RECENT WORK, ETC. 469 



allied species differ as regards their polystely or monostely, 

 but even in one and the same shoot we may find a transi- 

 tion from the one condition to the other, and back again 

 (e.g., Gunnera magellanica, Nephrolepis). 



It is unnecessary to follow the modifications of polystelic 

 structure in detail, as the views of Van Tieghem have lately 

 been embodied in an English text-book (see Vines, 1 1). 



Another departure from typical structure is described 

 by Van Tieghem under the name of astely (5, p. 764), 

 or more precisely, as schizostely (8, p. 285). This is 

 characterised by the fact that the stele completely breaks 

 up into the individual bundles, each surrounded by its own 

 peridesm 1 and endodermis, the cylinder thus ceasing to 

 exist as a whole. Typical examples are found in Equisetum 

 limosum, and other species, in Nympham, aquatic species 

 of Ranunculus, etc. The French anatomist now regards 

 all species of Equisetum as essentially astelic (except of 

 course in the embryonic stem). The apparently monostelic 

 structure in E. arvense, etc., is explained as arising from 

 a re-fusion of the distinct bundles, " dialydesmy " thus 

 becoming converted into " gamodesmy " (13). 



Astely, as Strasburger has pointed out (3, p. 312), is a 

 weak point in Van Tieghem's system. There is really no 

 essential difference between the monostelic and the astelic 

 Ranunculi, for example, except the presence in the latter 

 of a distinct endodermis around each bundle. It is un- 

 reasonable to regard the central ground-tissue of R. aquati- 

 lis as homologous with the cortex, while in R. repens it is 

 a true pith. Strasburger's view that there is a single stele 

 in all these cases, but that its limits have become indefinite 

 in the aquatic species, seems to do less violence to the facts 

 than the interpretation of Van Tieghem. 



The importance attached to the endodermis as a 

 limiting layer is in fact altogether exaggerated in Van 

 Tieghem's system. The appearance of a differentiated 



1 Peridesm is the conjunctive tissue at the periphery of any portion of 

 a subdivided stele, and is now distinguished from the pericycle which sur- 

 rounds an entire stele (see Van Tieghem, 12). 



