— 137 — 



tions the primary protostelic and siphonostelic structures of the 

 stem. The siphonostele often presents in the most primitive stage 

 an inner phloem; this tissue has disappeared in the higher or- 

 ganized forms. In such forms we find an inner ring of protoxylem 

 surrounded by a secondary tissue of xylem formed by cambial 

 action. 



Thus an evolutionary stage is represented by Lepidodendroid 

 plants. At a more advanced stage (Sigillaria) the primary xylem 

 breaks up into small parts which lie in the inwards directed angles 

 of the secondary xylem. At a still more advanced stage — in the 

 group of Equisetales (Calamités) — the protohadrom is very 

 slightly developed and the angles of the secondary xylem more 

 acute. Further is mentioned, how the medullary rays gain in im- 

 portance in respect to the evolution and effect the formation of 

 fibrovascular bundles in the higher plants. 



In these considerations it seems very strange that many old 

 forms, presenting entirely modern structures in respect to the 

 separation of the bundles, for instance other Calamités sp. and 

 other Equisetales, and among the Pteridospermeae Lyginodendron 

 Oldhamium, are not treated. 



As emphasized by Jeffrey the primary xylem in the first named 

 types, at any rate in the two first named, is centripetal, as is that 

 of the root, while the protoxylem in the higher plants, as is well 

 known, is centrifugal. As I later draw attention to this problem 

 I shall here be content to point out that it is not especially neces- 

 sary to start from these types — perhaps adapted to special 

 limited external conditions — when we e. g. find types in the group 

 of Filices, from which the structure of the stem in the Angiosperms 

 is much more easily derived. 



Though Jeffrey, as far as I know, was the first to draw attention 

 to the problems he was not the first author to deal with the subject 

 in botanical literature. The first author who dealt with this 

 problem was A. J. Eames in his paper of 1911: "On the origin of 

 the herbaceous type in the Angiosperms". 



Firstly he mentions the earlier explanations of the woody 

 cylinder as a fusion of separate bundles. On page 223 he writes: 

 "Instead of this, however, a reverse process seems to have occur- 

 red. A primitively solid cylinder has been reduced and dissected 

 to form the type characteristic of dicotyledonous herbs — a ring 

 of small separate bundles". 



