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water and solutes that it requires from the soil. Plants that are not self-fertile need pollen from others to set seeds. All whose pollen is not carried by wind or water are dependent upon animalsinsects, birds, bats, and othersto bring it to them, and many employ animals to disperse their seeds. In the living world as a whole, the most widespread mode of dependence of one organism upon another is for reproduction. Even those that as individuals can live quite alone would cease to exist as species without other individuals to fecundate their reproductive cells.

Unable to synthesize their own food from inorganic materials, all animals are strictly dependent upon plants, either directly or indirectly, as when they eat other animals that are themselves nourished, often also indirectly, by vegetation. The dependence of one organism upon another may be unilateral or reciprocal, exploitative or cooperative, in both relationships often complicated by competition for resources. The effects of these two modes of interaction upon the creatures involved in them are among the greatest contrasts that the living world presents.

Interactions among organisms fall into the following broad categories:

Exploitation of plants by plants

Exploitation of animals by plants

Exploitation of plants by animals

Exploitation of animals by animals

Cooperation among plants

Cooperation between plants and animals

Cooperation among animals

Let us briefly survey these relationships in this order.

Exploitation of Plants by Plants

In the vegetable kingdom probably the most frequent, and certainly the most conspicuous, mode of unilateral exploitation is that practiced by vines of all sorts, from slender twiners like morning

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glories to massive lianas of tropical forests, all of which, having lost the capacity to hold themselves erect, grasp other plants to raise themselves into the sunshine. Usually they reduce the amount of light that falls upon the supporting plants, decreasing their photosynthesis. The more aggressive of them spread a smothering blanket of foliage over the crowns of the highest trees of the forest. Their constricting coils may strangle the tree up which they spiral; but sometimes the trunk grows over them and embeds them in its wood.

In abandoned clearings in humid forests, a riotous growth of vines and creepers burdens young trees, weighing them down or breaking their branches or trunks, making it difficult for them to rise above the welter, and delaying for years the reestablishment of the forest. Certain trees that frequently start life in such clearings have developed special strategies to meet this situation: the young trunk bears no branches, but only big leaves that perform their photosynthetic task and fall, giving the creepers no permanent hold, until the slender tree rises above encumbering vegetation and forms a spreading crown, in the manner of cecropia and jacaranda trees in tropical America. Many vines flower beautifully on the roof of the forest or nearer the ground, often at the expense of the host's own bloom.

Instead of climbing from the ground up to the light, epiphytes germinate and grow on trunks and branches, above all in the humid tropics. They depend upon the host only for support, while they make their own food in green leaves and derive their minerals from decaying bark and the vegetable debris that lodges among them. Some epiphytes send long roots to the ground, whence they draw water and minerals, while others catch and store rainwater in specially modified organs. In size, epiphytes range from trees that perch on other trees through a vast array of ferns, aroids, bromeliads, and orchids large and small to tiny mosses and liverworts. A moderate load of epiphytes appears to have no ill effects upon the plant that supports them but, especially in cloud forests on tropical mountains, the burden of these air plants may become so heavy that it breaks large boughs. With a wonderful diversity of colorful

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flowers and floral bracts, bromeliads,