400 Chandler — On the Arrangement of the Vascular Strands 
these changes the vascular tissue has increased greatly in size, the xylem, 
as before, being still somewhat parenchymatous. The next leaf-trace 
is very quickly formed, exactly opposite the last ; the corresponding gap 
soon closes, and the following leaf-strand shows distinct indications of 
a double character which no doubt obtains in later leaf-traces. The repair 
of this gap results in the completion of the vascular ring, a condition which 
is maintained unaltered for some little distance. In connexion with the 
next leaf-trace, however, we have, just as in Alsophila , the formation of 
a typical ground-tissue pocket, the apex of the pocket in this case being 
blunt, and occupying a considerable portion of the central phloem. The 
actual course of events is as follows :• — Several cells, some eight or nine 
in number, and possessing thickened refractive walls, appear at the centre 
of the phloem and persist through four or five sections. These cells are no 
doubt endodermal in function, and are succeeded by a patch of parenchy- 
matous cells resembling those of the ground-tissue and surrounded by 
a definite endodermis, an isolated section presenting a c gamostelic 5 appear- 
ance. As in similar cases described, the inner and outer endodermis 
become continuous at the gap, and the pocket-like character of the central 
ground-parenchyma is demonstrated. 
Unfortunately, no older plants were available for examination. The 
great similarity between the above early transitional changes and those 
described as occurring in the young plant of Alsophila , leaves little doubt 
that subsequent changes in Dicksonia are essentially the same as those 
given in the excellent account of G Wynne- Vaughan, 
Conclusions. 
A discussion of the results arrived at in the above research involves 
a consideration of the stelar question. It would be serving no useful 
purpose, however, to follow in detail the various changes of thought which 
have followed upon the enunciation of Van Tieghem’s famous hypothesis, 
since the history of the question is of general knowledge, largely owing to 
the appearance, from time to time, of excellent resumes of the subject, 
prominent among which are those of Tansley 1 , Faull 2 , and Schoute 3 . 
At the present time, botanists are practically unanimous in regarding 
Van Tieghem’s conception of polystely as no longer affording a real 
explanation of observed facts. His work fails chiefly because the investiga- 
tion of individual types was not carried far enough, for there can be little 
doubt that, had Van Tieghem followed out the complete elaboration of the 
1 The Stelar Theory. Science Progress, 1896. 
2 The anatomy of the Osmundaceae. Botanical Gazette, xxxii. 
3 Die Stelar-Theorie. Groningen, 1902. 
