WETMORE: LEAF-STEM RELATIONSHIPS 19 



Between these cortical and pith ground meristems exists a ring of small-celled 

 tissue which Louis has designated prodesuiogcn. An examination of this ring, 

 however, shows it to be heterogeneous in nature, not homogeneous as Louis 

 supposed. Confronting each leaf buttress is a small-celled arc of differentially 

 staining tissue — the procambium of the leaf trace — which is continuous with 

 that of the developing leaf. On either side of this leaf trace bundle, in the ring, 

 is a narrow zone of residual meristem, — the primary ray. Immediately above 

 each outward-bending leaf trace is the highly vacuolated local break in the 

 ring, referred to above as the Icaj gap (Fig. 1, right). Louis' investigation of 

 Syringa does not include the development of procambium. However subse- 

 quent study by the writer indicates the continuity of this procambium with 

 differentiating primary vascular tissue below, its development being con- 

 tinuously acropetal. 



Allowing for variations according with the phyllotactic pattern, size of 

 leaf and number of leaf traces per leaf in the large number of Angiosperms 

 now investigated,* it would seem fair to state that in this large group of 

 plants vascular and cortical patterns are generally correlated with the forma- 

 tion of leaves at the apex. The pith by contrast seems to belong to the axis. 

 It was on the strength of such studies in his own laboratories that Priestley 

 and his associates propounded the idea of "the unit of shoot growth" for 

 Angiosperms, a modified phyton and a unit closely resembling the "Spross- 

 glieder" of Celakovsky. Each such unit consists of a leaf and a subtending 

 longitudinal sector of the stem — not a whole segment as earlier phytonists had 

 considered it. There are many interesting points in this hypothesis as Priestley 

 has developed it. Time does not permit their consideration. It is true that in 

 the Angiosperms investigated by Priestley the facts could be so interpreted. 

 It is perhaps equally pertinent to question whether the generalization which 

 he makes will hold for all cases in the Angiosperms. In this connection I 

 should like to call your attention to Hippiiris vulgaris, which Louis has also 



* Studies by Foster (1938, 1939b, 1940, 1941a, 1941b) and his student Gifford (1943), 

 Crafts (1940), Cross (1939, 1940, 1941), etc., would indicate that this statement is perti- 

 nent also for Ginkgo, the Cycads, the Conifers and Ephedra. 



Explanation of figures 1-5 



Figs. 1-4. Syringa vulgaris. Fig. 1. Longitudinal section of stem tip (X90). 

 Fig. 2. Transverse section of stem tip showing region of maximal area through buttresses 

 of first pair of leaves (X130). Fig. 3. Transverse section slightly lower througli 

 buttresses of first pair of leaves near region of minimal area; leaf gap almost confluent 

 with pith (X130). Fig. 4. Transverse section below attachment of first two pairs 

 of leaves to show the ring composed of procambium and primary rays or interfascicular 

 residual meristem (X130). Fig. 5. Longitudinal section of stem tip of Hippuris 

 ■vulgaris (X130). (Figs. 1-5 after Louis.) 



