Abscission
Abscission () is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed. In zoology, abscission is the intentional shedding of a body part, such as the shedding of a claw, husk, or the autotomy of a tail to evade a predator. In mycology, it is the liberation of a fungal spore. In cell biology, abscission refers to the separation of two daughter cells at the completion of cytokinesis. In plants Function A plant will abscise a part either to discard a member that is no longer necessary, such as a leaf during autumn, or a flower following fertilisation, or for the purposes of reproduction. Most deciduous plants drop their leaves by abscission before winter, whereas evergreen plants continuously abscise their leaves. Another form of abscission is fruit drop, when a plant abscises fruit while still immature in order to conserve resources needed to bring the remaining fruit to maturity. If a leaf is damaged, a plant may also abscise it to conserve water or photosynthetic efficiency, depending on the 'costs' to the plant as a whole. The abscission layer is a greenish-greyish color. Abscission can also occur in premature leaves as a means of plant defense. Premature leaf abscission has been shown to occur in response to infestation by gall aphids. By abscising leaves that have been made host to aphid galls, plants have been shown to massively diminish the pest population, as 98% of aphids in abscised galls died. The abscission is selective, and the chance of dropping leaves increases as the number of galls increases. A leaf with three or more galls was four times more likely to abscise than a leaf with one, and 20 times as likely to be dropped as a leaf without any galls. Process Abscission occurs in a series of three events: 1) resorption, 2) protective layer formation, and 3) detachment. Steps 2 and 3 may occur in either order depending on the species. Resorption Resorption involves degrading chlorophyll to extract the majority of its nutrients. Nitrogen is found in chlorophyll and is often a limiting nutrient for plants, which need large quantities to form amino acids, nucleic acids, proteins, and certain plant hormones. Once nitrogen and other nutrients have been extracted from chlorophyll, the nutrients will travel to other tissues of the plant. Resorption is what causes leaves in the fall to change colors. Carotenoids in the leaves are slower to degrade than chlorophyll, so autumn leaves appear yellow and orange. Protective layer formation Cells under the abscission zone divide and form a layer of cork cells. Situated on both sides of the abscission zone are layers of parenchyma cells, which produce and inject suberin and lignin under the abscission zone into the new layer of cork cells. Suberin and lignin create a durable and waterproof layer for the plant once the organ is detached. Detachment This step can occur in a variety of ways depending on the species but always occurs