JOHNSON: SHOOT APEX 53 



or lateral buds which are fairly common on large plants make excellent sub- 

 jects for study. Fortunate indeed is the botanist who can secure the tip of a 

 really old plant which must be sacrificed for want of space or other reason. 

 Greater difficulty will be experienced in obtaining material from the remaining 

 genera. Zamia and Cycas, however, will provide a satisfactory picture of the 

 shoot apex as seen in the cycads. 



The shoot apex, in mature plants, is deeply sunken and closely invested by 

 a protective armor of leaf bases and cataphylls. These must be removed with 

 considerable care so as not to injure the delicate tip within. The microtechnique 

 outlined by Ball (1941) for the shoot apex will yield excellent preparations. 



The apex is characteristically mound-like or broadly cone-shaped in out- 

 line and relatively enormous in size. That of Cycas revoluta measures 2018- 

 3305 fi in diameter, of Microcycas calocoma 500-2000 fi, of Dioon edule 1677- 

 1941 [x, of Encephalartos 856-1263 /x, of Bowenia serrulata 1246 /a, of Zamia 

 integrijolia 400-860 fx, and that of an embryo of Macrozamia Moorii 625 (i. 

 These dimensions surpass anything reported for other seed plants. Boke (1941) 

 gives 700-900 /x for Trichocerus spachiaitus which appears to be the largest 

 on record among angiosperms. 



Median or near-median longsections show that the shoot apex is divided 

 into three or four zones. This condition is indicated schematically in figure 1 

 and may be observed in the photomicrographs, figures 2 and 3. The summit is 

 occupied by a region of initiation (zone 1) ; immediately beneath is a core of 

 central mother cells (zone 2) which is underlaid by a rib meristem (zone 4). 

 The three axial zones are surrounded by a series of flanking or peripheral layers 

 (zone 3). Each zone may be distinguished by its contribution to the growth 

 of the shoot as well as by such characteristics as : cell size and arrangement, 

 degree of vacuolation, staining qualities of the nucleus, and thickness of cell 

 walls. 



Zone of initiation (zone 1). This zone consists of the relatively small, thin- 

 walled, densely protoplasmic cells in the upper part of the shoot apex. 1 The 

 actual number of cells involved depends upon the size and shape of the apex. 

 In small conical apices there are about a dozen superficial cells, in larger ones 

 from 50 to 100, while in the large dome-shaped apices of Cycas revoluta the 

 number will be much higher. There is nothing to suggest an apical cell or even 

 a clearly defined group of initials, for the superficial cells at the summit are in- 

 distinguishable from those of zone 3 except possibly by difference in size. 

 Periclinal divisions are sufficiently frequent in both the superficial layer and its 

 derivatives to build somewhat irregular files of daughters which converge 

 toward zone 2 (Fig. IB). In Microcycas calocoma anticlinal and oblique divi- 



1 In two instances I have found large vacuoles in the superficial cells of Encephalartos. 

 The specimens were in a dormant condition. 



