368 



University of California Publications in Botany [ V OL. 5 



size originate. The groove is fairly well developed (fig. 5) in young 

 buds whose corolla is only 3 mm. in length, but is not so deep as in 

 older buds. The cells of the separation zone at this stage are smaller 

 than cells on either side, but the difference is not so prominent as in 

 older flowers. In very small buds whose corolla is only 1 mm. in 

 length or whose calyx is only 2 mm. long, the groove is just beginning 

 to appear (fig. 4). In buds below this size (fig. 3) no groove or 

 differentiation in cell size can be detected. Abscission can occur in 

 these early stages, before the groove or differentiation in the size of the 

 separation cells has appeared, as well as at any other stage. In these 

 early stages the radial diameter of the cortex is much less, as com- 

 pared with that of the pith, than in older flowers. It is evident, 

 therefore, that the cells of the separation zone are small because they 

 retain their original small size while the rest of the cortical cells 

 increase in size. The fact that the groove is formed makes it probable 

 that there have been few cell division, or none, in the separation zone 

 of the cortex during the development of the bud. It was observed, 

 however, that the cells of the separation zone in the pith retain their 

 meristematic nature for a considerable period during the development 



r 



/ 



Fig. 3 



Fig. 4 



Fig. 5 

 a separation zone 



