2i 6 The Anatomy of Plants BOOK.II 



walls of its cells seems to mark out the endodermis as 

 especially a physiological layer, the function of which 

 indeed can be readily interpreted in the light of such 

 structure. It seems, however, as if the interpretation of 

 similar structure, seen in layers or sheaths, whose morpho- 

 logical position is not definitely determined from other 

 relationships, should have been made on the lines of 

 adaptation to function rather than on a forced morpho- 

 logical conception. The presence of a sheath with the 

 characteristic marking in the cell walls, delimiting a certain 

 strand of tissue, may well be taken to show that a certain 

 need in connexion with supply or transport of water is its 

 explanation rather than that an interpretation of it must 

 be forthcoming which shall satisfy a demand to associate it 

 with a particular morphological conception, and to bring 

 it into line with an original differentiation into particular 

 systems of tissue. 



The pericycle, the layer clothing the stele on the exterior, 

 was shown by many authors to be more constant. This 

 again may be due in a measure to the nature of its func- 

 tions, which are more definitely associated with the work 

 and relationships of the stele as a whole. Those of the 

 pericycle of the root are mainly three ; the development 

 of corky or periderm layers for protective purposes, the 

 origination of lateral roots, and the development of 

 a portion of the ring of meristem necessary for the 

 thickening of the roots. Those of the stem pericycle are 

 more varied, but it is difficult to speak of them freely on 

 account of the ill-defined limits of the layer. 



This layer was first defined by Naegeli and Leitgeb in 

 1867, if we except certain references to some isolated cases 

 by Sanio in 1863. The recognition of merismatic functions 

 associated with it led to its being called pericambium, 

 a name which was applied to it until the researches of 

 Van Tieghem led to the recognition of the stele. It was 



