400 Scott. — On Peduncle of Cycadaceae. 
than the inner surface. Here, then, the development goes on 
in two directions ; a part of the wood is centrifugal, i. e. de- 
veloped outwards towards the phloem, but the greater portion 
is centripetal, i. e. developed inwards, in the direction away 
from the phloem. This type of bundle may conveniently 
be called mesarch , a term suggested by Count Solms-Laubach 1 , 
implying that the starting-point of the wood, the protoxylem, 
lies in the interior of the xylem-strand. The structure was 
thoroughly investigated by Mettenius, to whose observations 
little has been added by subsequent authors 2 . 
The relative development of the two parts of the wood 
varies considerably ; in the finer veins of the foliage-leaves 
the centrifugal part may die out altogether, while in many 
sporophylls it greatly exceeds in bulk the centripetal portion, 
which may even be entirely absent. 
Among recent plants typical mesarch bundles have, until 
the present time, only been found in the leaves of Cycadaceae 3 . 
In the stem of these plants only normal collateral bundles 
have so far been found. Mettenius has shown in detail how 
the transition from the one type of structure to the other 
takes place gradually in the base of the leaf 4 . 
Among fossil plants mesarch structure was much more 
widely spread. Not only did it occur in families belonging 
to the Cycadean stock, as in Bennettiteae and Medulloseae, 
but it was general in the leaves of the Cordaiteae, a fourth 
order of Gy mnosperms, only known from the Primary Rocks 5 . 
The Cordaiteae, like the recent Cycadaceae, had normal 
bundles in their stems, but it is a remarkable fact that in 
certain other Palaeozoic plants the vascular bundles of the 
1 Fossil Botany, English Edition, p. 257. 
2 See the well-known figures of this type of vascular bundle in De Bary’s 
Comparative Anatomy of Phanerogams and Ferns, Figs. 158 and 159. 
3 Mr. W. C. Worsdell has quite recently shown that the bundles of the cotyle- 
dons and other leaves of Ginkgo biloba , and of Cephalotaxus, are mesarch, and 
finds reason to believe that in Conifers generally the ‘ transfusion-tissue ’ of the 
leaf is, in part at least, homologous with centripetal xylem. See his preliminary 
paper ‘ On the Origin of Transfusion-tissue,’ Journal of the Linnean Society, 1897. 
M. c. p.577. 
5 Renault, Tiges de la Flore Carbonifere, p. 295, 1879, Plate XVI, 
