COLLATERAL READING 651 



Some of the known causes of the abscission of leaves are: (i) a water 

 deficit in the plant, usually developed as a result of drought conditions, 

 (2) low temperatures, (3) reduced light intensity, (4) change in length of 

 the photoperiod, and (5) destruction or removal of all or most of the leaf 

 blade. 



Very little is known of the mechanism whereby any of these conditions 

 induces leaf abscission. The induction of abscission by removal of the leaf 

 blade, an excellent example of a growth correlation, can be easily demonstrated 

 in coleus plants. If the blade of a coleus leaf is pinched off, abscission of the 

 petiole will occur in a day or two (Sampson, 1918). Recent investigations 

 (LaRue, 1935) have shown that if a small portion of lanolin containing auxin 

 is affixed to the stump of a petiole from which the blade has been removed, 

 abscission of that petiole will be greatly delayed as compared with similar 

 petioles which have had only pure lanolin applied at the cut end. This 

 suggests that abscission of leaves is retarded by the migration of a growth- 

 regulating substance from the blade to the base of the petiole. Destruction 

 or removal of the blade eliminates the supply of this substance to the abscis- 

 sion layer, and hence induces abscission. 



Leaves, however, are not the only organs or parts of plants which abscise. 

 In compound leaves the individual leaflets usually drop off one by one, leaving 

 the petioles attached to the otherwise defoliated plant. Usually abscission 

 of the petioles follows within a relatively short time. Similarly bud scales, 

 inflorescences, petals, and fruits may be detached from the parent plant by 

 abscission. Segments of the woody stems of some species also abscise. In 

 many species of woody plants (examples are elm, cherry, birch, linden) abscis- 

 sion of the leafy stem tips occurs at the termination of the spring growing 

 period. In such species elongation of the stem continues the next season from 

 the lateral bud just below the point of abscission, such lateral buds function- 

 ing essentially as terminal buds. Many species of conifers bear their needle- 

 like leaves in fascicles, each fascicle being attached to a dwarf branch. In 

 the pines and other such species leaves are shed in bundles by the abscission 

 of the dwarf branches rather than by detachment of the individual needles. 

 In certain other woody species (oaks, cottonwood) segments of woody stems 

 of considerable age and diameter are often shed by abscission. 



Suggested for Collateral Reading 



Benecke, W., and L. Jost. Pflanzenphysiologie. Vol. II. G. Fischer. Jena. 



1924. 

 Pfeffer, W. The physiology of plants. 2nd Ed. Translated and edited by 



A. J. Ewart. Vol. II. Clarendon Press. Oxford. 1903. 



