218 PERIODICITY OF GROWTH 



however, cork layers may be used to cut off the supply of food to particular 

 tissues and organs, and so bring about their death and separation. 



The time of abscission is always dependent upon external circumstances, 

 and can be hastened or delayed by unusual conditions. It is hastened by 

 feeble illumination l , an insufficiency of water, and by high temperatures. 

 Sudden changes may have the same effect, especially upon the older leaves. 

 For example, it is mainly owing to the altered transpiration that Coleus, 

 Impatiens Sultani, Goldfussia anisophylla, and many other plants throw off 

 a portion of their leaves when brought from the moist air of a greenhouse 

 to the drier air of a room 2 . A sudden change of temperature or illumination 

 may produce similar results, and various factors may induce the abscission 

 of the roots of Azolla*. If, however, the change is gradually made no 

 stimulating shock-effect is exercised, and the leaves are not thrown off. 



Both permanent and transitory functional disturbances due to altered 

 demands may therefore induce the active abscission of organs. Hence 

 leaves fall and die in the absence of carbon dioxide, or when they are 

 prevented from exercising their normal functions. Similar factors, combined 

 with correlative influences, determine the fall of unfertilized flowers, of fruits 

 in which the ovules have been devoured by grubs, and of petioles deprived 

 of their laminae. 



The process of abscission may be induced in various ways, although 

 the mechanical separation is produced by the same means that are adopted 

 for the partial or total separation of individual cells. We do not, however, 

 know what part enzymes may play in the process, or whether the formation 

 of organic acids at the end of summer is of importance, which Wiesner in fact 

 supposes to be directly responsible for the fall of the leaves. 



1 Vochting, Organbildung inr Pflanzenreich, 1878, p. 232; Molisch, Sitzungsb. d. Wien. Akad., 

 1886, Bd. xcvi,I, p. 161. 



2 Watering with salt water acts similarly according to Schimper (Indomalayische Strandflora, 

 1891, p. 22). 



3 Pfeffer, Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. n, p. 213. 



