362 University of California Publications in Botany [Vol. 5 



separation zone are very small, not much could be determined in 

 regard to the dissolution of cell walls by means of thick, free-hand 

 sections. The best results along this line were obtained from the thin 

 paraffin sections of the separation zone, although in order to show the 

 cell wall in its normal thickness it was necessary to use the free-hand 

 sections. As a supplement to these sections, several points of interest 

 were brought out by washing off the isolated cells from the end of 

 freshly abscissed pedicels and mounting them for microscopic exam- 

 ination. 



In most of the work the paraffin sections were stained in safranin 

 and Delafield's haematoxylin. The free-hand sections were generally 

 mounted in water and stained in iodine. In special instances other 

 stains were used. Thus, in testing for chemical differences in the cell 

 walls of the separation cells, several other stains, such as erythrosin, 

 eosin, Bismark brown, gentian violet and Ruthenium red were used. 

 It was found that for demonstrating the dissolution of cell walls 

 aqueous methylene blue was an excellent stain to use. This stain was 

 allowed to act overnight and the sections destained slightly in alcohol. 

 Methylene blue was also an excellent stain for the isolated cells ob- 

 tained as noted above. By fixing these cells to the slide with albumen 

 fixative and staining with this stain, the thin membranous wall sur- 

 rounding the protoplast can be distinctly seen. 



Various methods, such as subjecting inflorescences to illuminating 

 gas and mechanical injury, were used to bring about abscission. The 

 best results were obtained in cases where abscission was induced by 

 inserting shoots under a bell-jar containing from 1.5 per cent to 

 3 per cent illuminating gas. By using illuminating gas in this way 

 and by taking sections of the pedicels at intervals it was possible to 

 determine just when the first signs of abscission appeared in a certain 

 percentage of gas. This time was definitely determined for certain 

 species so that it was possible to get material killed and fixed at any 

 desired stage in the process of abscission. It was found that the best 

 results were obtained by killing and fixing the pedicels at about the 

 time when abscission was known to be commencing. 



