THE STELA R THEORY. 137 



Van Tieghem had recognised the condition which he 

 afterwards described as " astely ". 



The "membrane rhizogene," now considered, under the 

 name of pericycle, as forming merely the external layer of 

 the conjunctive tissue of the cylinder, was at that time 

 treated as a region external to, and distinct from, the rest 

 of the parenchyma, to which the name " conjonctif " was 

 given. But the clear recognition of the existence of an 

 individualised stem cylinder, forming a direct continuation, 

 tissue for tissue of that of the root, was the first and funda- 

 mental step in the evolution of the stelar idea. 



Little progress was made during the next ten years in 

 the development of this conception. 



Falkenberg (3), in 1876, showed that the " Aussen- 

 scheide " in monocotyledonous rhizomes corresponds with 

 the " pericambium " in roots, both in position and role ; 

 and Mangin (4) in 1882 entirely confirmed his results and 

 showed that not only adventitious roots but also the 

 " reseau radicifere " arises from this layer, which he calls 

 the " couche dictyogene ". 



In 1882 Van Tieghem published a short paper (5) in 

 which, a propos of the Cucurbitacese, he gives conclusive 

 reasons, based upon grounds of comparative anatomy, for 

 regarding the fibres in the stem, hitherto called primary 

 "bast fibres," as really belonging to the "membrane 

 rhizogene ". With these extended limits, this layer forms a 

 complete investment of the stem cylinder, just as the peri- 

 cambium does of the root cylinder. Since the one layer is 

 the direct continuation of the other, and the two correspond 

 very largely in function as well as in position, it is clearly 

 desirable that they should have a common name. For this 

 purpose Van Tieghem introduced the word pericycle, which 

 was to supersede the various terms "pericambium," 

 " Aussenscheide," "membrane rhizogene," "couche dictyo- 

 gene," etc., applied by various writers to the same layer in 

 various plants and parts of plants, according to its various 

 histological characters and functions. The importance of 

 this introduction of the conception of the pericycle was of 

 course very great, since it fixes more accurately the external 



