350 University of California Publiaitions in Botany [Vol. 5 



SUMMARY OF THE LITERATURE 



Since the literature on abscission is rather voluminous, it seems 

 best to present the following discussion under several different head- 

 ings corresponding, to a certain extent, with the six main topics of 

 interest mentioned in the introduction. The summary below is largely 

 confined to the literature on axial abscission, although that on lateral 

 abscission is considered in so far as it has a direct bearing on the most 

 important aspects of the abscission problem. 



1. Histology of the Pedicel 



a. POSITION OF THE SEPARATION LAYEE 



Hoehnel (1880), discussing the fall of catkins in Populus and 

 Salix, locates the separation layer at the base of the catkin. The gen- 

 eral region at the base of the catkin, in the distal part of which the 

 separation layer is located, he calls the "separation zone." In Salix, 

 actual separation occurs in the -separation layer, but in Populus it 

 occurs in the parenchyma entirely outside the separation laj^er. 

 According to Balls (1911), the separation layer in the cotton flower 

 is located at the base of the pedicel. The layer is located by Hannig 

 (1913) at the base of the pedicel in Nicotiana Tahacum, X. rustica, 

 N. accuminata, N. sylvestris, Datura, and Atropa, and at the tip of 

 the pedicel in Nicotiana Langsdorffii, Salvia Aloe, Cuphea, and Gasteria. 

 He finds it occurring at the middle of the pedicel in Impatiens Sultani, 

 Solanum tuhcrosum, Lycopersicum, Asparagus, and Begonia. Gort- 

 ner and Harris (1914) and Lloyd (1914&), working on the abscission 

 of internodes as the result of injury in Impatiens Sultani, locate the 

 separation layer at the first node below the injury and just above the 

 axillary bud. Occasionally, according to the latter investigators, ab- 

 scission may occur at the second or third node below the injury and 

 in these cases the buds at the first or second nodes seem to be abortive. 



The separation layer, according to Hannig (1913), may occur at 

 the base of the complete inflorescence in Impatiens and Oxyhaphus. 

 According to Lloyd (1914a), the separation layer occurs at the base 

 of the pedicel in cotton and at the base of the ripened ovarv' in grape 

 "shelling." In the abscission of internodes and tendrils in Vitis and 

 Ampelopsis, Lloyd (1914a) locates the layer near but not exactly at 

 the base of the internode. A peculiar case illustrating the result of 

 displacement of the stem on the location of the separation layer is 



