134 SCIENCE PROGRESS. 



by the majority of anatomists, notwithstanding its accept- 

 ance by the most brilliant of German contemporary in- 

 vestigators. 



In England, though these ideas have recently been 

 made familiar to the student by more than one of our lead- 

 ing botanists, their discussion has still the interest of com- 

 parative novelty. And although the general idea of the 

 stele as a morphological unit is simplicity itself, yet the 

 application of this idea is in some cases by no means easy, 

 so that not only does Strasburger's interpretation of certain 

 facts differ from Van Tieghem's, but the author of the 

 theory has himself been led to modify his original views in 

 an important manner. The possibility of such a difference 

 in the interpretation of facts which are undisputed seems to 

 spring, if we may say so without presumption, from a 

 certain want of definiteness in the apprehension of the 

 criteria legitimate to their interpretation. 



To investigate these criteria and to endeavour to as- 

 certain their relative validity is one of the primary objects 

 of the present paper. 



We shall begin with an account of the development of 

 the stelar doctrine. 



HISTORY OF THE STELAR DOCTRINE. FIRST PHASE— 

 THE IDEA OF THE CENTRAL CYLINDER. 



In 1 8 70- 1, Van Tieghem published, in the Annales des 

 Sciences Nattirelles, a memoir (1) which was to have been 

 the first of a series entitled " Recherches sur la syme.trie 

 de structure des plantes vasculaires". This instalment con- 

 sisted of a general introduction setting forth the plan of 

 the whole work, followed by 274 pages devoted to an ex- 

 tended anatomical account of the root, in vascular plants. 



The introduction is of the Greatest interest. The 

 author tells us how he wished to obtain anatomical defini- 

 tions of root, stem, and leaf, in order to give a basis to the 

 study of comparative anatomy. These definitions are to 

 be framed in accordance with the different kinds of sym- 

 metry exhibited in the arrangement of the vascular strands 

 in the three organs, to each of which a separate memoir 



