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now to the cooperative interactions of organisms, we find that among plants they are chiefly passive. By growing close together, plants may maintain an environment favorable to one another, as among the trees of tropical rain forests. The mycorrhiza that envelop the feeding roots of many of these trees, helping them to absorb indispensable elements from the soil, are nourished by carbohydrates supplied by the trees in a mutually advantageous association. A similar symbiosis occurs between leguminous plants and the bacteria that form nodules on their roots, absorbing free nitrogen from the air that permeates the soil and supplying their hosts with nitrates in return for carbohydrates.

Cooperation between Plants and Animals

Plants, which for long ages have suffered uncomplainingly from the depredations of animals, respond magnificently when animals cooperate with them. The flowers that brighten woods and meadows, adorn our festive occasions, express our sympathy with the sick and the bereaved, and provide the motifs for so many paintings and household decorations are displayed by plants to advertise the availability of nectar to the bees, butterflies, other insects, birds, and other creatures that convey pollen from the anthers of one to the stigmas of another. Fragrance makes flowers more attractive to insects as well as to the people who delight in them. Sweet nectar is sometimes enriched with vitamins and amino acids; while pollen,

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produced in excess of the plant's needs for fertilization, offers a nutritious food for the many kinds of bees that busily collect it in special baskets on their hindlegs, then carry it back to their hives and mix it with nectar to make ''beebread."

Wind pollination is a wasteful but adequate method for plants that grow in almost pure stands, as in grasslands and northern woods. In rich tropical forests where trees of one kind tend to grow scattered among many other kinds, only insects or birds that, at least temporarily, confine their visits to a single species can efficiently pollinate them. We owe such forests to cooperation between animals and plants. We, who enjoy the colors and fragrance of flowers, the beauty, interest, and diverse products of tropical woodlands, are incidental beneficiaries of a mode of cooperation between animals and plants highly advantageous to both of them.

Although by hooks or other means of attaching inedible fruits to fur or clothing many plants exploit animals unilaterally to disperse their seeds, a great number of them reward the dispersers with food. Frugivorous birdsin contrast to seed predators like parrotsare by virtue of their numbers and mobility the chief disseminators of seeds, especially in tropical woodlands where dispersal by wind plays a subordinate role. After swallowing a many-seeded berry or a small, one-seeded drupe like a cherry, a bird rapidly digests the soft pulp. Small frugivores like tropical American euphonias and manakins may void the seeds in their droppings ten minutes or less after swallowing the fruit; larger species, like Eurasian Blackbirds, within half an hour. If, instead of passing the seeds through the length of the alimentary canal, the bird regurgitates them from its crop, it eliminates them much more rapidly. Such brief residence inside the bird does not injure seeds adapted for avian dispersal, which germinate where they fall, often at a distance from the plant that bears them (Snow and Snow 1988). We owe the colors, aroma, and nutritive value of a great diversity of fruits to a tacit compact between fructiferous plants and frugivorous birds. By selection, people have increased the size and flavor of fruits originally adapted for dispersal by birds, among which we might include cherries, currants, and strawberries. The wild ancestors of certain larger fruits

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improved and esteemed by humansavocados in the New World, mangos in the Oldwere probably disseminated by mammals. Again, we have benefited greatly from a mutually beneficial arrangement between plants and animals.