DURATION, in plants, the period for which any given part of the plant remains alive. The duration of the plant as a whole is condi tioned primarily by that of the roots; an annual lives for only one vegetative period, a biennial for two and a perennial for a number of such periods. The same plant, such as wheat, may be an annual if planted early and a biennial if planted late. Similarly, plants such as the cas tor oil bean, which are perennials in an equable climate, are often annuals where there is a win ter and vice versa. In ground left to itself the perennials tend to drive out the annuals. In biennials and herbaceous perennials, the part of the plant which survives the winter or dry season is generally underground and the leaves and stalks die away each year and are produced anew.
In shrubs or trees the duration of the stalks may be measured in centuries, or even in mil lerua. However, the original live wood be comes dead wood after a few years. Leaves are of much shorter duration, ranging from days in the case of cotyledons or some xero phytic leaves to perhaps ten years in the coni fers. In deciduous trees the fall of the leaf is
due to the formation of a special zone of frac ture at the base of the leaf stalk, which gives a scar covered by cork before or after the stalk breaks. In such trees as the oak or beech this zone of fracture is imperfectly formed, so that the dead leaves remain on the trees throughout the winter. The duration of flowers is usually relatively brief and may be limited to a few hours, as is the case with the night-blooming cereus, but there are certain orchids such as odontoglossum which may remain open for almost three months if they are not pollinated. It will be noted that tissues which have a per ennial duration are generally woody or other wise mechanically strengthened and protected, whereas delicate, leafy or herbaceous structures are unable to survive periods of climatic ad versity.