Leaf Fall

Leaf Fall

 

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

References in classic literature ?
Here have I been a dweller on the earth for four-score and six changes of the seasons, and all that time have I look'd at the growing and the dying trees, and yet do I not know the reasons why the bud starts under the summer sun, or the leaf falls when it is pinch'd by the frosts.
Find a solution to leaf fall on rail lines IT amazes me that in this world of high technology we are unable to find a solution to the problem of 'leaf fall' on railway lines.
This will save you hours when the annual autumn leaf fall starts.
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Translink NI Railways is working hard to minimise disruption caused by leaf fall, which can cause problems at this time of year.
Get your compost bins or bags ready for collecting leaf fall which makes superb compost after leaves have broken down.
It will have survived the winter on fallen leaves and appears as scab marks on leaves from mid-spring until leaf fall in autumn.
Lee Chapman, professor of climate resilience at the university, has worked with technology transfer company Alta Innovations to develop the device to help better predict where and when problems caused by poor weather and leaf fall will occur.
Extra tree surgery and vegetation clearance takes place ahead of the leaf fall season to reduce the amount of leaf fall on Metro corridors during the autumn months.
The span of leaf fall causes the collection effort to be repeated often.
Intensive rain and the widespread incidence of abnormal leaf fall are the reasons for decline in production.