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A TEXTBOOK OF THEORETICAL BOTANY 



we pointed out that the procambium usually makes its first appearance in 

 the ring of residual meristem, at separate points in the ring, each point lying 

 at the base of a leaf primordium. These vertical strands of the earliest pro- 

 cambium extend themselves continuously both downwards in the tissue of 

 the internode below and also in an upward and outward direction into the 

 leaf primordium itself. This latter extension is due to the appearance in the 

 primordium, at a very early stage, of a median band of meristem, connecting 

 the end of the procambium advancing from the stem with the apical meristem 

 of the primordium, which is still active in this early phase of development 

 (Fig. 967). The rapid elongation of these meristem cells to form procambium 



Fig. 967. — Types of relationship between meristem and the procambial supply to the leaf 

 rudiments. A, Microphyllous with protostele. B, Megaphyllous with protostele. 

 C, Polycycly through invasion of stele by leaf gap parenchyma. D, Dictyostele with 

 medulla formed by fusion of leaf gap parenchyma. E, Dictyostele with pith formed 

 endogenously. F, Osmunda type. G, Dicotyledonous type. H, Monocotyledonous 

 type. {After Kaplan.) 



in the leaf is probably the reason for the relatively rapid growth in length which 

 occurs in the young rudiment. This growth in length is accompanied by 

 radial growth in thickness, so that the primordium assumes the form of a 

 slender cone, somewhat flattened on the axial side. This constitutes the 

 petiolar-midrib region of the leaf, which is dift'erentiated before the lamina 

 makes its appearance. The petiole does not, therefore, arise as a structure 

 intercalated between the lamina and the leaf base, as older accounts maintained, 

 but is one of the primary structures of the leaf. 



The lamina begins to difl"erentiate from the upper or adaxial portion of this 

 petiolar-midrib structure, in the form of two thin marginal ridges of meristem 

 which appear before the rudiment is as much as i mm. long. Very little 

 is known about the early development of compound leaves, but it has been 



