676 
PEOFESSOE W. C. WILLIAMSON ON THE OEGANIZATION 
( loc . cit. p. 58). The plants thus defined are obviously such as I should have considered 
to possess some modification of exogenous growth, though Cotta points out certain 
features in which he considers that they differ from the stems of true Dicotyledons. 
The above publication was followed in 1845 by Corda’s noble work entitled “ Beitrage 
zur Flora der Vorwelt.” In this admirably illustrated volume he described and figured 
a large number of hitherto unknown forms, and included in his respective genera all 
those previously described by Cotta, the whole being thrown into two primary groups. 
One of these groups was composed of what he regarded as Monocotyledonous and 
Dicotyledonous plants, the other of Ferns. These groups he further divided into 
secondary natural families. The only one of these latter belonging to his first division 
which concerns me now is that of the Palmee. The Ferns he divided into seven families 
and more than forty genera, the latter being too often based upon the most insufficient 
characters. The undue multiplication of genera by this distinguished botanist was very 
properly objected to by M. Brongniart, who says, “M. Corda, dans son essai sur la 
flore de l’ancien monde, me parait avoir trop multiplie, pour l’etat actuel de nos 
connaissances, les genres fondes sur les tiges des Fougeres, dont nous ne connaissons 
generalement la structure que d’une maniere trop imparfaite pour etablir des divisions 
bien definies ” (‘ Tableau des genres de Yegetaux Fossiles,’ p. 34, 1849). He then 
proceeds to throw many of Corda’s genera into more comprehensive generic groups, 
still retaining sixteen genera, nine of which, however, he regards as merely provi- 
sional ones. 
The second great difficulty with which I have had to contend springs out of Corda’s 
multiplied genera even as reduced in number by Brongniart. They are all based chiefly 
upon variations in the form of the vascular bundles, a character which a study of living 
forms demonstrates to have no generic value whatever; and his definitions further take 
no cognizance of the immense differences which generally subsist between the form of 
those bundles in the primary and in the secondary petioles. Thus, on the one hand, 
there is a close resemblance between the transverse sections of the bundles in the petioles 
of BicTcsonia rubiginosa, Pteris umbrosa, Adiantum trapeziforme and macropliyllum, and 
Asjpidium molle , whilst, on the other, nothing can be more distinct than the vascular 
arrangements in Pteris umbrosa and P. aguilina , in Adiantum trapeziforme and A. 
cuneatum , and in numerous other examples of a similar kind which might be quoted. 
Such being the case, it is obvious that no guides can well be less trustworthy than trans- 
verse sections of these vascular bundles in establishing generic distinction amongst the 
fossil ferns. 
These difficulties are not diminished by the fact that in the fossil petioles the tissues 
forming the sheath of each bundle, and the elongated cells which are enclosed along with 
the vessels, are rarely, if ever, so distinctly marked as in the living one. Indeed in 
many of them the cortical cells graduate uninterruptedly up to the scalariform vessels, 
which latter constitute the only tissue that is sharply differentiated from the cortical 
ones ; in other words, such bundles appear to be vascular rather than fibro-vascular. 
