382 



Reed and Smoot : Polygonum virginianum 



At the time when the gynoecium is pollinated, it has been raised 

 by intercalary growth nearly 0.5 mm. above the place where the 

 zone of short cells arose. Simultaneously, a well-defined separa- 

 tion-layer may be distinguished in the midst of this zone. It con- 

 sists of two rows of very short cells (fig. 4), which come in con- 

 tact with the epidermis of the pedicel at the constriction which 

 appeared somewhat earlier. This layer has the general form of a 

 very low, flat-topped dome (fig. 2). All the tissues of the ped- 

 icel, except the fibro-vascular bundles, are intersected by this 

 double row of cells. As the superficial notch deepens, the mar- 



Fir,. 4. Section showing the separation-layer shortly after it is differentiated. 



gins of these layers are brought into contact with the epidermal 

 layers, and their cells become cutinized. 



Following the differentiation of the separation-layer, there is 

 instituted a series of changes in the tissues of the pedicel. The 

 pith-cells for some distance on the proximal side of the layer begin 

 to elongate, and at the same time their walls become considerably 

 thickened. The cell-elongation and wall-thickening appear to 

 start about the same time ; ultimately the greatest wall-thickness 

 is reached in the immediate vicinity of the separation-layer. The 

 result is that the separation-layer is pushed upward in the middle, 

 giving it the form of a higher and more sharply pointed dome than 

 formerly (fig. 3). Some notion of the amount of elongation may 

 be formed by noting the increase of curvature in the separation- 

 layer as the time of maturity approaches (figs. 2 and 3). Figure 

 6, T, represents the thickness of walls of pith-cells in the imme- 



