in trees and shrubs, the fall of leaves; it occurs more rarely in herbs (nettle and impatiens). The leaves may fall together at a certain time or one at a time gradually over a long period. Plants that lose all their leaves over a certain period of time are called deciduous. Deciduous trees in humid tropical forests sometimes stand without leaves only for a few days. Evergreen plants bear leaves throughout the year, changing them periodically or gradually.
Leaf fall is a normal physiological process associated with the aging of leaves. A leaf usually lives less than a year, infrequently up to two to five years, very rarely 25 or more years (Schrenk spruce). Profound biochemical, physiological, and structural changes occur in leaves before leaf fall. The green pigment, chlorophyll, is usually destroyed; the more stable yellow and orange pigments—carotenoids—are preserved longer, causing the autumnal colors of leaves. Nutrient substances, especially proteins, from the cells of the aging leaf flow out to the storage organs, to the growing points, and to young, growing leaves. Certain salts often accumulate in old leaves and calcium oxalate crystals are formed.
The mechanism of leaf fall is associated with the appearance at the base of the leaf of a separation layer of readily dissociated parenchymal cells. The vascular bundles that keep the leaf on the stem are torn under the weight of the leaf and by the wind. In some cases, for example, when there are abrupt decreases in temperature, drought, or the appearance of harmful gases in the air, the leaves may die quickly before the separation layer has time to form; as a result, they remain on the plant in a dried state. Deciduousness is primarily characteristic of plants in climatic zones with a vividly expressed unfavorable season, such as winter in the temperate zone and dry periods in the subtropical and tropical zones. Leaf fall is an adaptation that was acquired in the process of evolution to reduce the surface area of the above-ground organs under unfavorable conditions. It decreases the loss of moisture and in winters with heavy snows prevents breakage of branches under the weight of the snow.
In woody plants of temperate latitudes, preparation for autumn leaf fall begins long before the onset of frosts, owing to changes in the length of the solar day. In many evergreen plants, periodic leaf fall results from the unfolding of young leaves (spring and summer).
T. I. SEREBRIAKOVA