Why are so many species of orchids rare, while dandelions, shepherd's purse, and other weeds occur all over the world? Why are some plants confined to single continents, mountain ranges, or small niches occupying less than a hectare (2.47 acres) of land? The answers to these questions involve many different factors, including climate, soil, the adaptability of the plant, and its means of seed dispersal. How fruits and seeds are transported from one place to another is the subject of the following sections. Other factors are discussed in Chapters 13 and 25.
Fruits and seeds have a variety of adaptations for wind dispersal (Fig. 8.22). The samara of a maple has a curved wing that causes the fruit to spin as it is released from the tree. In a brisk wind, samaras may be carried up to 10 kilometers (6 miles) away from their source, although usually most are relatively evenly distributed within a few meters of the tree. In hop hornbeams, the seed is enclosed in an inflated sac that gives it some buoyancy in the wind. In some members of the Buttercup and Sunflower Families (Ranunculaceae and Asteraceae), the fruits have plumes, and in the Willow Family (Salicaceae), the fruits are surrounded by cottony or woolly hairs that aid in wind dispersal. In button snakeroots and Jerusalem sage, the fruits are too large to be airborne, but they are spherical enough to be rolled along the ground by the wind.
Seeds themselves may be so tiny and light that they can be blown great distances by the wind. Orchids and heaths, for example, produce seeds with no endosperm that are as
The adaptations of fruits and seeds for animal dispersal are legion. Birds, mammals, and ants all act as disseminating agents (Fig. 8.23). Shore birds may carry seeds great disfine as dust and equally light in weight. In catalpa and tances in mud that adheres to their feet. Other birds and jacaranda trees, the seeds themselves are winged rather than mammals eat fruits whose seeds pass unharmed through their the fruits, which remain on the branches and split, releasing digestive tracts. Some bird-disseminated fruits contain laxa-their contents. Dandelion fruitlets have plumes that radiate tives that speed their passage through the birds' digestive out at the ends like tiny parachutes; these catch even a slight tracts. In blackbirds, the seeds may remain in the tract as lit-breeze. In tumble mustard and other tumbleweeds, the tle as 15 minutes, but in mammals, the period is more com-whole aboveground portion of the plant may abscise (sepa- monly about 24 hours. In the giant tortoises of the Galápagos rate from the root) and be blown away by the wind, releas- Islands, seeds do not pass through the tract for 2 weeks or ing seeds as it bumps along. more, and the seeds usually will not germinate unless they
have been subjected to such treatment (see Chapter 11). Some seeds and fruits are gathered and stored by rodents, such as squirrels and mice, and then are abandoned. Blue jays, woodpeckers, and other birds carry away nuts and other fruits, which they may drop in flight and abandon.
Many fruits and seeds catch in or adhere to the fur or feathers of animals and birds. Bedstraw and bur clover fruits are covered with small hooks that catch in fur (or a hiker's socks). The large capsules of unicorn plants have two giant curved extensions about 15 centimeters (6 inches) long. These catch on the fetlock of a deer or other animal that happens to step on the fruit, and the seeds are scattered as the animal moves along. Twinflowers and flax have fruits with
sticky appendages that adhere to fur on contact, and those of the puncture vine penetrate the skin and stick by means of hard little prickles (Fig. 8.24).
Bleeding hearts, trilliums, and several dozen other plants have on their seeds appendages that contain oils attractive to ants (Fig. 8.25). The Scandinavian scientist Sernander once estimated that more than 36,000 such seeds were carried by members of a single ant colony to their nest, where the ants stripped off the appendages for food but did not harm the seeds themselves.
Some fruits contain trapped air, adapting them to water dispersal. Many sedges, for example, have seeds surrounded by inflated sacs that enable the seeds to float (Fig. 8.26). Others have waxy material on the surface of the seeds, which temporarily prevents them from absorbing water while they are floating. Sometimes, a heavy downpour will create a torrent of water that dislodges masses of vegetation along a stream
Figure 8.26 Sedge adaptation to water dispersal. A. A sedge plant. B. A sedge fruit. The seed is enclosed within an inflated covering that enables it to float on water.
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