Anatomy of Solenostelic Ferns. 737 
cally distinct from the cortex ; or as cortical, in which case 
the stele, or the separate portions of it, must possess a definite 
internal as well as an external limit. Supposing the central 
parenchyma in question to be really stelar, then it must have 
come into existence by the substitution in the stele of paren- 
chymatous elements for those that were previously vascular. 
To take the case of the Cyatheaceae and Polypodiaceae in 
particular ; if the course of development actually did take 
place in the manner suggested above (p. 717), then each cell 
of the central parenchyma must have belonged in previous 
generations successively to the xylem, phloem, pericycle and 
endodermis before attaining its present condition. On the 
other hand, if the central parenchyma be regarded as cortical, 
then the stelar elements at certain points in the stele through 
successive previous generations must have undergone fewer 
and fewer divisions as they developed, while the divisions in 
the cortex opposite these points must have correspondingly 
increased. Therefore the cortical tissue at these points would 
eventually encroach upon the vascular, and would in time 
come to occupy the greater part of the centre of the stele. 
It has been very generally assumed that if the distinction 
between stele and cortex is really a morphological one, the 
two regions must of necessity be marked off from one another 
by the earliest cell-divisions at the apex, or by the so-called 
histogenetic layers. But recent researches upon apical meri- 
stems 1 have shown that the earlier tangential divisions in the 
segments of an apical cell, and even the histogenetic layers 
of Hanstein, are not only very inconstant and unreliable, but 
also that they bear no invariably fixed relations to any of the 
subsequent tissue-differentiations in the mature regions of the 
plant, and that in consequence they have little or no general 
value as morphological criteria. It does not appear to me 
that the regional significance of the stele is in any way bound 
up with the maintenance of these distinctions. The con- 
sideration of the morphology of the stele can only begin when 
1 Schoute, Die Stelartheorie, Groningen, 1902 : also Tansley, Proceedings of 
the Linnean Society, Nov. 20, 1902. 
