BSC 314 Lecture Notes - Lecture 14: Aquatic Plant, Abscission, Venus Flytrap
26 Jun 2018
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Leaf Abscission and Movements
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 zoneat 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.
Leaves and the Environment
Sun and shade leaves. The architecture of leaves changes depending upon the light
intensity in which they grow, even on the same plant. Sun leaves usually are smaller
and thicker with more and better defined palisade cells, and more chloroplasts. They
frequently have more hairs as well. Sun leaves rarely have chloroplasts in their
epidermal cells, but chloroplasts are common in the epidermises of shade leaves.
Day length. The presence or absence of light (as well as particular wavelengths)
influences the production of plant hormones and the development of plant organs. For
example, leaves do not develop normally in the dark, and chloroplasts don't turn green
unless exposed to light; the tissues are yellow and said to be etiolated. The duration of
the light also influences the shape of many kinds of leaves; leaves produced during the
short days of spring are different from those produced during the long days of summer.
Water effects
The presence or absence of water in the environment has profound effects on the
structure of plant stems, roots, and leaves, so much so that three types of plants are
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Document Summary
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 zoneat 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. 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. The whole leaf droops as the pulvini cells become flaccid.
