Vol. 44 T O R R E Y A January 1945 



On the Shoot Apex of the Cycads* 



Mariox A. John sox 



The structure of the shoot apex has been and is being widely studied be- 

 cause of its bearing on growth and ultimate origin of the organs and tissues 

 of the shoot. The pioneer investigators in this field traced cell lineage from 

 the base of the shoot apex to the summit as a converging cellular network. Un- 

 fortunately both botanists and zoologists thought that the tissues in widely 

 divergent groups could be derived from identically placed apical cells, histogens, 

 or germ layers, as the case might be. Accordingly several theories were pro- 

 posed to account for cellular configuration and growth in the shoot apex of 

 seed plants. The discovery and wide-spread occurrence of an apical cell in 

 bryophytes and pteridophytes, together with the histologic technique of the 

 times, led several investigators to portray apical cells in both gymnosperms and 

 angiosperms. Subsequent studies, particularly those of Hanstein (1868), re- 

 vealed the shoot apex as containing the three familiar histogens : dermatogen, 

 periblem and plerome, each of which was perpetuated by a series of superim- 

 posed initials. The dermatogen, a uniseriate layer, produced the epidermal 

 system, the periblem of one or more layers, the cortex, and the plerome, the 

 procambium and pith. More recently Schmidt (1924) has interpreted the shoot 

 apex in certain dicotyledons (the theory has been extended to include certain 

 grasses, gymnosperms and species of Lycopodium) as comprised of two zones. 

 The outer, tunica, consists of one or more cell layers in which division is peri- 

 clinal and which as a consequence is characterized by surface growth. The inner 

 zone, corpus, is characterized on the other hand by division in all planes and 

 by increase in volume. Still other investigators have considered the shoot apex 

 to be occupied by a primordial meristem characterized by a homogeneous, sim- 

 ple, undifferentiated structure. Further details concerning these theories will 

 be found in two critical surveys by Foster (1939a, 1941a). 



The shoot apex in the cycads is of particular interest because its structure 

 can not be interpreted by these proposals and, furthermore, comparative stud- 

 ies of the shoot apex must of necessity include the cycads because of their low 

 position among living seed plants. Representatives from seven of the nine 

 genera have been studied recently. Foster (1939b, 1940, 1941b, 1943) has 

 described and illustrated the structure in Cycas, Dioon and Microcycas while 

 Johnson (1939, 1944) has investigated Zam'xa, Encephalartos, Bowenia and 

 Macrozamia. 



Material of Zamia may be grown easily from seed or obtained from plants 

 growing in the field in southern Florida. Two-year old plants of Cycas revoluta 



^Presented in greater detail at the meeting of the Torrey Botanical Club on May 17, 

 1944 at The New York Botanical Garden. 



52 



