1918] Kendall: Abscission of Flowers and Fruits in SoUinaceae 373 



for example, the process originates on the ventral side and may even 

 spread through the large area of storage cells in the axil of the pedicel 

 before reaching the dorsal side. The distance distal from the bottom 

 of the groove at which separation appears is also subject to variation. 

 This variation, however, is not typical of certain species, since it may 

 occur at different times in the same species, evidently as a result of 

 an abnormal stimulation to abscission. 



The second part of the process subject to variation has to do with 

 the amount of tissue that may be concerned in actual cell separation. 

 Abscission first becomes complete in a narrow plane between two or 

 three tiers of cells across the pedicel and the flower can be easily 

 shaken off at that time. If, however, the flower remains on the stem, 

 and is kept turgid by the water rising in the unbroken tracheae, cell 

 separation spreads more and more widely through the tissues of the 

 pedicel, especially in the pith and cortex. It is the extent to which this 

 spreading normally proceeds that varies in the different species. When 

 the process has spread to a considerable extent, a white ring formed 

 by the isolated masses of cells can be seen with the naked eye at the 

 base of the pedicel and a casual inspection indicates that the amount 

 of this white substance varies in the different species. In most 

 hybrids, except F^ H179, there is more spreading in normal abscission 

 than in pure species. In Nicotiana quadrivalvis, N. Bigelovii, and 

 other similar species in which abscission very seldom occurs, no spread- 

 ing takes place. Spreading, however, occurs to a remarkable extent in 

 .V. Tahacum "Maryland." 



&. LYCOPEBSICUM 



We may say that, in general, abscission in Lycopersicmn corre- 

 sponds to that in Nicotiana and that the main points of distinction 

 between the two arise only from the original differences in the separ- 

 ation zones (cf. page 364). In addition, attention must be called to 

 the fact that quite frequently, in individual plants of the tomato, no 

 true abscission occurs in normal flower-fall. In these cases the flower 

 seems to be detached from the plant by a process which compares 

 closely with that called exfoliation. There is no active cell separation 

 and the flower simply wilts and dries back to the groove, where it 

 hangs until broken off by some mechanical agency. The first indica- 

 tion of the process is the loss of chlorophyl in the pedicel, which 

 gradually turns yellow, commencing at the tip and spreading proximal 

 to the separation zone. It is possible that most of the flower-fall 



