Although it is a matter of common observation that many plants are capable of detaching portions of the body, the underlying cause and the actual mechanism which bring about such separation are only slightly understood. The process has often been described as of self-pruning by which the plant rids itself of useless portions of its body. Since abscission is sometimes confused with exfoliation, it seems desirable here to distinguish definitely between the two phenomena. It can be said that, in general, exfoliation is preceded by drying and death of the part to be cast off and that actual separation of the organ is accomplished by a mechanical break through dry, dead tissues. Abscission, on the other hand, is usually not preceded by drying and death of the organ concerned and its detachment is accompIished by a separation along the plane of the middle lamellae of aetioe living cells. Abscission may be either axial or lateral. Axial abscission includes the abscission of portions of...

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