Unit Seven. Plant Life
During seed formation, the flower ovary begins to develop into fruit. The evolution of flowers was key to the success and diversification of the angiosperms. But of equal importance to angiosperm success has been the evolution of fruits, which aid in seed dispersal. Fruits form in many ways and exhibit a wide array of modes of specialization.
Three layers of the ovary wall can have distinct fates and account for the diversity of fruit types, from fleshy to dry and hard. There are three main kinds of fleshy fruits: berries, drupes, and pomes. In berries—such as grapes, tomatoes (figure 34.7a), and peppers—which are typically many-seeded, the inner layers of the ovary wall are fleshy. In drupes—such as peaches (figure 34.7b), olives, plums, and cherries—the inner layer of the fruit is stony and adheres tightly to a single seed. In pomes—such as apples (figure 34.7c) and pears—the fleshy portion of the fruit forms from the portion of the flower that is embedded in the receptacle (the swollen end of the flower stem that holds the petals and sepals). The inner layer of the ovary is a tough, leathery membrane that encloses the seeds.
Fruits that have fleshy coverings, often black, bright blue, or red (as in figure 34.7d), are normally dispersed by birds and other vertebrates. By feeding on these fruits, the animals carry seeds from place to place before excreting the seeds as solid waste. The seeds, not harmed by the animal digestive system, thus are transferred from one suitable habitat to another. Other fruits that are dispersed by wind, or by attaching themselves to the fur of mammals or the feathers of birds, are called dry fruits because they lack the fleshy tissue of edible fruits, and their ovaries form hard layers rather than fleshy tissue. Dry fruits can have structures that aid in their dispersion, as seen in the plumed dandelion in figure 34.7c or the spiny cocklebur in figure 34.7f which catches onto fur (or socks or pants!) and is carried to new habitats. Still other fruits, such as those of mangroves, coconuts, and certain other plants that characteristically occur on or near beaches or swamps, are spread from place to place by water.
Figure 34.7. Types of fruits and common modes of dispersion.
(a) Tomatoes are a type of fleshy fruit called berries, which have multiple seeds. (b) Peaches are a type of fleshy fruit called drupes; they contain a single large seed. (c) Apples are a type of fleshy fruit called pomes, which contain multiple seeds. (d) The bright red berries of this honeysuckle, Lonicera, are highly attractive to birds. Birds may carry the berry seeds either internally or stuck to their feet for great distances. (e) The seeds of this dandelion, Taraxacum officinale, are enclosed in a dry fruit with a "parachute" structure that aids its dispersal by wind. (f) The spiny fruits of this cocklebur, Xanthium strumarium, adhere readily to any passing animal.
Key Learning Outcome 34.5. Fruits are specialized to achieve widespread dispersal by wind, by water, by attachment to animals, or, in the case of fleshy fruits, by being eaten.