296 University of California Publications in Botany [Vol.5 



tissue, through some part of which separation is always seen to take 

 place, was not present from the early bud stage, or if partially ma- 

 tured fruits fell as readily as buds and open flowers, it might be con- 

 ceivable that the formation of the abscission zone occurs only immedi- 

 ately following the particular "stimulus" or lack of "stimulus" upon 

 which abscission is dependent. 



Returning now to the length of time occupied by the actual process 

 of abscission, we may note that two types of reaction were investigated. 

 First, the abscission following non-pollination or unsuccessful polli- 

 nation, and second, the abscission due to injury, "spontaneous abscis- 

 sion"; in this case the effect of illuminating gas in the air surrounding 

 the plant. In the first, the tissues of the pedicels of numerous cas- 

 trated flowers of all degrees of development were examined histolog- 

 ically to determine the time of first appearance of loosening or sepa- 

 ration in the cells of the abscission zone. At least six and apparently 

 not more than ten hours are necessary for the actual process of ab- 

 scission caused by non-pollination or unsuccessful pollination. In the 

 second case above the reaction is much more rapid, taking place in 

 from one to four hours. These latter figures are based upon direct 

 observation of the behavior of flowers on a freshly cut shoot placed 

 in water surrounded by air into which illuminating gas had been 

 introduced. 



The results of a rather extensive histological and cytological inves- 

 tigation as to the position and condition of the abscission zone and 

 the nature of the process of abscission proper are summarized below: 



1. In N. Tabaciim, N. sylvestris and the F^ hybrids between them 

 the abscission zone is always to be found at the base of the pedicel. 

 This is also ti'ue of varieties of N. Langsdorffii, our observations on 

 this species differing from those of Hannig (1913), who states that 

 the abscission zone is located at the tip of the pedicel just below the 

 calyx. 



2. A conspicuous grooved ridge or ring of tissue stands out around 

 the base of the pedicel in Nicotiana species. This grooved ring may 

 be taken to indicate the position of motor tissue or merely of a node. 

 The position of the abscission layer is independent of the groove, 

 although the position of the general abscission zone occurs under it. 

 The actual abscission layer ordinarily is found from five to seven rows 

 of cells distal to the groove. 



3. The abscission zone is seen in the cortex and pith to be composed 

 of small isodiametric cells which grade up gradually to the size and 



