402 University of Calif or nia Publications in Botany [Vol.5 



the section. Abscission became complete in sections 0.3 mm. or more 

 in thickness, the separation taking- place in such a way that a slight 

 bending or pulling motion sufficient to break the tracheae divided the 

 section into equal halves. In thinner sections, ranging from 0.3 mm. 

 to 0.17 mm., abscission starts in the normal position but does not pro- 

 ceed to completion, the extent to which the process takes place depend- 

 ing, as has been said, upon the thickness of the section. In sections 

 much below 0.17 mm. no signs of abscission appear. Also, if the 

 thicker sections are shortened in length to any considerable extent by 

 cutting off portions of the tissues from either side of the separation 

 laj^er, abscission will not occur. 



The process of abscission as it occurs in these sections corresponds 

 exactly to the process in an entire pedicel. Cell separation starts 

 independently in the pith and in the cortex, appearing first in that 

 part of the cortex corresponding to the ventral region of the pedicel 

 where, it will be remembered, abscission starts in the entire flower. 

 When mounting the sections on an object slide for microscopical 

 examination, the isolated cells in the pith lie in position but can be 

 easily washed out with a small jet of water. In the cortex a break 

 soon appears in the epidermis as the result of manipulation in mount- 

 ing and a cavity is formed at that point as the result of the isolated 

 cells of the cortex floating out in the water. 



Experiment 7 was repeated in the case of Datura with similar 

 results, except that in this case abscission was more active since it 

 involved more cells, a situation which one might be led to expect 

 because of the differences between the two species in the normal abscis- 

 sion of entire flowers. It will be remembered that the separation cells 

 of the cortex in Datura are in no way distinguishable from other 

 cortical cells ; yet even in these sections separation occurs in a definitely 

 predetermined position corresponding entirely Avith the position in 

 abscission of the entire flower. It was even noticed that abscission 

 started in these sections in the same tissues and in the same manner 

 as in normal floral abscission. 



After the thickness of the sections best adapted to obtaining results 

 liad been determined, the following experiment was performed on 

 sections cut from different x^arts of the pedicel. 



Experiment 8. — In this experiment a series of longitudinal sections 

 of the pedicel were cut so that the plane of the sections was at right 

 angles to that of the sections cut in Experiment 7. The first section 

 was tangential, on the ventral side of the pedicel, and contained only 

 the epidermis and a few tiers of cortical cells. Section 2 was also 



