DURATION. In plants, the length of the vegetative period. One of the chief ecological classifications is based on the duration of the various plants or their organs. Perhaps the most important classification of this kind is that into monocyclie, dieyelie, and pleiocyclic plants: or. as they arc more popularly called, annuals, biennials. and perennials. Whether or not these types may be regarded as the product of certain ecological conditions. it is certain that they are often associated with particular environ ments. Annuals and biennials. for example, are most common in new habitats. as in gardens allowed to run wild, and after fires, or in xero phytic situations, as on shores and deserts. Perennials. on the other hand, most of the world's vegetation, and dominate particu larly in habitats that are ecologically rich or that have been long established. It seems, there fore. that the perennial habit is, on the whole, the most successful. and it is not difficult to see why, since commonly spread vege tatively as well as by seed. Each year, there fore, the perennials arc on the ground at the beginning of the season, while annuals and bien nials are obliged to start from seed. A garden patch allowed to run wild shows. as a conse quence. fewer and fewer annuals and were and more perennials each year until finally the an nuals are gone. In desert regions and along
shores the vegetation is sparse. and annuals or biennials may exist ahnost indefinitely. An imals are scarce in Alpine and Arctic regions, perhaps because the vegetation periods are too short for their development.
Another important classification based on dura tion is that into deciduous and evergreen forms; in these plants, however, the critical point is not fully expressed when these terms are employed. Deciduous forms may be defined as plants which shed their organs, and especially leaves, regu larly: whereas evergreens are those plants which shed their leaves irregularly. Evergreen leaves are much longer lived than deciduous leaves. The sOiificanee of these types will be discussed under the head of FOREST (q.v.). The duration of stems depends largely on their struetural na ture: in temperate climates most aerial her baceous stems are annual, whereas woody stems arc perennial. Flowers. in contrast to other organs. have an ephemeral duration. There seems thus to be a harmonious relation between duration and structure, organs with long life having resistant structures. while short-lived or gans are commonly delicate. See FOREST; STEM; VEGETATIVE PROPAGATION; BIENNIAL; ANNUAL; PERENNIAL.