids have seeds that are as tiny and light as dust and so float in the air.
• 4. Explosive. Many fruits, such as those of mistletoe, explode when ripe, propelling the seeds as far as 50 feet (about 15 m) away at speeds of 60 miles (about 100 km) per hour.

Water-Dispersed Fruits and Seeds Some plants produce floating fruits, such as the coconut, that can be dispersed by rivers or oceans.

Animal-Dispersed Fruits and Seeds Animals act as dispersers simply by carrying or eating fruits.

1. Carried. Some plants produce fruits that have sharp or clingy burrs on the outside. These burrs catch on the legs or fur of animals (including humans) and are carried away until the animal grooms itself and removes the fruit, leaving the seeds to germinate and grow in the new location. Burrs were the inspiration for Velcro.

2. Consumed. Many fruits are sugar- and nutrient-laden fleshy structures eaten by animals, usually vertebrates. In some cases (such as the peach), the fruit develops from a single ovary; in others (such as blackberries or pineapple), it forms from multiple ovaries from numerous flowers. It does not benefit a plant to have its seeds dispersed before they are ready. Consequently, fruits generally do not ripen and become sweet and edible until the seeds are fully developed. In fact, most fruits remain green until the seeds are ready for dispersal, at which point the fruit sweetens and turns a bright color to attract the attention of dispersers.

The seed’s coating is tough enough to pass through an animal’s digestive tract without being destroyed, and the seed can germinate and begin growing wherever the animal defecates (or, in the case of birds, regurgitates).

The various animal-dispersed fruits include tomatoes, grapes, cherries, olives, apples, pears, squash, zucchini, all beans and peas, corn kernels, and avocados. Because there appears to be no species of animal alive today that disperses avocado seeds, researchers have speculated that the avocado plant evolved in concert with some large species of animal that has since become extinct. Today, avocado plants rely on humans to disperse their seeds. Humans are also the sole disperser of the seed of the Osage-orange tree, whose fruit was once eaten (and seeds dispersed) by the now-extinct mastodon (FIGURE 18-24).