364 University of California Publications in Botany [VOL. 5 



der. The pith and cortex (fig. 1, p and c) are composed of large 

 parenchyma cells which in the cortex are two or three times as long 

 as wide, but in the pith are more nearly isodiametric. There is no 

 mechanical tissue to be found in the floral pedicel but, as will be noted 

 in more detail later, wood fibres are formed as soon as the fruit begins 

 to develop. The epidermis of the pedicel (fig. 1, e) is typical but with 

 a poorly developed cuticle, especially in the groove (fig. 1, #), where 

 the cells are also much reduced longitudinally. Beneath the epidermis 

 is a layer of small cells with very large intercellular spaces and an 

 abundance of chloroplasts (fig. I,/). This tissue stops a short dis- 

 tance proximal to the separation zone and does not continue in the 

 pedicel. The layer of collenchyma which is commonly found in cer- 

 tain species just beneath this chlorophyl tissue is entirely absent in 

 Nicotiana, or at least is very poorly developed. 



Corresponding with the general region of the groove is an area of 

 medullary and cortical cells which are smaller than corresponding 

 cells on either the proximal or distal side of the groove. This region 

 of small cells is homologous with the separation zone (fig. 1, &) and it 

 extends across the base of the pedicel. The smallest cells are in the 

 center of the region, in a plane with the bottom of the groove, and 

 grade in size to the larger cells of the pith and cortex on either side 

 (plate 49, fig. 1). The zone of small cells is ten to fifteen tiers of 

 cells thick on the dorsal side but is wider on the ventral side, where it 

 spreads out into the large area of storage cells found in the axil of 

 the pedicel. The separation layer (fig. 1, h) is located five to seven tiers 

 of cells distal from the bottom of the groove. Hanning reports this 

 layer as occurring at the tip of the pedicel in Nicotiana Langsdorffii, 

 but in all my experiments on two varieties of this species I find separa- 

 tion invariably occurring at the base of the pedicel in the position 

 described above. All the species and varieties of Nicotiana examined 

 show a structure of the pedicel corresponding with the above descrip- 

 tion except that in some varieties, as in those of N. Bigelovii, the sep- 

 aration zone is much thinner on the dorsal side. In such cases it is 

 also noted that the groove is poorly developed. 



The cells of the separation layer are in no way morphologically 

 differentiated from those making up the remainder of the separation 

 zone. Indeed, any cell of the zone seems capable of functioning as a 

 separation cell. The separation cells are smaller than normal cortical 

 cells and spherical in shape except in the vascular bundles, where they 

 do not seem to.be differentiated in size and are elongated parallel to 

 the longitudinal axis of the pedicel. The cell walls are slightly thicker 



