412 University of California Publications in Botany [Vol.5 



Histology and Cytology of the Pedicel 



1. The separation layer arises in all the species listed above, except 

 Lycopersicum and Solawwm tuberosum, at or near the base of the 

 pedicel. In the latter two species the layer is located near the middle 

 of the pedicel, but even in these cases, if one considers the pedicel to 

 be composed of two internodes, the layer occurs at the base of the 

 most distal internode. 



2. The separation layer is preformed, ready to function at any 

 stage in the development of the flower and represents (cf. Kubart's 

 first type, page 350) a portion of the primary meristem which has 

 retained some of its originally active condition. 



3. In all the species except Datura the separation cells are char- 

 acterized by their small size, isodiametric shape, large amount of 

 protoplasm and somewhat collenchymatous appearance. A stud}' of 

 the early histological development of the pedicel indicates that the 

 small size of the separation cells does not necessarily bear any relation 

 to abscission. This statement is supported by the fact that in Datura 

 there is absolutely no visible difference between the separation cells 

 and any other cells of the pedicel. 



4. Various tests with stains, acids, and alkalis fail to indicate any 

 chemical difference between the cell walls of the separation cells and 

 the walls of neighboring cortical cells which do not separate. How- 

 ever, the middle lamellae of cell walls in the general region at the 

 base of the pedicel seem somewhat more easily hydrolysed by acids 

 than in the more distal portions. 



5. A study of the early histological development of the pedicel in 

 Nicotiana and Lycopersicum shows that the grooves near which the 

 separation zone arises do not necessarily bear any relation to abscis- 

 sion. The grooves are formed because, in the development of the 

 pedicel, certain cells do not increase in size so fast as the neighboring 

 cells on either the proximal or distal side. 



6. The development of mechanical tissue in the pedicel of Nicotiana 

 continues through the separation layer, thus frequently holding the 

 fruit on the plant in spite of the fact that abscission commonly occurs 

 in the cortex. In most of the berry-forming species of the Solanaceae 

 this mechanical tissue does not become continuous through the separa- 

 tion layer and thus offers no impediment to fall when abscission occurs 

 in that region. 



