556 



THE STRUCTURE OF ECONOMIC PLANTS 



tissues of the pedicel are not continuous through the separation 

 layer as in Nicotiana so that there is a break in a plane parallel 

 to the bottom of the groove. It is possible, therefore, for abscis- 

 sion to cause the fall of the fruit at any stage in its development, 



but this rarely occurs ex- 

 cept at about two or three 

 days after anthesis. 



On the basis of his in- 

 vestigation, Kendall agrees 

 with Kubart (lo) that the 

 separation layer represents 

 "merely a portion of the 

 primary meristem which 

 retains its original physio- 

 logical capacities." Dur- 

 ing the development of the 

 fruit, there is a very definite 

 increase in the size of the 

 pedicel which is restricted 

 almost entirely to the por- 

 tion distal to the groove; 

 and its diameter in the 

 fruit may be two or three 

 times greater than at the 

 time of the anthesis of the 

 flower. This results from 

 r- ... If the enlargement and divi- 



FiG. 193. A, diagrammatic longisection or young o 



flower just prior to anthesis, showing development sion of the Cells of the COt- 

 of cauline ovules and location of abscission layer; ^^^ ^^^ ^y^^ diameter of 

 B, diagrammatic longisection or portion of pedicel 

 in which abscission zone is located ; C, transection the 

 of ovary of bicarpellate pistil showing fleshy central 

 placentae which have grown around and embedded 

 developing ovules: a, abscission zone; b, vascular 

 strands; c, epidermis ; d, separation layer; ?, pith; separation and isolation of 

 /, chlorenchyma; e, collenchyma; ped, pedicel. , 11 • 1 



(B, redrawn after Kendall, Univ of Calif Pub ) ^^e Cells in the Separation 



zone. Frequently, the 

 process is initiated in the pith as well as the cortex, and proceeds 

 independently in the two regions until the final break occurs 

 through the vascular tissue and the epidermis. The separation 

 begins in two tiers of cells at the bottom of the groove, but may 

 later affect three or four layers. It takes place in such a way 



pith remains practi- 

 cally constant. 



Abscission involves the 



