THE ANATOMY OF THE SHOOT 8i 



the stem and leaf, and emphasizing the earlier statement of Sachs 

 (31), that 



"the morphological conceptions of stem and leaf are correlative; 

 one cannot be conceived without the other ... in other words, the 

 expressions stem and leaf denote only certain relationships of the 

 parts of a whole — the shoot." 



In connection with the actual extension of the leaf primordium 

 from the surface of the stem apex, Schiiepp (36) has pointed out 

 that the frequency of cell division is approximately the same in all 

 portions of the shoot apex. In other words, the number of cell 

 divisions of the cells of the tunica would be approximately the same 

 as those in the corpus. Since the divisions in the corpus occur in 

 all planes, while those of the tunica are anticlinal, except during 

 the formation of a leaf or bud primordium, the development of both 

 regions produces folds in the superficial tissues or tunica as a 

 necessary adjustment to the differences in the manner of cell divi- 

 sion of the two regions. 



In the development of the leaf at the foliar buttress or node, 

 the first stages in ontogeny result in an elongation of the primor- 

 dium so that it frequently becomes a tapered cone that is somewhat 

 flattened on its adaxial surface. As Avery (5) and others have 

 pointed out, this growth in length is apical and results from con- 

 tinued periclinal and anticlinal divisions of a subepidermal cell 

 or cells. The apical growth is not limited to subepidermal cells 

 in all cases, and subapical growth has also been reported; so that, 

 again, it is clear that there is no strict uniformity in the ontogeny 

 of the leaf of angiosperms. In most cases, apical growth is 

 retarded or ceases altogether early in development, and the ensuing 

 extension of the primordium is due to intercalary activity in addi- 

 tion to some unlocalized cell division which may result in an 

 increase in the radial thickness of the primordium. It has been 

 found that the broadening of the base of the leaf and the early 

 development of the petiole in many dicotyledonous leaves is the 

 result of the activity of meristematic tissues lying on the adaxial 

 surface of the leaf base. The development of the lamina, which 

 follows the formation of the petiolar midrib region, is initiated 

 by the activity of marginal ridges of meristem that develop on the 

 terminal and adaxial portion of the primordium. 



Variations occur with respect to the layers involved in the 

 production of the marginal meristems that form the young blade. 



