1918] Kend-all: Abscission of Flowers and Fruits in Solmvacme 367 



which separation does not occur. The separation layer here is located 

 about 8 mm. distal to the base of the pedicel, with absolutely no ex- 

 ternal indication of its position. Microchemical tests, which in Nico- 

 tiana gave different reactions in the case of the separation zone and in 

 the case of normal cortical cells, here fail to show any corresponding- 

 condition of differentiation. 



Abscission has never been found to occur in Petunia or Salpiglossis, 

 so that it is of interest to examine the histological condition of the 

 base of the pedicel in these two species. They are practically identical 

 with regard to the structure of the pedicel, so that the description 

 given below can be taken as applying to both genera. The cortical 

 cells of the pedicel pass into those of the stem without any groove or 

 small-celled region. On the ventral side, however, is the region of 

 small cells in the axis of the pedicel, which is more or less common to 

 all flowers. The tissues of Petu7iia are not so soft and succulent as 

 those of Datura, Nicoiimia, and Lycopersicum. They tend rather to 

 be dry and tough. The cells in the cortex and pith are also not so 

 nearly isodiametric as in Datura, but are much elongated in a direc- 

 tion parallel with the long axis of the pedicel. 



The condition in the other species mentioned above will be given 

 only a general description. Abscission occurs in all the other species 

 except Salpichrora and Lycium which, however, do not differ, in 

 respect to the histology of the base of the pedicel, from any of the 

 others. Solatium tuberosum resembles Lycopersicum. All the other 

 species are similar in regard to the structure of the separation zone. 

 There is in every case a general region of small cells extending 

 across the base of the pedicel where the separation layer occurs. 



3. Development of the Separation Zone in Lycopersicum 



AND Nicotiana 



a. LYCOPERSICUM 



The development of the separation zone could be followed better 

 in Lycopersicum than in Nicotiana because in the former the zone is 

 not so close to the main axis of inflorescence. The problem here 

 resolves itself into an effort to determine, by means of longitudinal 

 sections of very young pedicels, how early in the development of the 

 flower the groove and the differentiation in cell size of the separation 

 cells appear. It was found that the development of the separation zone 

 indicates the method bv which the groove and differentiation in cell 



