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Plants and Geography
Although different plants have varying environmental requirements because of physiological differences, there are certain plant species that are found associated with relatively extensive geographical areas. The distribution of plants depends upon a number of factors among which are (1) length of daylight and darkness, (2) temperature means and extremes, (3) length of growing season, and (4)precipitation amounts, types, and distribution.
Daylight and darkness are the keys by which a plant regulates its cycle. It is not always obvious how the triggering factor works, but experiments have shown day length to be a key. A case in point is that many greenhouse plants bloom only in the spring without being influenced by outside conditions other than light. Normally, the plants keyed to daylight and darkness phenomena are restricted to particular latitudes.
In one way or another, every plant is affected by temperature. Some species are killed by frost; others require frost and cold conditions to fruit. Orange blossoms are killed by frost, but cherry blossoms will develop only if the buds have been adequately chilled for an appropriate time. Often the accumulation of degrees or the direction of temperatures above or below a specific figure critically affects plants. Plant distributions are often compared with isotherms to suggest the temperature limits and ranges for different species. The world's great vegetation zones are closely aligned with temperature belts.
Different plant species adjust to seasonal changes in different ways. Some make the adjustment by retarding growth and arresting vital functions during winter.This may result in the leaf fall of middle latitude deciduous trees. Other plants disappear entirely at the end of the growing season and only reappear through their seeds. These are the annuals, and they form a striking contrast to the perennials, which live from one season to another.
Precipitation supplies the necessary soil water for plants, which take it in at the roots. All plants have some limiting moisture stress level beyond which they must become inactive or die. Drought resistant plants have a variety of defenses against moisture deficiencies, but hydrophytes, which also are adapted to humid environments, have hardly any defense against a water shortage.
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