STELAR THEORIES 339 



entiation of the xylem-mass. At any rate, if one regards the 

 pith or other central tissue as having arisen in the first place 

 by the transformation of potential tracheides into other tissue- 

 elements, these latter should be treated morphologically as part 

 of the stele." For these reasons, the pith is regarded as being 

 stelar, and morphologically distinct from the cortex. 



Gwynne-Vaughan (29, p. 737) does not consider that the 

 inconstancy of the apical cells and meristems has any bearing 

 on the main question. He remarks that " the consideration of 

 the morphology of the stele can only begin when that region is 

 definitely and satisfactorily delimited. It is not directly con- 

 cerned with any question as to which particular segment it is 

 in which the tangential wall appears that first of all delimits it. 

 All that is required is that a definite delimitation should actually 

 be possible at one point or another during the course of its 

 development in a majority of cases sufficiently great to render 

 the statement general." 



He is of the opinion that it is better to consider the central 

 parenchyma, or pith, as stelar, and to regard the internal 

 endodermis as not being strictly homologous with the external 

 endodermis. But inasmuch as the cortical origin of the central 

 parenchyma is theoretically possible, it should not at once be 

 rejected as inherently improbable. 



Tansley and Lulham (61, p. 514) regard Jeffrey's view of the 

 intrusion of the cortex into the stele as having " a merely 

 metaphorical value." 



It is pointed out that in order to show that the pith originated 

 by an invasion of the cortex, it must be proved that the cortical 

 elements actually pushed their way into the stele during the 

 development, or that such had been the case during the course 

 of descent. They consider that the pith " is morphologically 

 an entirely new tissue, formed in the centre of the stele, in 

 place of vascular tissue, which preceded it in ontogeny and 

 phylogeny, but is always, in the Ferns, separated from the latter 

 by endodermis." 



As regards the difficulty connected with the occurrence of 

 the internal commissural strands of vascular tissue in the 

 Marattiacese and Matouia, Tansley and Lulham remark that, 

 if their view of the pith be correct, then these accessory struc- 

 tures of such polycyclic ferns must also be regarded as new 

 developments, which in part replace the pith in much the same 



