THE STELA R THEORY. 149 



These bundles are accompanied by a certain amount of 

 closely associated parenchyma belonging to the external 

 conjunctive of Flot, a tissue which in the leaf Van Tieghem 

 now calls peridesm (16). The bundles are sometimes 

 arranged in a ring, and the whole may be, though com- 

 paratively rarely, surrounded by an endodermis. The 

 petiole is then, according to Van Tieghem (10, p. 842), 

 monostelic. In the commoner case where each bundle has 

 an endodermis of its own the petiole is astelic. 



Strasburger prefers the term schizostelic (15), since the 

 stelar tissue of the petiole represents a separated portion or 

 portions of that of the stem. To such a portion he gives the 

 name schizostele or schistostele \ at the same time denying the 

 existence of monostelic petioles in Phanerogams on the 

 ground that the apparent pith of the petiole is continuous 

 with the cortex, and not with the pith, of the stem. This 

 last contention brings forward a difficult position. Is it de- 

 sirable to introduce the question of continuity at all ? If we 

 have in the petiole a structure apparently identical with that 

 which we have agreed to call monostelic in the stem, should 

 we be satisfied to call it monostelic here also, without con- 

 sidering the connections of its parts with those of the stem ? 

 The strength of Strasburger's position lies in the fact that 

 the continuity, region for region, of the cylinder of root 

 and stem is really the basis of the stelar idea. The origin 

 of the difficulty is to be found in the tendency of a petiole, 

 where it is subject to the same conditions as a stem, to 

 assume the characters of a stem, and among them the 

 arrangement of its vascular tissue according to a radially 

 symmetrical type. We might, perhaps, fitly call such a 

 structure a pseudostele. 



The mesophyll of the leaf (corresponding with the cortex 

 of the stem) which surrounds the smaller vascular bundles, 

 often has its innermost layer or phloeoterma, which abuts 



1 Van Tieghem has since (17, p. 285) used the word meristele for 

 Strasburger's " schizostele," and applied the latter term to the portion of 

 stelar tissue enclosed by each special endodermis in an "astelic" stem. 

 This seems an unwarrantable diversion of the meaning of Strasburger's 

 term. 



