356 University of California Publications in Botany [Vol. 5 



tion commences, Bismark brown and Ruthenium red fail to stain the 

 primary and secondary cellulose membranes of the separation cells, 

 although, when abscission does not occur, the entire cell wall is stained 

 in the normal manner. The cells when separating seem, furthermore, 

 to be surrounded only by the thin tertiary membranes. Lloyd, in his 

 work, figures cells in the process of separation which show the disso- 

 lution of the primary and secondary membranes of the cell wall. 



Various interpretations are given to the repeatedly observed 

 occurrence of cell divisions preceding and accompanying abscission 

 Mohl (1860) expresses the opinion that cell divisions are generally 

 necessary before abscission can occur. Investigators since his time 

 have disproved the universal occurrence of cell divisions because they 

 find more and more cases where no cell divisions occur. Lloyd 

 (1914a) maintains that cell divisions are not of necessity correlated 

 with abscission, but are merely evidences of renewed growth and 

 wound responses. As evidence he states that cell divisions are some- 

 times absent and sometimes present in the same species. He cites 

 (1916&) the cotton plant as a typical example in which cell divisions 

 are present in the abscission of older flowers in which the reaction to 

 stimulus is slow. In young flowers and flower buds abscission may 

 proceed without cell division. He further notes (1914a) that cell 

 divisions sometimes precede and at other times follow abscission in a 

 given species. 



c. AGENCIES ACTIVE IN BRINGING ABOUT THE DISSOLUTION 



OF THE MIDDLE LAMELLA 



Very few theories have been proposed to account for the dissolu- 

 tion of the middle lamella and practically no evidence of any kind 

 has been submitted. "Wiesner (1905) claims that in leaf -fall an 

 organic acid, produced as a result of lessening of cell activity and 

 stagnation of cell contents, acts on the middle lamella. His evidence 

 for this statement has to do with obtaining acid reactions with litmus 

 from cells at the base of the petiole during abscission. Kubart (1906) 

 also obtains acid reactions at the base of the corolla in Nicotians dur- 

 ing abscission and, although agreeing with Wiesner that an organic 

 acid probably causes the dissolution of the middle lamella, he also 

 admits the possibility that an enzyme plays a part in the process. 

 Lloyd (1916&) makes the statement that the dissolution of the middle 

 lamella is a process of hydrolysis and although making no definite 

 statement on the subject appears to take it for granted that an 



