208 The Anatomy of Plants BOOK n 



gated the stele theory some years later. This claimed 

 to supply something which was entirely absent from De 

 Bary's position, viz. the recognition of a fundamenta 

 unity of plan in the structure of the axis as a whole. De 

 Bary presented the conception of the root-part of the axis 

 with a single large bundle in its centre, sinuous in outline, 

 with a number of projecting points of origin of differentia- 

 tion of its wood, while an equal number of strands of bast 

 occupied the spaces between them. There was thus a solid 

 core of wood with a number of strands of bast lying along 

 indentations or sinuosities of its periphery. This was called 

 a radial bundle. A totally different plan of construction 

 was recognized in the stem in the shape of a number of 

 conjoint bundles of wood and bast placed separately in 

 the stem in the form of a hollow cylinder. 



De Bary recognized that the central cylinder of the root 

 is encircled by a particular limiting layer of special con- 

 struction, which he called the endodermis, a name previously 

 applied by Oudemans to a particular case. He recognized 

 this layer also in such stems as possess an axial vascular 

 cylinder. 



Though these views might represent the structural appear- 

 ances exhibited by sections, they did not convey the idea 

 that the stem had anything in common with the root as 

 far as the plan of its construction was concerned. Herein 

 lay the weakness of De Bary's presentation of the subject. 



The first successful attempt to form a coherent scheme 

 of the anatomy of the axis of vascular plants was made 

 by Van Tieghem in 1870 and the following years. He 

 published first a sketch of the internal structure of the 

 root, and showed that its vascular tissue forms a central 

 cylinder, containing near its periphery strands of phloem 

 which are arranged round the zone alternately with strands 

 of xylem, the whole being united together by conjunctive 

 tissue. The xylem strands meet in the centre when they 



