404 University of California ftiolieati-ons in Botany [Vol. 5 



2. Abscission induced by the above methods in isolated pieces must 

 be independent of transportation of material from the rest of the plant. 



3. The fact that abscission cannot be induced in thick cross-sections 

 of the separation zone shows that cell separation cannot be induced 

 by the action of the external factor directly on the separation cells. 



4. It is necessary that a certain proportion of the tissues of the 

 pedicel be in intercellular connection with the cells of the separation 

 zone before cell separation will occur, but this proportion is surpris- 

 ingly small (Expts. 7, 8, and 9). 



5. There is evidently increase in turgor in all the cortical cells of 

 the pedicel during abscission induced by the above method (Expt. 8). 



2. Action of Acids on the Separation Cells op Kicotiana 



Under this heading a description will be given of the effect of 

 mineral acids on small isolated pieces such as were used in experiments 

 6, 7, 8, and 9. It was stated above (page 364) that by the use of two 

 mineral acids together with several stains, no chemical difference could 

 be detected between the cell walls of the separation cells and those of 

 normal cortical cells. The present work represents an attempt to 

 determine, by experimental means and by watching through the micro- 

 scope the action of acids on cell walls, whether the cell membranes 

 of the separation cells are more subject to hydrolysis than those of 

 normal cortical cells. 



Experiment 10. — Small pieces of the pedicel were prepared as in 

 figure 10. These pieces were boiled for one or two minutes in 4 per 

 cent hydrochloric acid and then washed in water. Upon examination 

 it was found that the pieces could be separated into halves through 

 the separation zone by a slight pulling or bending motion. Microscopic 

 examination of the separation surfaces showed that the break through 

 the cells of the separation zone had taken place along the plane of the 

 middle lamellae of their walls. This same type of separation was 

 brought about without boiling when 10 per cent nitric or hydrochloric 

 acid was allowed to act on the pedicels for approximately five minutes. 

 When longitudinal sections are used in place of entire pedicles, the 

 same results are obtained but much more rapidly. It was also noticed 

 that separation under these latter conditions takes place more quickly 

 in younger pedicels than in older ones. In the pedicels of fully 

 developed fruits no separation could be induced, but in those of 



