1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 375 



After the break in the epidermis separation continues in the layer 

 of collenchyma just beneath. The cells of the collenchyma layer, 

 which are much elongated parallel to the long axis of the pedicel (five 

 to eight times as long as wide), separate for a distance of about 0.3 

 mm. up and down the pedicel, involving only a few tiers of cells. It 

 is evident that the cells of this tissue separate without difficulty, 

 although not by any means as freely as the small spherical cells de- 

 scribed above. The large, isodiametric, parenchyma cells of the cortex 

 separate for a distance of 2 or 3 mm., involving many tiers of cells. 

 The cells of the starch sheath, which are small and spherical, separate 

 for a distance of 1 cm. or more, thus causing a longitudinal cavity to 

 be formed just outside of the vascular bundles. In the latter, separa- 

 tion involves only two or three tiers of cells. Separation originates 

 and continues in the pith independent of the process in the cortex, 

 but involves about the same number of cells as in the parenchyma of 

 the latter tissue. "When separation has thus become complete, the 

 weight of the flower is very often sufficient to break the tracheag and 

 cause the flower to fall to the ground. 



Several important facts are brought out by this examination of 

 abscission in Datura. In the first place, it shows that floral abscission 

 can take place without any structure which might possibly be inter- 

 preted as a morphologically differentiated separation layer. In the 

 second place, it indicates that cell separation is possible in several dif- 

 ferent types of living cells. It also shows that separation takes place 

 more readily in small cells than in large ones and more readily in 

 isodiametric cells than in elongated ones. The theory that the separa- 

 tion layer is not a morphologically differentiated structure, but repre- 

 sents a physiological condition (Lloyd and Loewi), could certainly be 

 well applied in this case. 



d. OTHER GENEEA 



The process of abscission in the other species listed on page 365 is 

 essentially the same throughout. No indications were noted of cell 

 divisions or elongations accompanying abscission. Separation is 

 brought about by means of a separation of small and active cells 

 located in the general region at the base of the pedicel. In all these 

 forms the separation surface of the pedicel is convex in outline, so 

 that the separation layer must lie in more or less of a crescent in the 

 stem at the base of the pedicel. The main difference between these 

 forms and the three that have been described in detail above is found 



