1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 371 



that such a condition of affairs very often exists. It will be found in 

 many cases, especially on older plants, that although abscission has 

 occurred in the cortex, as evidenced by the presence af a white, 

 powdery substance at the base of the pedicel, the capsule is yet firm 

 on the stem. Indeed, in certain hybrid tobaccos it is common to find 

 most of the capsules in this abscissed condition. The fruit is supported 

 in these cases by the tough mechanical elements of the wood, which 

 also prevent the breaking of the tracheae and protect the intraxylary 

 phloem. In the pith the tissues may be in a somewhat abscissed con- 

 dition, but since there is no way for these cells to escape through the 

 sheath of wood they remain for some time in position before finally 

 collapsing. 



The development of mechanical tissues takes place in Lycopersicum 

 in much the same manner as in Nicotiana but with the distinct differ- 

 ence that in the former the wood-fibre tissue does not become con- 

 tinuous through the separation layer. That is to say, in the tomato 

 a break is .left in the mechanical tissue in a plane with the bottom of 

 the groove. It is evident here that abscission would cause fall of the 

 fruit in any stage of its development, although in this case it happens 

 that abscission very rarely occurs after two or three days past anthesis. 

 A condition resembling this one in the tomato was observed in other 

 berry-forming species of the Solanaceae such as Oestrum fasciculatum 

 and Solanum verbascifolium, which often drop their immature fruits 

 by abscission. Abscission, however, very seldom occurs in mature 

 berries of these species, the fruit generally falling away from the 

 receptacle above the calyx. 



THE PROCESS OF ABSCISSION 



1. GENERAL DESCRIPTION OP THE PROCESS IN SEVERAL GENERA 



a. NICOTIANA 



The process of abscission in all the species of Nicotiana investi- 

 gated conforms to the usual type involving separation and isolation of 

 cells. Further details of the process were briefly discussed in a pre- 

 liminary paper (Goodspeed and Kendall, 1916) for certain F : species 

 hybrids of Nicotiana. It was there noted that cell separation starts in 

 the dorsal side of the pedicel, in the cortex a short distance distal to 

 the groove (pi. 49, fig. 1) and spreads from this point around to the 

 ventral side. The first external indication seems to be a bulging of 



