1918] Kendall: Abscission of Flowers and, Fruits in Solanaceae 405 



immature fruits separation oeeurred in the cortex but failed to take 

 place within the vascular cylinder. 



Experiment 10 at first glance would seem to indicate that the cell 

 walls of the separation cells are more subject to hydrolysis than normal 

 cortical cells. Another interpretation is possible, however. Actual 

 separation which takes place through the separation zone may be due 

 to the fact that the cells in this zone are small and have a tendency 

 to be isodiametric, whereas the remaining cells of the cortex are larger 

 and are elongated parallel to the long axis of the pedicel. Hydrolysis 

 of the cell walls may go on with equal rapidity in all the cortical cells 

 at the base of the pedicel, yet upon bending or pulling separation may 

 take place through the region of isodiametric cells because of the inter- 

 locking of the elongated cells in the rest of the cortex. An attempt 

 was made to gain further evidence on this point by observing through 

 the microscope the action of acids on the cell walls of the tissues con- 

 cerned. When the action of the acids is thus observed, the walls are 

 seen to soften and to swell to two or three times their normal thick- 

 ness. This effect is all the more noticeable if the walls initially are 

 comparatively thick. Now, since the cells of the separation zone are 

 small and somewhat collenchymatous, or at least have thicker walls 

 than normal cortical cells, the process of swelling in the cell wall is 

 most conspicuous in that region. Indeed, hardly anj"^ swelling can be 

 perceived as a result of the acid treatment in the cell walls of normal 

 parenchyma cells of the cortex. However, when a form such as 

 Lycopersicum is examined in which there is a distinct layer of col- 

 lenchyma beneath the epidermis for the entire length of the pedicel, 

 this collenchyma appears to be affected at the same time and in the 

 same manner as the cells of the separation zone of Nicotiana. Also 

 in Nicotiana there seems to be a certain amount of similarity in 

 reaction to acids between the smaller cells of the cortex just beneath 

 the epidermis and those of the separation zone. The conclusion can 

 thus be drawn that the cell walls of the separation cells are no more 

 readily hydrolyzed than those of normal collenchymatous tissues. Of 

 course, the fact still remains that the collenchyma of the cortex may 

 be more subject to hydrolysis than the cortical parenchyma. Now 

 the small cells of the separation zone not only extend across the base 

 of the pedicel but also spread throughout the general region at the 

 base of that organ; it was therefore noticed that the swelling of cell 

 walls was by no means confined to cells of the separation layer but 

 was more or less prominent throughout the whole general region at 

 the base of the pedicel. 



