Answer :

Apart from physiological changes the other anatomical mechanism involved in the abscission of leaves are as follows:

i. Structural: In deciduous trees, an abscission zone also known as separation zone is formed at the base of the petiole. It is composed of top layer and bottom layer. The cells in the top layer have weak cell walls and the bottom layer expand in winter and break the cell walls of the top layer, this result in shedding of leaves.

ii. The loss of chlorophyll may also result in abscission process.

iii. Hormonal: Abscisic acid hormone stimulates abscission.

An abscission layer is formed at the junction of the leaf and stem, it prevents transport of substances to and from the leaves and causes death of leaf cell thereby shedding of leaves takes place.

Anatomically, the cells of abscission zone are thin-walled and without deposition of lignin or suberin. At the time of abscission, the middle lamella may dissolve between the cells of two middle layers but the primary wall remains intact. The middle lamella as well as the primary walls of the adjacent cells is dissolved. Ultimately the whole cells of middle layer found in the abscission layer gets dissolve completely. Thus, there is separation of plant organ, i.e., leaf from the plant, wherever there is rainfall or wind.

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