238 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
there is no difficulty in establishing the conjoint existence of the two tissues; hence I 
venture to affirm that, in Diploxylon cycacleoideum , we have two distinct forms of 
medullary rays: 1st, the larger lenticular ones, which are primarily composed of barred 
cells, but through which the vascular bundles escape to the surface of the woody zone ; 
and 2nd, of smaller ones, in which similarly barred cells are chiefly, though not invari- 
ably, arranged in tangential sections in single vertical rows, often not containing more 
than two or three cells in each vertical series, but which constitute true medullary rays. 
In the preceding memoir I have designated the large lenticular spaces primary me- 
dullary rays, to distinguish them from the smaller or secondary ones. Those botanists 
who, like Mr. Carruthers, wholly repudiate the existence of any parallelism between 
these fossil Cryptogams and the more highly developed Phanerogamic Exogens, con- 
sistently deny that any of these cellular horizontal communications between the interior 
and exterior of the woody zone are representatives of or entitled to be called medullary 
rays ; but Brongniart, than whom it would be difficult to quote a higher authority, so 
designated both the larger and the smaller ones in Stigmaria fcoides, as well as illus- 
trated them by what are found in Zamia integrifolia and other Cycads (“ Observations sur 
le Sigillaria elegans ”), and I am convinced that he is right in so doing. It is as impossible 
to separate these Cryptogamic forms of medullary rays from those of the Cycadese on 
the one hand, as it is to disjoin the latter from those of the higher Conifera on the other. 
Professor King quotes the late Dr. Lindley’s opinion that no vascular bundles ever issued 
through medullary rays. This may be true in the case of Phanerogamic Exogens, but 
it does not follow that it must be equally true of these Cryptogamic modifications of the 
exogenous type of woody zone. One thing is clear, viz. that the large lenticular spaces 
(my primary medullary rays) are but modifications of the smaller or secondary ones, 
enlarged to serve a special teleological purpose ; i. e. the transmission of vascular bundles 
to the leaves and rootlets. At their upper and lower extremities these large elliptical 
cellular rays are undistinguishable from and merge in the smaller ones. However large 
and thick in their central portion, they diminish in size upwards and downwards, both 
in the Diploxylons and in Stigmaria , until they contract into laminae consisting of a 
single thin vertical layer of cells. Such teleological modifications are universal amongst 
animals ; and I fail to see why we should refuse to recognize their existence amongst 
plants. At all events until some better reasons for doing so are furnished by those who 
differ from me than they have hitherto advanced, I shall continue to follow the example 
of M. Brongniart, and employ the terms adopted in the preceding pages. 
Having thus obtained additional light respecting the Diploxylons, I again turned to 
the more highly organized of the stems described by Mr. Binney under the name of 
Sigillaria vascularis , and which I have already represented in Plate XXV. figs. 8-11. 
I made a fresh series of carefully prepared dissections, and succeeded in demonstrating 
the existence in this plant of a series of primary and secondary medullary rays, the 
former containing large foliar bundles, precisely identical with those of Diployxlon cyca- 
deoideum. I have not succeeded in discovering in the former plant the cellular layer 
