APICAL MERISTEMS 



23 



result from ditt'ereiiees in the direction of growth and plane of 

 cell division in these zones. In the tunica, surface growth accom- 

 panied by repeated antivlimd divisions predominates, resulting 

 in the maintenance at the summit of the apex of a more or less 

 regular and constant series of shell-lilve layers. On the sides or 

 flanks of the apex, however, the distinctness of the inner tunica 

 layers is somewhat lost, chiefly because of the periclinal and 

 oblique divisions which appear in them during the initiation of 

 foliar structures and lateral buds. In contrast to the tunica, 

 growth in volume is characteristic of the corpus, and the sequence 

 in the successive planes of cell division is variable and usually 

 very irregular. Tunica and corpus thus represent two interdepend- 

 ent zones in the shoot apex, and their extent and behavior may 

 be expected to fluctuate, depending upon the systematic position 

 of the plant in question as well as upon the phase of development 

 of the plant itself. Modern studies have shown that the number 

 of tunic layers varies from one in grasses and Scrophularia 

 nodosa to as many as five or six in Hippuris. Unfortunately, no 

 detailed survey along broad systematic lines has yet been at- 

 tempted, so that the phyletic significance, if any, of differences 

 in the number of tunic layers is quite obscure at present. The 

 classical "Histogen Theory" of Ilanstein (1868) attempted to 

 assign specific destinies or "prospective values" to the various 

 layers and to the central core of the shoot apex. In contrast, the 

 concept of tunica and corpus, which originated with Schmidt 

 (1924), is non-committal wdth respect to the nature of the tissues 

 produced by these two zones. Recent studies justify Schmidt's 

 cautious viewpoint. In certain angiosperms [e.g.. Viburnum 

 rufidulum. Cross (1937)], the corpus is exclusively concerned 

 with the production of the pith, while in other plants Carya 

 Bnckletji var. arkansana, Foster (1935) ; Morus alba, Cross 

 (1936), the provascular tissue and inner region of the cortex, 

 as well as the pith, originate from the corpus zone. In Hippuris 

 and Myriophyllum, the corpus gives rise to the central pith-less 

 stele of the axis (Louis, 1935, pp. 128-130, PI. IX, Figs. 77-78), 

 simulating in this respect the histogenesis characteristic of many 

 roots. The ' * prospective significance ' ' of the various layers of the 

 tunica also varies, particularly with respect to their role in the 



