i8o HISTORY OF BOTANY 



his researches by giving a comparative account of the 

 anatomy of the root and showed that De Bary's " radial 

 bundle " was in reality a cylinder composed of several 

 bundles, the strands of phloem alternating with the 

 strands of xylem, which latter might meet in the centre 

 or leave a more or less lignified pith. These strands 

 were connected with each other by conjunctive tissue, 

 and the whole was enclosed by a pericycle abutting on 

 the outside on an endodermis which Van Tieghem regarded 

 as the innermost layer of the cortex. To this central 

 core, from the pericycle inwards, he gave the name of 

 " stele," a column. In succeeding papers Van Tieghem 

 applied the idea to the stem and claimed that there also 

 a " stele " could be recognised, but that the conjunctive 

 tissue was subdivided into medullary rays and pericycle, 

 and that these tissues were quite distinct in origin from 

 the cortex. At the same time he pointed out that the 

 outline of the " stele " in the stem was not so clearly 

 marked, owing to the disturbance caused by entry of 

 leaf traces and to other causes. 



Van Tieghem, who had associated with himself one of 

 his pupils, Douliot, next proceeded to work out all the 

 different conditions he could find among plants and 

 attempted to fit them into his scheme. When the axis 

 possessed a large medulla he found that the endodermis 

 and pericycle might push their way in between the 

 bundles and isolate them from each other, the " mono- 

 stelic " condition thus becoming " astelic," and the 

 conjunctive tissue of the medulla becoming confluent 

 with and indistinguishable from the cortex. Should 

 these isolated cords again fuse the condition known as 

 " gamodesmy " resulted. In Ferns and in a few Angio- 

 sperms, the " stele " in the hypocotyledonary axis 

 divided into two or more branches and for this condition 

 Van Tieghem coined the term " polystely " or " dialy- 

 stely." Finally, the fragments of the "stele" that 

 entered the leaf he described as " meristeles." In 1891 



