THE STELA R THEORY. 221 



become recognisable in comparatively bulky apices (owing 

 to the early ceasing of longitudinal divisions, and the stretch- 

 ing of its cells), before the outer limit of the young cylinder 

 is defined. On the other hand, in the slender stems of 

 many water plants, Hanstein's scheme applies with dia- 

 grammatic precision, the outer limit of the cylinder being 

 clearly marked at the apex, before there is any sign of a 

 differentiation between pith and bundle ring. But these 

 differences of precocity in the development of the various 

 regions of the cylinder, depending, as they do, upon the 

 subsequent duration and size relations of the regions are 

 clearly of little importance to morphology. The important 

 fact which remains is the clear separation, slightly sooner, 

 or slightly later, of the young cylinder from the cortex, in 

 at any rate the vast majority of cases. 



The separation thus made in development is, as a rule, 

 more or less clearly maintained in the adult stem, though 

 sometimes it is lost altogether. There is the possibility 

 of a complete loss of a visible boundary between cylinder 

 and cortex by the occurrence of irregular cell divisions in 

 the young pericycle and inner cortex, together with a 

 "shifting" (Verschiebung) of the original walls separating 

 the two ; unfortunately we do not know if this takes place 

 in some cases or not. But apart from such an occurrence 

 the distinction between cylinder and cortex, once made, is 

 always made, and the layer of cells which once abutted on 

 the young cylinder is still the phloeoterma, not merely 

 "theoretically," but in substance and in fact, however im- 

 possible it may become to distinguish it from the surround- 

 ing tissue. 



It is these facts which form the real developmental basis 

 of the stelar theory. 



The phenomena (supposing them to be established) of 

 real importance in the opposite sense, would be the occur- 

 rence of stems in which the external limit of the cylinder is 

 never clear, of stems, in a word, which never possess a 



tion of the term. The further pursuit of the theoretical implications of this 

 statement would lead us into very deep waters, but it is clear that an ex- 

 tended comparative investigation of the origin of the pericycle is needed. 



