Although different plants have varying environmental requirements because ofphysiological differences, there are certain plant species that are found associatedwith relatively extensive geographical areas. The distribution of plants dependsupon 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） precipitationamounts, types, and distribution. Daylight and darkness are the keys by which a plant regulates its cycle. It is notalways obvious how the triggering factor works, but experiments have shown daylength to be a key. A case in point is that many greenhouse plants bloom only in thespring without being influenced by outside conditions other than light. Normally, theplants keyed to daylight and darkness phenomena are restricted to particular latitudes. In one way or another, every plant is affected by temperature. Some species arekilled by frost; others require frost and cold conditions to fruit. Orange blossomsare killed by frost, but cherry blossoms will develop only if the buds have beenadequately chilled for an appropriate time. Often the accumulation of degrees or thedirection of temperatures above or below a specific figure critically affects plants. Plantdistributions are often compared with isotherms to suggest the temperature limits andranges for different species. The worlds great vegetation zones are closely aligned withtemperature belts. Different plant species adjust to seasonal changes in different ways, Some makethe adjustment by retarding growth and arresting vital functions during winter. Thismay result in the leaf fall of middle latitude deciduous trees. Other plants disappearentirely at the end of the growing season and only reappear through their seeds. Theseare the annuals, and they form a striking contrast to the perennials, which live from oneseason to another.
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