210 The Anatomy of Plants bookii 



owing to variations in the development of the endodermis 

 and the pericycle. 



Exceptions to this suggested typical structure were the 

 subject of further investigations carried out by Van 

 Tieghem and Douliot, which were published in 1886. In 

 the first case, in certain medullated axes they said the endo- 

 dermis and pericycle sometimes bend in between the bundles, 

 separating them completely from one another and becoming 

 broken into segments, which fuse in front of the bundles, 

 completely isolating them. The monostele is in this way 

 broken up into as many pieces as there are bundles, 

 and the parenchyma of the pith appears to be indistin- 

 guishable from that of the cortex. The appearance pre- 

 sented is that of a number of steles immersed in and 

 supported by a mass of conjunctive tissue. The outlines 

 of a central cylinder, though an interrupted one, can still 

 be distinguished. Van Tieghem spoke of this condition as 

 Astely. In another type of structure, seen in certain 

 species of Equisetum, these separate steles subsequently 

 fuse laterally, the endodermis and pericycle disappearing 

 between them but forming a continuous ring both within 

 and without the circle which the steles now form. This 

 condition he named Gamostely. 



In stems of the genus Auricula another type is found. 

 The cylinder or stele of the hypocotyl is seen to bifurcate, 

 sometimes more than once, giving rise to two or more 

 strands each like the original one. This condition, known 

 as Polystely or Dialystely, is represented also in the Ferns, 

 which were investigated by Leclerc du Sablon in 1890 ; he 

 showed the structure commonly associated with their stems 

 to be due to the splitting of an original monostele in the 

 hypocotyl, whereby several distinct cylinders are formed. 



Van Tieghem included the dialystelic and the gamostelic 

 conditions together under the term Polystely. He recog- 

 nized, therefore, three main types, the monostelic, astelic, 



