Plant Biology

All leaves have a definite life span and are dropped following receipt of internal or environmental signals. The process is termed abscission and is facilitated by the formation of an abscission zone at the base of the petiole. Plants that drop all of their leaves within a short time resulting in a temporarily bare, leafless plant are called deciduous plants. Those that drop leaves a few at a time throughout the life of the plant are called evergreen plants (they appear to be fully leafed at all times).

Hormones trigger the formation of the abscission layer. Severance of the leaf is aided by anatomical changes in the abscission zone where two tissue zones differentiate; the one nearest the stem accumulates suberin in the cell walls—blocking the flow of materials—while cells of the separation layer on the blade side simply disintegrate. The suberized zone left on the stem after the leaf falls is called the leaf scar; visible within it are bundle scars, the remnants of the vascular strands.

Some leaves have anatomical specializations that make possible responses almost as fast as those of animal movements. In one sensitive plant, a Mimosa, touching the leaves causes a change in the permeability of the membranes of the large, thin‐walled parenchyma cells in the pulvini (singular, pulvinus)—the swollen glands at the bases of the petioles—and an almost instantaneous water loss. The whole leaf droops as the pulvini cells become flaccid. Pulvinus‐mediated movements of other taxa are slower, but also serve to move the petioles.

Some plants change the position of their leaves daily, dropping them to a vertical position at night, elevating them back to horizontal at dawn. These sleep movements, one of many circadian rhythms in plants, are called nyctinastic and the process, nyctinasty.

Insectivorous plants such as the Venus flytrap have perfected a combination of anatomical and physiological leaf specializations to attract, catch, and digest insects.