562 Bancroft.—A Contribution to our 
significance. It is only possible to say that if the theory of the asterostele 
be tenable, typical examples of R. cylindrica represent a stage much in 
advance of the primitive condition. 
According to Lignier, 1 the primitive vascular system was a solid 
exarch xylem mass ; during the course of evolution this underwent a 
process of dissection and of subsequent concentration which, to a certain 
extent, recalls the behaviour of Bertrand’s asterostele. Lignier’s theory, 
again, would place R. cylindrica far from the original type. 
It is therefore impossible to say with certainty whether the stele 
of R. cylindrica is more or less highly organized than those of related 
species; it is, however, reasonable to conclude that typical examples are 
not primitive according to any of the theories mentioned above. 
IL The Homology of the Leaf. 
A comparison of the methods of stem-branching and leaf-production 
in R. cylindrica provides evidence in favour of the view, suggested by 
Bower 2 in 1884, that stem and leaf are homologous branches of a primi¬ 
tively undifferentiated and dichotomous system. This view is now the 
basis of the hypothesis set forth by Potonie, 3 Hallier, 4 Lignier, 5 Tansley, 6 
and Bertrand 7 ; according to Tansley it ‘carries with it the necessity 
of looking upon the branching away of the leaf-trace from the vascular 
system of the stem as in origin a separation of the vascular strand into 
branches of equivalent morphological status \ 8 
Stem-branching and leaf-production have been described in R. cylin¬ 
drica , and it is evident that the two processes are essentially the same 
in origin. In branching, however, the completion of both branch steles 
is ensured by the formation of metaxylem elements below the actual level of 
their separation (see PI. XXVI, Figs. 5 and 9; Text-fig. 7); in leaf-production, 
on the other hand, only the stem stele is completed in this way, for at the 
1 Lignier, O. : Organisation progressive du parcours des faisceaux libero-ligneux dans le meri- 
phyte des Phyllinees. Bull. Soc. Bot. de France, t. 58, 1911, p. 29. 
Lignier, O.: Essai sur les transformations de la stele primitive dans l’embranchement des 
Phyllinees. Bull. Soc. Bot. de France, t. 58, 1911, p. [87]. 
2 Bower, F. O.: On the Comparative Morphology of the Leaf in the Vascular Cryptogams and 
Gymnosperms. Phil. Trans. Roy. Soc., B, vol. 175, 1884, p. 565. See p. 605. (Bower has now 
abandoned this theory.) 
3 Potonie, H. : Ein Blick in die Geschichte der botanischen Morphologie und der Pericaulom- 
theorie. 1903, p. 33. 
See also Lehrbuch der Pflanzenpalaeontologie, 1899, pp. 156-9 ; and other references. 
4 Hallier, H.: Beitrage zur Morphogenie der Sporophylle und des Trophophylls in Beziehung 
zur Phylogenie der Kormophyten. Jahrb. der Hamburgischen wissenschaftlichen Anstalten, 19, 
1901 (published 1902). See p. 45 and p. 104. 
5 Lignier, O. : Essai sur revolution morphologique du Regne vegetal. Bull. Soc. Linn* de 
Normandie, ser. 6, vol. 3, 1908-9, p. 35. Reprinted, 1911. See other references also. 
6 1 . c., p. 1 of reprint. 7 1 . c. (’ 09 ), pp. 260, 261; (’ 12 ), p. 278* 
8 1 . c., p. 3 of reprint (cf. New. Phyt., vol. 6, 1907, p. 26). 
