JOHNSON: SHOOT APEX 



55 



The facts presented. in this brief survey of the cycad shoot apex, together 

 with the more complete accounts given by Foster and Johnson, suggest the fol- 

 lowing considerations which are of general morphological interest and possibly 

 may have phylogenetic significance. 



None of the seven genera examined show an apical cell in plants old enough 

 to have a well-developed terminal meristem. One-year old seedlings of Micro- 

 cycas calocoma and two-year seedlings of Cycas revoluta have already estab- 

 lished the growth pattern which is more fully developed in adults (Foster, 

 1939b, 1943). In Zamia the zonal structure was discovered in an embryo prior 



Fig. 1. Zamia silvicola. Shoot apex showing zonation : 1, initiation zone, B, detail of 

 same ; 2, central mother cell zone, cells containing cytoplasm in B and A are from edge of 

 this zone ; 3, peripheral tissue zone, outer portion is derived from zone 1 and the super- 

 ficial layer as seen in C, inner part is from zones 1 and 2 ; 4, rib meristem, which originates 

 from zone 2 as seen in A. A, B, and C taken from A', B', C respectively, X 400. 



to emergence from the seed coat. A large cell, in longisection resembling an 

 apical cell, is often seen at the tip of the shoot apex in embryos and seedlings 

 No recent "observer, however, has been able to demonstrate that segments are 

 produced from such a cell in the definite sequence required by the apical cell 

 theory. The superficial cells at the exact geometric center of the apex do not 

 differ from their neighbors in either structure or planes of division. Further 

 studies, however, are necessary in the embryology and early growth of the 

 seedling before the possibility of an apical cell can be ruled out. 



