— 141 — 



of phloem and xylem appears very early as a distinct unit, just as 

 we find in the Angiosperms. A concentric vascular bundle with 

 central protohadrom and exarch protoleptom seems as though it 

 might quite naturally be considered as the prototype of the modern 

 vascular bundles. It is scarcely necessary to assume that these 

 are formed by the breaking off of a solid cylinder on a later stage 

 of the evolution. It seems much more natural to assume that these 

 bundles conserved in the solid cylinder, appear isolated in the her- 

 baceous plants as parts of the original system of bundles, parti- 

 cularly as isolated bundles are really very old organs (they occur 

 in the fossil Equisetales and the Pteridospermeæ). It is also pos- 

 sible, that the root-structure in the wood of Lepidodendroid forms 

 might be considered as a fusion of concentric bundles. On the whole 

 it seems surprising to begin with these perhaps specialized forms 

 when modern structures are found in groups as old as the Lyco- 

 podiales. 



Then I shall deal with the development of the vascular bund- 

 les in dicotyledonous plants. In these one can distinguish between 

 vascular bundles, which are directly leaf-trace bundles, and such 

 which are connected with these without passing into the leaves. 

 As shown by the classic investigations of Lestiboudois, Hanstein 

 and Nägeli, the system of bundles in the stem not only in herbaceous 

 but also in woody plants is characteristic of the species and appears 

 as the fundamental structure, around which the rest of the con- 

 ducting and sclerenchymatic tissues are arranged. 



The first author who investigated the development of the first 

 annual ring in woody plants was Sanio (Botan. Zeitung, 1863). 

 Later on Sachs in his "Lehrbuch der Botanik" and de Bary in 

 his "Vergleichende Anatomie" give, on the basis of Sanio's and their 

 own investigations, many exhaustive details concerning this ques- 

 tion. 



The primary stage is an annular formation of cells, in which 

 initial cell-groups of the leaf-traces and other vascular bundles can 

 early be discerned; slightly later, the cambium is formed, the fasci- 

 cular as well as the interfascicular. The first stage is thus an annu- 

 lar formation of cells, distinctly separated from the pith and the 

 cortex which by divisions within distinct areas develops into vas- 

 cular bundles (de Bary indicates that in some cases the interfas- 

 cicular tissues are of considerable extension). I have investi- 

 gated sections through young buds of different plants, among 



