1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 403 



tangential but contained a few tracheae on one surface. Section 3 was 

 more or less radial, containing two strands of vascular tissue on either 

 side. Sections 4 and 5 were similar to sections 1 and 2. On subject- 

 ing these sections to illuminating gas it was noticed that abscission 

 started first in sections 1, 2, and 3, appearing last in sections 4 and 5. 

 This result is exactly parallel with the process as it occurs in normal 

 abscission, where the process starts first in the ventral cortex" and in 

 the pith. 



In passing, mention might be made of the peculiar reaction of the 

 tangential sections 2 and 4, which were made up almost entirely of 

 cortical cells with a few vascular elements on one side. When abscis- 

 sion occurred in these sections, a bending or bowing of the section was 

 alwa.ys noticed. This bending was always such that the tracheal tissue 

 was on the concave side, as if the cells of the cortex had undergone 

 considerable expansion while the cells of the vascular tissue retained 

 their original size. From the work of Richter and others, it may be 

 expected that subjection of portions of plant tissues to illuminating 

 gas would cause an increase in turgor in the cells concerned. Thus, it 

 is probable that the bending of the sections, as described above, is due 

 to the increase in turgor of the cortical cells caused by the narcotic 

 effect of the illuminating gas. The extent of the bending was such 

 that most of the cells in the cortex as well as the separation cells must 

 have been involved in the process. On repeating the above experiment 

 with Datura, a similar bending of the tangential sections was even 

 more pronounced than in Nicotiana. 



Experiment 9. As mentioned above, efforts to induce abscission 

 failed in thin sections. The sections in Experiment 9 were cut so that 

 they were thin in the separation layer but thick on either side. Both 

 surfaces of these sections were thus cut slightly concave so that the 

 sections were thickest at the ends and thinnest in the middle, where 

 the separation zone was located. The sections were then subjected to 

 7 per cent illuminating gas as in Experiment 7. It was not possible 

 to cut very thin free-hand sections of the shape described, but it was 

 demonstrated without a doubt that abscission occurred in sections of 

 this peculiar shape which were thinner in the separation zone than 

 thoso in Experiment 7 where abscission had failed to occur. 



" Certain conclusions which can be drawn from experiments 5, 6, 7, 

 8, and 9 are given below. 



1. Abscission can be induced by allowing the external factor to act 

 directly upon the cells in the vicinity of the separation zone (Expts. 

 6, 7, and 8). 



