1916] Goodspeed-Kendall: Partial Sterility of Nicotiana Hybrids 297 



shape of the cells characteristic of these tissues in the remainder of 

 the pedicel. The cells of the abscission zone appear to be somewhat 

 collenchymatous and in many cases contain a greater number of starch 

 grains than do the neighboring cells without the abscission zone. The 

 abscission zone extends in a ring completely around the cortex and 

 across the pith. On the outer or ventral portion of the cortex it is 

 ordinarily composed of from fifteen to twenty rows of cells, while on 

 the inner or dorsal side the zone is wider and spreads out into the 

 considerable area of storage cells found in the axil of the pedicel. 



4. Abscission apparently may take place in almost any portion of 

 the abscission zone distal to the groove. In both cortex and pith the 

 cells taking part in abscission and most distant from the vascular 

 cylinder may be from five to ten rows in thickness, but as the vascular 

 tissues are approached the abscission layer is gradually reduced in 

 thickness until separation takes place along only one row of cells. 

 In the cortex this behavior results in a wedge-shaped ring of abscission 

 cells, the wedge being widest below the epidermis, while in the pith a 

 circular mass of cells is involved. 



5. Abscission starts in the cortical tissues just beneath the epider- 

 mis on the ventral side and extends around the cortex, taking place 

 finally on the dorsal side. The first external indication is a bulging 

 out of the epidermis just above the tissue in which the abscission 

 process is taking place. This process apparently originates independ- 

 ently in the pith and simultaneously with the start of abscission in 

 the cortex. There is no indication of cell division accompanying 

 abscission in the tissues involved, nor is there evidence of any alter- 

 ation of the cell walls by the dissolution of the middle lamellae or 

 elongation and softening of the entire wall. The cells of the abscission 

 layer become separated one from another over a considerable area or 

 along only two rows of cells, depending, as noted above, on the prox- 

 imity to the vascular tissues. This separation appears to be initiated 

 simply by an increased turgor, as a result of which the cells round up, 

 intercellular spaces increase in size and the contact points are suc- 

 cessively reduced. The distention of the epidermis at the start of 

 abscission, the entire collapse in the pith of all the cells concerned and 

 the fact that a wilted shoot with less than the normal tur-gidity retains 

 its flowers following injury but sheds them as soon as turgid — these 

 facts seem to favor this view. This question as to tlie physiology and 

 mechanics of cell separation in the abscission zone is still receiving 

 attention; tlu^ stateiiiciils iiiadi' in the ])receding sentences represent 



