This very large family has, according to some authorities, more than 35,000 species that are widely distributed. Like members of the Lily Family, they are especially abundant in the tropics. In many genera, the number of individual plants at any one location may be quite small—sometimes limited to a single plant.
The flowers are exceptionally varied in size and form, and the habitats of the plants are equally diverse. The flowers of one Venezuelan species have a diameter of less than 1 millimeter (1/25 of an inch), while those of a species native to Madagascar may be more than 45 centimeters (about 18 inches) long. One species of Dendrobium orchid from Java has flowers that are so delicate they wither within 5 or 6 minutes after opening. Many orchids are epiphytic on the bark of trees. During its 5-month flowering season, one epiphytic species of Malaysia and the Philippine Islands produces 10,000 flowers on plants that weigh more than 1 metric ton. Others are aquatic or terrestrial, and a saprophytic species (see Fig. 8.1) native to western Australia grows and flowers entirely underground.
Orchids have three sepals and three petals, with one of the petals (the lip petal) differing in form from the other two (Fig. 24.30). The stamens and pistil are united in a unique
single structure, the column (see Fig. 23.9). The stigma usually consists of a sticky depression on the column. The anthers contain sacs of pollen called pollinia and are covered with a cap until they are removed by an insect or other pollinator. The specific adaptations between orchid flowers and their pollinators are extraordinary and sometimes bizarre (as illustrated in Fig. 23.18).
Orchids have minute seeds that are often produced in prodigious numbers (e.g., a single fruit of certain orchid species may contain up to 1 million seeds). Each seed consists of only a few cells, and in order for a seed to germinate, it must become associated with a specific mycorrhizal fungus that produces substances necessary for its development. Once a seed has germinated, it may take from 6 to 12 or more years before the first flower appears.
Contrary to popular belief, some orchids can be grown relatively easily on a windowsill that has bright light, but not direct sunlight (see Appendix 4). Because orchids are among the most beautiful and prized of flowers, a large industry has grown up around their culture and propagation, discussed in the section on mericloning and tissue culture in Chapter 14. One species, the vanilla orchid, is grown commercially in the tropics for its fruits, which are the source of true vanilla flavoring.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.