348 University of California Publications in Botany VVoiu. 5 



INTRODUCTION 



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 

 one 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 these 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 separa- 

 tion 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 

 accomplished by a separation along the plane of the middle lamellae of 

 active living cells. 



Abscission may be either axial or lateral. Axial abscission includes 

 the abscission of portions of stems, shoots, entire flowers or fruits. 

 Lateral abscission includes the abscission of leaves, petioles, sepals, 

 petals or styles. Considerable attention has been given by investi- 

 gators to the abscission of flowers because of the theoretical detriment 

 to crops caused by the fall of the flower before the fruit is formed. 



The cause of leaf-fall in deciduous species is connected with peri- 

 odic changes in the physiological condition brought about by changes 

 in the environment. In the case of some herbaceous plants and occa- 

 sionally in trees, sudden changes in environmental conditions result- 

 ing in a loss of physiological equilibrium often cause the throwing 

 off of leaves, flowers or even small shoots. In certain species, any- 

 thing which tends to loss or completion of function within or peculiar 

 to an organ causes the organ to be thrown off. Thus, staminate flow- 

 ers are commonly throv^ii off soon after anthesis and pistilate flowers 

 generally fall when fertilization is prevented. Similarly, certain 

 species — e.g., Impatiens Sultani and Mirahilis Jalapa — throw off por- 

 tions of their stems which have been rendered useless as a part of the 

 conducting system because of injury or removal of distal buds or 

 leaves. 



The following definitions of terms, which will be used throughout 

 this paper, are made necessary because of a notable lack of uniformity 

 in their usage by various investigators who have dealt with abscission. 



