706 Gwynne- Vaughan. — Observations on the 
at the present time. For his conclusion lies essentially at the 
base of the various theories of 4 phytons/ ‘ rejuvenescence/ 
and 4 segmentation ’ that have been advanced by Gaudichaud, 
C. H. Schultz, Delpino, and others. The most recent modifica- 
tion of these theories is that set forth by Celakovsk ^ 1 in his 
paper upon the segmentation of the stem. He also arrives at 
the same conclusion as Cesalpino, from very different premises, 
but by an analogous process of deduction. So far as the 
Ferns are concerned, it is fairly clear that any apparent 
segmentation that may occur in this group is to be regarded 
as a late development rather than a primitive feature, because 
it is becoming more and more probable that the dictyostelic 
species, in which the so-called segmentation is most obvious, 
are to be derived from solenostelic forms, and these in turn 
from forms with a solid central cylinder. That is to say, 
so far as the structure of the stem is concerned, there is less 
indication of segmentation in the primitive types than in the 
more advanced. Von Mohl 2 , with his habitual freedom from 
external influences, was the first to establish the cauline nature 
of the vascular system of the Tree-fern stem ; the leaves being 
supplied by branches given off from the stem system. He 
admitted that the vascular strands in the 4 pith * of the stem 
ran out directly through the leaf-gap into the centre of the 
petiole, but in spite of this, he utterly rejected the comparison 
of the Tree-ferns with the Monocotyledons which had hitherto 
been advanced with enthusiasm by Link and others. From 
now on the controversy settled upon the nature and course of 
these central strands of the leaf-trace. Karsten 3 , who next 
investigated them, observed that their course in the stem was 
similar to that of the central vascular bundles of a Palm, and 
in consequence revived the Monocotyledonous comparison, 
rendering it so much support that it continued to exercise 
1 Die Gliederung der Kaulome, Bot. Zeit., Bd. lix, p. 79, 1901. 
3 Uber den Bau des Stammes der Baumfame, Vermischte Schriften, p. 108. 
Tubingen, 1845. First published in Martins’ ‘ leones plantarum cryptogamicarum 
Braziliae,’ 1833. 
3 Die Vegetationsorgane der Palmen, Abhandl. d. K. Acad. d. Wissensch. z. 
Berlin, p. 186, 1847. 
