352 University of California Publications in Botany [ V L. 5 



work Mohl gives the general impression that it is necessary for a sep- 

 aration layer to be formed from a secondary meristem before abscis- 

 sion can occur. Wiesner (1871), working on leaf -fall in general, 

 observes that the separation layer is not generally of type b, as Mohl 

 believes, but more often of type a. According to Becquerel (1907), 

 the separation layer is formed in the pedicel of Nicotiana from a sec- 

 ondary meristem (type b). In the cotton flower Balls (1911) finds 

 that the separation layer is of type Z>, but according to Lloyd (1914a 

 and 1916&) there is doubt as to this conclusion, since in the case of 

 very young cotton flowers in which abscission occurs very suddenly, 

 he finds only rarely that cell divisions do not precede abscission. 

 Hannig (1913), for flower-vfall in general, states that a separation 

 layer of type a is always present but in certain species a secondary 

 layer of type b may also be formed, through which separation may or 

 may not occur. Hannig, differing from Becquerel ( 1907 ) , points out 

 that the separation layer in Nicotiana is of type a. Lloyd (1914a) 

 and Loewi (1907) indicate that in general a layer of cells through 

 which abscission is possible is more often of type a than of type b. 

 They believe, however, that the separation layer is not a definite 

 morphological structure but represents merely a physiological con- 

 dition. 



c. CYTOLOGY OF THE SEPAKATION LAYEE 



Mohl (1860) describes the separation cells in the flower stalk as 

 young, active, small cells which generally contain no starch. He also 

 states that in most cases cell divisions are characteristic of the sep- 

 aration layer, i.e., that the separation layer is meristematic. Hoehnel 

 (1880) finds that cell divisions are characteristic of the proximal por- 

 tion of the separation zone in Salix and Populus but in the distal 

 portion, where the separation layer is developed, these divisions are 

 not so numerous. In some cases he finds separation taking place in 

 the parenchyma, entirely outside the "zone" where there had been 

 no cell divisions. The separation cells in Nicotiana are described by 

 Becquerel (1907) as small, practically undifferentiated cells with 

 large nuclei. In Begonia, Fuschia, Mirabilis, and Impatiens Hannig 

 (1913) describes the tissue as secondary meristem (type b) with the 

 cells rectangular in shape and arranged in more or less definite rows. 

 In contrast to the above observations, he describes the cells as small, 

 irregularly arranged and spherical in Salvia, Solanum nigrum, and 

 Nicotiana Tabacum. In Solanum nigrum the separation layer consists 



