326 SCIENCE PROGRESS 



This older view, however, was replaced, for a time at any 

 rate, and in many quarters, by the view first put forward by Van 

 Tieghem (63, 69). He regarded the whole of the axial vascular 

 system as a single structure, of which the bundles were parts. 



Thus in the young sunflower stem, described above, the 

 zone of vascular bundles, together with the pith and the inter- 

 fascicular parenchyma or soft-walled tissue between the bundles, 

 is morphologically distinct from the other tissues and forms a 

 morphological unit, the stele, which is separated from the 

 cortex by the endodermis, the innermost layer, according to 

 Van Tieghem, of the cortex. Hence the central parenchyma- 

 tous mass, enclosed by the vascular bundles, is part of the 

 stele, and is thus morphologically distinct from the cortex. 



The simplest possible case, according to this first theory of 

 the stele, may be found, for example, in roots and in the stems 

 of young ferns, where the endodermis immediately surrounds 

 a solid vascular cylinder, or stele, which has for its external 

 limit the pericycle. To this type of structure Van Tieghem 

 gave the term monostele, and he derived all other vascular 

 arrangements from it. 



Thus a medullated monostele is formed by the aggregation 

 of parenchyma in the centre of the original structure ; the 

 medulla, or pith, may extend outwards so as to break up the 

 surrounding ring of vascular tissues into separate parts which 

 are termed the bundles. This condition obtains in some ferns, 

 e.g. Osmunda, and the majority of Dicotyledons. 



It is to be observed that the term " bundle " is used by Van 

 Tieghem in a more restricted sense than that employed by 

 De Bary. Thus the root-structure referred to above was 

 spoken of by De Bary as being a radial bundle, while Van 

 Tieghem treats it as representing several bundles. The term 

 "meristele," much used at the present time, is synonymous with 

 the term "bundle" employed by Van Tieghem. 



In other cases the endodermis and pericycle may extend 

 inwards towards the pith, so as to enclose completely each meri- 

 stele. There is now direct continuity between the medulla and 

 the cortex through the medullary rays. The original stele is thus 

 broken up, as it were, and ceases to exist as such. This condition 

 is consequently termed astelic, and may be found in certain 

 species of Equisetum, e.g. E. limosum, and in some aquatic plants, 

 e.g. Nymphcea (fig. 1). 



