— 138 — 



As support for this view he refers to the stem-structure of the 

 previously mentioned a. o. fossil species and certain recent fami- 

 lies. Thus the structure of the stem in woody and herbaceous spe- 

 cies of Rosaceæ is stated in support of this view, e. g. the stem 

 structure of Potentilla palustris, which in its primitive stages 

 presents a typical continuous annular cylinder. 



The reason for such a change is due to the leaf-traces, in par- 

 ticular to the parenchyma connected with these. The first stage 

 in this process of evolution is represented by the formation of com- 

 pound rays i. e. aggregate rays connected with parenchyma. At 

 an earlier stage great areas of the central cylinder are transformed 

 into parenchyma with the exception of the protoxylem. In this 

 way a vascular structure consisting of alternate segments of xylem- 

 tissue and parenchyma is formed. In the parenchymatical parts, 

 xylem representing the leaf-traces is found at the inner side. 

 Eames seeks a support for this view in the fact, that fossil miocène 

 species of Quercus do not possess compound, but only uniseriate 

 and aggregate rays. The broad medullary rays are here a later 

 acquirement. The occurrence of uniseriate rays in seedlings of 

 these species also supports this view. 



In a paper (1911): 'The evolution of the annual ring and me- 

 dullary rays of Quercus", Percy Groom has opposed this ex- 

 planation of the Quercus-rays. He means that it here is impossible 

 to decide whether uniseriate or compound rays were the older. 



Eames does not take into consideration that all fibrovascular 

 bundles in the herbaceous stem are leaf-traces, either median or 

 lateral, which often pass through many internodes, before the con- 

 nection with the bundles of the petiole is established. In his 

 opinion the larger of the above named segments cannot be compa- 

 red with the fibrovascular bundles of the herbs. 



In a paper of the same year: "The relation of the leaf-trace 

 to the formation of compound rays in the lower Dicotyledons", 

 I. W. Bailey also touches on this subject. Distinctly and without 

 possibility of misconstruction he writes (p. 237): "In Angiosperms 

 the primitive condition of the stele is a tubular cylinder .... In 

 the development of the latest dicotyledonous plants, the proto- 

 xylem elements of the solid tubular cylinder have become gra- 

 dually more or less localized, first into a primary ring with locali- 

 zed thickened areas, separated by areas from which the proto- 

 xylem has nearly disappeared, and finally into a dissected cylin- 



