380 University of California Publications in Botany [Vol. 5 



times be absent. Such cases might be explained by the absence of 

 any considerable amount of starch in the cells concerned. Indeed, 

 the starch grains usually noted in tlie separation layer can not at 

 times be observed. This might also explain the fact that the bulging 

 of the epidermis and collapse of cells in the pith usually accompany- 

 ing abscission are sometimes absent. Also, starch grains are rarely 

 observed in the separation cells of Lycopersiciim and Datura and in 

 these forms xexy little bulging of the epidermis occurs. Although 

 humid conditions favor abscission and drought prevents the process, 

 it has also been observed that drought has to be very severe before it 

 produces such a result. Other evidences for increased turgor derived 

 from the turgid appearance of the cells are mostly obtained after 

 abscission has started and, granting that the cells are isolated by dis- 

 solution of the middle lamella, more or less expansion due to release 

 of pressure is to be expected. 



A critical examination of the separation cells during abscission 

 brings out several facts, other . than those mentioned in the above 

 paragraph, which oi themselves render inadmissible the theory that 

 cell separation is brought about by increased turgor. These are 

 as follows: 1. There seems to be no perceptible change in cell 

 shape or size during separation. 2. The increase in size of the inter- 

 cellular spaces does not necessarily take place first between the walls 

 at the /'corners" of the cells, but may appear first as a longitudinal 

 streak between the lateral walls of the cells (pi. 51). 3. Cell isola- 

 tion may be incomplete- in large numbers of cells still remaining 

 attached to each other. 4. Cell separation first becomes complete in 

 a narrow plane between only two tiers of cells before spreading later 

 to a larger number of cells. 5. The spreading of cell separation 

 itself is obviously hard to explain on the basis of the turgor theory. 



In view of the facts brought out in this discussion and the positive 

 evidence for the dissolution of the primary membranes, it should be 

 clear that increase in turgor, at least in the Solanaceae, is not the 

 direct cause of cell separation. Undoubtedly there is often great 

 increase in turgor during abscission, especially in certain types of 

 cells, but this increase, instead of being the direct initiating factor, 

 probably serves merely to hasten and facilitate the process. 



