54 TORREYA 



sions at the top "deflect" the files thereby producing a conspicuous fan-like 

 appearance. 



Central mother cells (zone 2). The central portion of the apex is occupied 

 by a core of tissue roughly spherical, obovoid, cylindrical or fan-like in shape, 

 which is concerned with increase in volume and generation of new cells from 

 its base and sides. Zone 2 originates from the lower part of zone 1, Fig. IB. 

 Divisions are in all planes but since the daughter cells tend to remain enclosed 

 by the original mother wall a complex arrangement of vertical files and blocks 

 of cells result having the general appearance of a "massiges Meristem." In 

 Cycas revoluta vertical files may occur at any level in the central tissue thus 

 making it difficult or impossible to distinguish the central mother cells from the 

 rib meristem. The enlarged mother cells contain thin cytoplasm with large 

 vacuoles and faintly staining nuclei. Their walls are considerably thickened, 

 especially where several cells touch, and display prominent primary pit fields. 

 When the majority of the cells enlarge at one time zone 2 becomes the most 

 conspicuous feature in the apex, (Fig. 2). Following enlargement the mother 

 cells may divide sporadically throughout the zone, while at the periphery they 

 give rise to the rib meristem, (Fig. 1A) and most of the peripheral layers. 

 Apices of Zamia which are gorged with starch fail to show a vacuolate central 

 mother cell zone (Fig. 3.) 



Peripheral tissue zone (zone 3). The zone of initiation and the central 

 mother cells are flanked by this tissue. It is clearly separated into an inner and 

 outer portion. The outer part consists of relatively small, thin-walled, deeply 

 staining, mitotically active cells which owe their origin to divisions in the edge 

 of the initiation zone and to periclinal divisions in the surface layer. The deriva- 

 tives from the latter may divide in any plane, thereby producing a complex 

 cellular pattern in which cell lineage can not be traced. However, periclinal divi- 

 sions may predominate and build short files at right angles to the surface 

 (Fig. lc) . Leaf primordia originate from the base of the outer peripheral tissue. 

 The inner layers of the peripheral tissue radiate from and have their origin in 

 the edge of the core of central mother cells. The cells here are larger, more 

 vacuolate and appear to divide less frequently than those in the outer layer. 



Growth in the peripheral tissue zone is centrifugal. Epidermis, leaf primor- 

 dia, cortex, pro-vascular tissue, and generally some of the pith are differen- 

 tiated from peripheral tissue. 



Rib meristem (zone 4). The rejuvenated cells at the base of the central 

 mother cell zone build files of daughters which through progressive enlarge- 

 ment ultimately mature as pith. The thickened walls of zone 2 prove to be 

 temporary and do not persist in the rib meristem. The rib meristem is a con- 

 spicuous feature in all the cycads examined except in Cycas revoluta where a 

 distinction between central mother cells and rib meristem cannot be observed. 



