JOHNSON: SHOOT APEX 57 



shoot apex but, again, growth is nicely coordinated and the summit continues 

 to be elevated above the maturing regions. 



It is tempting to speculate on the mechanism which permits the maintenance 

 of growth zones. What factors control the enlargement of the central mother 

 cells while they are surrounded by actively dividing neighbors? What agents 

 are responsible for rejuvenation at the periphery of the central mother cell 

 zone ? Why should the sides of the apex contain the simplest and least differen- 

 tiated part of the apex? Undoubtedly the answers to these and other similar 

 questions are to be found in cellular physiology. The location of the various 

 zones with relation to each other and the readiness with which metabolic sup- 

 plies and products can be obtained and removed are factors of first importance 

 and deserve serious cooperative study by morphologists and physiologists. 

 Future studies on the shoot apex should not only include problems in cell line- 

 age, the cytohistological features of zones and strata, and coordinated patterns 

 of growth, but also an experimental approach which may reveal something 

 concerning the chemical web underlying growth systems. 



Zoned apices are by no means confined to the cycads. Foster (1938) has 

 shown that the shoot apex in Ginkgo biloba is composed of apical initial, cen- 

 tral mother cell, rib meristem and peripheral zones. These zones are compar- 

 able in many respects to those in cycads. Among the Coniferales superficial 

 initials at the summit of the apex divide periclinally to produce a relatively 

 small zone of irregularly arranged cells, which in turn give rise to peripheral 

 and central tissue zones (Foster 1939a, 1941a, Cross 1943). Certain angio- 

 sperms likewise display a condition approaching that in cycads, (Foster, 1939a). 

 Several investigators have described the corpus as consisting of peripheral and 

 central tissue zones arising from a subterminal apical zone composed of irreg- 

 ularly placed, enlarged and much vacuolate cells, (Boke, 1941, in Cactaceae and 

 Hsu, 1944, in bamboo). The nature of the initiation zone, and extensive devel- 

 opment of the central mother cells and peripheral zone in the cycads tend to 

 isolate them from other seed plants. Great caution must be exercised in the 

 construction of phylogenetic schemes illustrating possible evolutionary lines 

 for the shoot apex. The smaller apices of the advanced seed plants with their 

 greater degree of cellular stratification and simpler zonation, may well be ad- 

 mitted as evidence for considering the cycad apex as primitive. Indeed I am 

 strongly inclined to follow the lead of Foster (1940) who has provisionally 

 suggested that the large apex of Cycas revoluta with its "wide range of varia- 

 tion in sequence and plane of divisions, especially in the zone of initiation and 

 central tissue zone" would seem to represent a primitive type. A more positive 

 statement cannot be safely made until more extensive comparative studies 

 have been completed throughout vascular plants. 



Department of Botany, Rutgers University 

 New Brunswick, New Jersey 



