34 MORPHOLOGY OF THE ANGIOSPERMS 



the information now available, number of tunica layers seems to have 

 no phylogenetic significance. A comparison of doubtful importance has 

 been made with the reduction in number of layers in the nucellus, 

 where the larger number of cell layers seems to represent primitive 

 structure (Chap. 7). Morphological significance has been seen in 

 manner and place of origin of leaf initials, but a leaf primordium may 

 arise wholly from cells of the tunica or from initials in both tunica and 

 corpus. 



Angiosperm shoot apices seem to have two types of organization: 

 one, in which the cells of the tunica divide anticlinally except in the 

 center of the apex, where some periclinal divisions occur; the other, in 

 which no periclinal divisions occur. Evolutionary progress in specializa- 

 tion in the zonation of the apex has apparently been from a tunica 

 with layers in which periclinal divisions are general throughout, to 

 those with periclinal divisions restricted to the apex, and to those with 

 anticlinal divisions only. Recognition of timica and corpus as descriptive 

 units has been of much value in studies of growth activities, especially 

 in cytohistology, but use of the terms must be considered primarily 

 topographical. 



The presence of a fairly well-defined zonation — tunica and corpus — 

 despite inconstancy in number of layers in the timica and perhaps 

 absence of such zonation (as reported for some rosaceous taxa), seems 

 to characterize the angiosperms. The zonation of apical meristems 

 should not be interpreted too rigidly, as it was in the earlier years of its 

 study; the apices must be recognized as dynamic structures, responding 

 within limits to various growth conditions. 



The shoot apex from which a flower will develop (the reproductive 

 or floral apex) has been claimed to differ fundamentally from the 

 vegetative apex, to be a completely different structure and, like the 

 sporophylls borne upon it, a structure sui generis. But this concept has 

 been shown by several critical studies to be without foundation. In the 

 transformation of a vegetative into a floral apex, the vegetative axis 

 undergoes, as it elongates, a gradual change in zonal pattern. The 

 change is in proportion of zones and is, in large part, the result of the 

 bringing close together of the nodes and the many appendages of the 

 flower. 



The interpretation of the floral apex as morphologically distinct from 

 the vegetative apex was based largely on the following claims. First, 

 the vegetative apex has a tunica and a corpus; the flower axis has a 

 "parenchymatous core" — not highly meristematic — and a thick, "heavy," 

 meristematic "mantle." But differences in gross structure between the 

 two types of apices are in degree only and are associated with basic 

 structure. In the floral apex, the nodes are crowded together, and the 



