DEVELOPMENT OF VEGETATIVE AND FLORAL BUDS 263 



of mitotic activity, as the first sign in an effect of the single photo- 

 period, so that the initiation of ribs or files of cells of the pith can now 

 be seen to be separated from the latter by several rows of cells where 

 only three or four rows were previously present. Though they are not 

 easily visible here, numerous mitotic figures can be seen in practically 

 every section. At least 3 were present in Fig. 8. It should be pointed 

 out that this first sign of induction by photoperiodic treatment was 

 found in every species of plant studied, whether long-day or short-day 

 type. 



In Fig. 9, with 1 short day of photoperiodic treatment and 8 

 subsequent days in the greenhouse, more extensive multiplication of 

 cells in the subcentral cell zone is evident. Generally, too, the apex 

 is beginning to show this increase in cells by appearing more exten- 

 sively mounded. It must be pointed out that the cells in this region of 

 mitotic activity soon begin a period of cell enlargement as do those of 

 the rib meristem region below, thereby giving definite extension to the 

 pith of the newly forming flower and inflorescence axis. 



In Fig. 10, after 10 days in the greenhouse, the inception of a 

 flowering apex is well advanced. Much has happened in the last 2 

 days. As mentioned, the multiplication of cells below the central region 

 has continued, but so has the collateral process of cell enlargement 

 with the progressive vacuolation of many of these cells, the result 

 being that the top of the pith seems to be close to the region of the 

 central zone. The progressive steps of this change become apparent. 

 The central cefls have divided anticlinally, as is indicated by the 

 smaller components; this row has also been extended well down the 

 flanks of the mound by like-appearing cefls from recent divisions in 

 the cefls of the remaining peripheral part of the flanking meristem, so 

 that the tunica-like second row of cells and even parts of a regular 

 third layer can be followed easily, though the cells are somewhat 

 smaller and less vacuolated than earlier. Anticlinal divisions were 

 present in the section but they are hard to detect at the magnification 

 of Fig. 10. 



The next series of four figures. Figs. 11 to 14 inclusive, is of plants 

 which were exposed to two photoperiods before being removed to the 

 greenhouse. Figure 1 1 is the section of an apex of a lateral bud at the 

 time of removal from the light room. Even at the end of two photo- 



