﻿BOTANICAL GAZETTE 



In young stems and leaves the same procedure seems to be 

 followed, although, because of the delicacy of the organs and the 

 rapidity with which abscission is consummated, it is more difficult 

 to follow. There is undoubtedly, however, a great deal of irregu- 

 larity. Instances have been seen in which the epidermis was 

 fractured and the wound gaped open, in the manner suggested by 

 Hannig's fig. 9, before the separation was completed in the pith. 



CHEMICAL ALTERATION IN THE WALLS OF THE ABSCISSION CELLS 



Proof that separation is preceded by a chemical alteration in 

 the cell wall is the following. 



(1) If a section (preferably a fairly thick one) is treated with 

 strong KI/I, the walls of the abscission cells will appear to have 

 an indefinite pale greenish hue. If the reagent is washed away 

 gradually, as the yellow of the iodine disappears, the walls show 

 a pale blue, which fades away as the washing is prolonged. An 

 open diaphragm is necessary. This color reaction 8 is much more 

 pronounced if the section is first boiled for a minute in weak (4 per 

 cent) HC1, which, because of a previous alteration of the cell walls, 

 attacks them but does not affect the remainder. Not merely the 

 cells actually involved in separation, but all the cells below the sepa- 

 ration plane for 0 distance oj 0.5 mm. (in a large node) show the 

 blue reaction, but in less and less degree the farther from the 

 separation plane. Above this the transition is sudden, only a por- 

 tion of the adjacent cell walls of the cells immediately above being 

 altered. 



(2) In earlier stages of abscission, however, the blue reaction is 

 not visible, but an indication of chemical alteration is to be seen 

 in the failure as compared with the adjoining, unaltered cells, to 

 color strongly with iodine. 



(3) The staining capacity of the wall of the separation cells 

 is obviously reduced. They will not hold Bismarck brown (pi. 

 rig. 9), so that they become almost colorless, while the unaltered 



8 The reaction was observed under circumstances of evident physiological sig- 

 nificance by Green (The soluble ferments and fermentation, p. 97) and by myself 

 (Development and nutrition of the embryo .... in the date). Ann. Rep. Mo. Bot. 

 Card. 21:103. 1910. 



