56 T O R R E Y A 



It is significant that while the apex is divided into zones (Fig. 1), the. 

 zones merge more or less and are not sharply delimited as in stratified apices 

 (Figs. 2 and 3). For instance the limits between the initiation zone and the 

 central mother cells fluctuate. The same is true for the central mother cells and 

 the peripheral tissue zone and rib meristem which surround them. The ex- 

 treme is reached in Cycas revoluta where it is impossible to separate the cen- 

 tral mother cells, rib meristem and maturing pith. Obviously a histogen theory 

 demanding discrete layers can not be applied to the cycads studied to date. 

 The surface layer continues periclinal divisions throughout the life of the 

 shoot and, therefore, can not be considered as a dermatogen. The concept of 

 a tunica and corpus is likewise untenable. Furthermore the marked zonation 

 definitely precludes considering the apex as a primordial meristem lacking in 

 cellular "differentiation." 



Conclusions concerning the similarity of zonation throughout the cycads 

 must remain premature until Ceratozamia and Stangeria have been investi- 

 gated. There is, however, a pronounced uniformity in the seven genera studied, 

 if the preliminary account of Johnson (1944) on Encephalartos, Bowenia and 

 Macrozamia is supported when additional material can be examined. In all, the 

 tissues of the shoot are ultimately derived from the initiation zone. Likewise 

 there is a marked similarity in the central mother cell zones with their large, 

 highly vacuolate, irregularly arranged cells which often have much thickened 

 walls. Upon rejuvenation these cells, with the exceptions noted for Cycas 

 revoluta, contribute to the peripheral zone and rib meristem. 



The cycad shoot apex furnishes an excellent study in the coordination of 

 growth. Each of the zones increases in size but does not disrupt the general plan 

 or shape of the apex. Growth converges from the initiation zone to the core of 

 central mother cells (a particularly striking feature in Dioon and Microcycas). 

 The production of new cells accounts for the increase in size in this case. 

 Growth from the central mother cell zone, on the contrary, is divergent with 

 increase in size through cell division and enlargement in the peripheral tissue 

 and young pith cells. A series of actively meristemmatic centers, arranged in 

 the form of a spiral at the base of the apex in Zamia, Dioon, Cycas and Micro- 

 cycas, become the leaf primordia. These centers encroach upon the sides of the 



Explanation of Figures 2 and 3 



Fig. 2. Zamia latifoliata. Note : conspicuously vacuolate central mother cells, irregular 

 files of cells in zone of initiation capping the summit, outer part of peripheral zone with 

 files of cells from periclinal divisions and inner part radiating from central mother cells,, 

 also rib meristem at the base of central mother cells. X 230. 



Fig. 3. Z. umbrosa. Shows apex gorged with starch. Note : absence of vacuoles in 

 central mother cells also planes in which division has occurred, and vertical alignment 

 of cells in zone of initiation. X 230. 



