WETMORE: LEAF-STEM RELATIONSHIPS 17 



to this field— Priestley (1928, 1929, 1935, 1936) and his associates (Griffiths 

 and MaHns, 1930; Majumdar, 1942; Scott and Priestley, 1925), Esau (1938, 

 1939, 1940. 1942, 1943), Cross (1937, 1939. 1940, 1941, 1942) to name a few' 

 These studies bear directly on any interpretation of the leaf-stem relationship. 



Louis reported in detail on the development of the stem apices in nine 

 Angiosperms and one Gymnosperm — Taxits baccata. In his study of Syrincja 

 vulgaris, with opposite leaves, he called attention to the general flat appearance; 

 of the apical meristeni upon which the paired leaf primordia are elevated 

 (Fig. 1). In their early appearance these peg-like protuberances are composed 

 of cells like those of the meristeni itself. At this level a transverse section of 

 the tip indicates an oval or near rectangular shape, with each end of the 

 rectangle called a leaf buttress, — Majumdar's leaf foundation (1942) — on 

 which originates an erect leaf primordium. Such a transverse section Louis 

 (following Schmidt, 1924) considered as made through a region of maximal 

 area ( Fig. 2 ) . If another transverse section be cut immediately below this pair 

 of leaf primordia, a circular outline is obtained ; Louis considers such a circular 

 section as cut through a reg'.on of minimal area (Fig. 3). A section through 

 the bases of the next lower pair of leaves provides another region of maximal 

 area, with its major axis at right angles to the first. Thus the stem tip is 

 composed of alternating zones of maximal and minimal area, each buttress 

 of the former always bearing a leaf primordium. Obviously then a leaf buttress 

 is topographically a part of the stem with no clear boundary between it and its 

 elevated leaf primordium. 



Studies of successive leaves proceeding downward from the apex give one 

 a progressive picture of developmental changes in the leaf. As Louis points 

 out, such studies point to a general increase in vacuolation on the outer or 

 abaxial side of each young primordium and its buttress. Shortly thereafter in 

 each leaf an adaxial area appears as equally vacuolated. Thus in Syringa there 

 is left between the two vacuolated areas, continuous with the apical meristeni, a 

 liand of tissue as seen in transverse section (Figs. 2. 3, 4). 



Careful examination shows this band to be heterogeneous, and comprised 

 of a leaf bundle in procambial stage flanked by residual meristeni (Esau, 1943). 

 As one follows the sections downward it is seen that the first two pairs of leaf 

 buttresses completelv surround the stem, at which level the four abaxial ground 

 meristems, appearing extensively vacuolated, now envelop the stem as the in- 

 cipient cortex (Fig. 4). Successive sections also show that eacli adaxial, highly- 

 vacuolated ground meristeni has become continuous with the pith, thereby 

 forming the so-called leaf gap. 



Thus at a level below the second pair of leaves (Fig. 4) the stem of Syringa 

 consists externally of a protoderm surrounding a potential cortex. A pith is 

 clearly evident as an early vacuolated, central ground meristeni (Fig. 1, 4). 



