THE STELA R THEORY. 217 



the instructive fact is that "there's such divinity doth 

 hedge " a stele — indeed any morphological conception, as in 

 almost every fresh case to prevent for a time our realisation 

 of the truism that " Nature knows no sharp boundaries ". 

 In the stelar doctrine, we have, no doubt, a classification 

 that enables us to perceive a little more closely the direc- 

 tions along which the various types of vascular system in 

 the higher plants have been evolved, and that after all is 

 the most we can expect. 



DEVELOPMENTAL EVIDENCE BEARING ON THE STATUS 



OF THE STELE. 



We have now to consider the developmental basis 

 of the stelar theory. Let us take the Phanerogams first. 

 It is well, as Dr. Scott (20) has already pointed out 

 in this journal, to draw a distinction between de- 

 velopment from the embryo, and development of the 

 various axes from their permanently embryonic grow- 

 ing points. It is clear, on reflection, that the former 

 alone is comparable to ontogenetic development in animals, 

 though it would be a mistake to suppose that the latter is 

 not of importance to morphology. In the comparatively 

 few types of monostelic plants with the anatomy of whose 

 embryos we have a sufficient acquaintance, it appears 

 that both in the plumule and radicle there is really a 

 clear separation at the apex between central cylinder 

 and cortex (plerome and periblem). But it is certainly 

 open to doubt whether this distinction, as Hanstein 

 thought, is really maintained at the growing points of the 

 various axes throughout the life of the plant. Into the 

 history of the differences of opinion on this point we need 

 not enter. The inherent difficulties of arriving at valid 

 conclusions from observations have been nearly as powerful 

 as the subjective causes which have evidently influenced 

 the views of the observers in creating the extraordinary 

 discrepancies which exist between the various accounts. 



The method employed by Ludwig Koch (21 and 22), 

 who recognised that the state of things at the growing 

 point was likely to differ at different epochs of growth, and 



