early Floral Stage 1 — see Fig. 5-8). Only one row of mother zone cells 

 is obvious, but these and the tunica still have large, relatively clear nuclei. 

 There are now 3 to 6 rows of cells in the transition zone, the primary site 

 of activity at this early stage. D. Floral Stage 3, clearly a floral bud (in- 

 florescence primordium). The most striking event has been the enlarge- 

 ment of the rib meristem cells along with some of the transition cells, 

 beginning to produce the characteristic shape of the cocklebur stamin- 

 ate floral head. The cells with light nuclei in the tunica and mother cell 

 zone are still evident, but they will gradually disappear as the bud 

 develops. C and D were drawn to the scale of A and B from photo- 

 graphs in R. H. Wetmore, E. M. Gifford, Jr., and M. C. Green, 1959, 

 in R. B. Withrow (editor) Photoperiodism and Related Phenomena in 

 Plants and Animals, 255-273, American Association for the Advance- 

 ment of Science, Washington, D.C. 



