DEVELOPMENT OF VEGETATIVE AND FLORAL BUDS 257 



one may still utilize the term tunica to designate the outer layer or 

 layers, with the characteristic anticlinal divisions, and apply the term 

 corpus to the remainder, the main body of the apical meristem, with 

 no regular orientation of mitotic figures, these terms are probably 

 only of topographical value. Recent workers tend rather to recognize 

 cytohistological zonation patterns in the apical meristems of angio- 

 sperms (Buvat, 1952, 1955; Gilford, 1956; Philipson, 1947, 1949; 

 Popham, 1951), gymnosperms (Camefort, 1956; Foster, 1938, 

 1941 ), and even of certain ferns (Steeves, 1951) and lycopods (Free- 

 berg and Wetmore, in preparation). These zones are usually three in 

 number; they have been designated the central zone, the peripheral 

 zone, and the pith rib meristem zone (Foster, 1938), irrespective of 

 the variation in their extent and position in different species (Figs. 1- 

 5 ) . Even if we assume a common set of morphogenetic interrelations 

 of zones in which all shoot meristems participate, we fail to find histo- 

 logical uniformity to correlate with this morphogenetic unity. 



Recent trends toward recognizing patterns of zonation as possibly 

 significant in the material and energy turn-over in the apex command 

 serious attention. That the cells of the central zone, however dis- 

 tributed, and in whatever patterns, have in common certain charac- 

 teristics is probably more than coincidence (Figs. 1-5). They habit- 

 ually are of larger size, are more highly vacuolated, divide less fre- 

 quently, and often, perhaps characteristically, possess larger nucleoli 



Figs. 1-6. Vegetative and flowering apices. 



1. Vegetative apex of Nicotiana tahacum. Note central zone with a 

 mitotic metaphase near the top. Xl50. 



2. Vegetative apex of Ginkgo biloha. Note conspicuous cup-shaped 

 central zone of very large, vacuolated cells; mitosis showing near bottom 

 of zone. XlOO. 



3. Vegetative apex of Xanthium pensyhanicum. Note central zone of 

 a layer of cells in the middle of the second row capped by a few con- 

 spicuous cells in the top row. x 100. 



4. Vegetative apex of Chenopodium album. Note less conspicuous 

 V-shaped central zone of about 7 cells each of first two rows, 4 to 5 cells 

 in the third row, and 2 cells in fourth row. Xl50. 



5. Vegetative apex of seedling Papaver somnifenim. Note faintly out- 

 lined V-shaped central zone of large cells, the outer three layers conspicu- 

 ously in rows, the peak of the V less conspicuously so. X200. 



6. Flowering apex of P. somniferum. Note absence of central zone at 

 top of apex. X50. 



