354 University of California Publications in Botany [Vol.5 



several genera, including Cinnamomum and Euonymus, notes and 

 figures cell elongations similar to those figured by Lloyd (1916a). 

 These cell elongations he finds so frequent and conspicuous that he 

 proposes a distinct type of abscission, calling it a " Schlauchzell 

 mechanismus. ' ' 



Loewi, on the basis of his studies, seeks to classify the methods of 

 cell separation in abscission under six different headings, which per- 

 haps would be more appropriately presented under the next subject 

 of consideration (the methods of cell separation) ; but since the author 

 gave them as distinct methods of abscission they will be considered 

 here. They are: (1) "round cell" mechanism; (2) dissolving of the 

 middle lamella; (3) maceration; (4) turgescence; (5) cell elonga- 

 tions; (6) "hard cell" mechanism. They are to be considered merely 

 as factors which, singly or in combinations, may enter in as a part of 

 the normal process of cell separation. Loewi also claims that by con- 

 trolling the temperature, humidity, and various other factors sur- 

 rounding the plant he can influence it to such an extent as to change 

 its method of cell separation. 



6. METHOD OF CELL SEPARATION 



It has been held by various investigators that the cell separation, 

 almost universally connected with abscission, can be caused either by 



(a) chemical alteration and dissolving of the middle lamella or by 



(b) increase in cell turgor. This whole matter has received consider- 

 able attention, although very little direct evidence has been obtained. 

 Wiesner (1871 and 1905) states that cell separation is caused by the 

 dissolution of the middle lamella and by increased turgor. Kubart 

 (1906) and Loewi (1907) agree entirely with "Wiesner on this point. 

 Strasburger (1913), Tison (1900), Lee (1911), Hannig (1913), and 

 Lloyd (1916a and b) believe that cell separation is accomplished by 

 the dissolution of the middle lamella. Practically all investigators 

 have noticed the turgid appearance of the cells after separation, 

 although this of course does not constitute evidence that the separa- 

 tion is due to increased turgor. Fitting (1911) claims that the sep- 

 aration is accomplished, at least in some cases, solely by an increased 

 turgor of the separation cells. He bases his claim on the fact that 

 abscission is very often too rapid to allow time for the dissolution of 

 the middle lamella. He also mentions the fact that the separation 

 cells are very often small, spherical cells, the type of cell which would 

 respond most readily by an increase in cell turgor. On account of its 



