710 TRANSACTIONS OF SECTION K. 
Aneimia and in some species of Dicksonia, in which the leaf-traces have more 
or less hooked ends. 
Variations of the usual methods of pinna-supply appear in Loxsoma 
Cunninghami R. Br., Onoclea sensibilis, L., Hymenophyllum demissum (Forst.), 
Sw., and Nothochlena affinis (Mett.) Moore, suggesting intermediate stages 
between the marginal and extramarginal types. In some species of Pteris and 
Hypolepis and in some of the Cyatheacee there is a combination of marginal and 
extramarginal types. 
The supply to the ultimate pinne is always marginal, and the supply to the 
pinne in the earliest leaves is also marginal. 
5. The Centripetal and Centrifugal Xylem in the Petiole of Cycads. 
By M. J. LeGoc, B.A., Ph.D. 
The cycads possess a number of characters which may be considered as sign- 
posts pointing out the path followed by some of the higher plants in the course 
of their evolution from more primitive forms. A noteworthy example is the 
peculiar structure of the foliar bundle with its double xylem arrangement, one 
portion centripetal (cpx.), the other centrifugal (cfx.). 
Two main interpretations of this structure have been given by different 
investigators. 
(1) The foliar bundle is mesarch, agreeing closely with the Lyginopteris 
type. 
(2) The bundle is mesarch only in appearance. In reality it is normal, and 
is the result of the xylem curving backwards into an inverted omega-like 
structure. 
Observations.—My observations bear mainly on the transition from the cfx. 
into the cpx. at the base of the petiole. 
In an adult petiole, though the bundles at the very base assume different 
forms, they possess, however, a common and constant character, i.e. the xylem 
is strictly and exclusively centrifugal ; it is like the continuation of the stelar 
bundle. But this character is gradually lost as one passes higher up in the 
petiole; some of the rows of xylem are lateral, while here and there appear 
lignified cells decidedly centripetal. With the constant increase of the cpx. 
there is a corresponding decrease of the cfx., which becomes very much reduced 
and consists only of a few cells scattered in a matrix of parenchyma. In a 
young petiole the amount of lignified cfx. is small at the base, while disappear- 
ing altogether a short distance higher up, where the lignification of the cpx. has 
begun. Wounds produced in a young petiole have induced interesting structures. 
Large bundles made of transfusion cells have been formed in order to bring the 
severed bundles of the upper part into communication with the lower part of 
the petiole. In addition a considerable number of the neighbouring bundles 
have been influenced by the wound; the activity of the cambium has been 
increased, the cfx. and cpx. are joined laterally, and secondary xylem has 
been produced: all round the bundles, which consequently have a concentric 
appearance. 
Interpretation.—The arrangement of the xylem, cambium, and phloem 
elements in regular rows, separated by medullary rays at the base of petiole, has 
led some writers to suppose that the cfx. is of secondary origin. But this alone 
is no sufficient proof, because such structures are met with in primary tissues. 
If, however, we have constantly found the number of lignified cells in an adult 
bundle to exceed the number of cells included between the cambium and the 
first cpx. cell in a young petiole, we are necessarily led to the conclusion that 
secondary growth occurs at its base. 
On the other hand, the cpx. is primary wherever it occurs; there is no trace 
of cambium in the vicinity, and the cells are well defined before undergoing 
the process of lignification. 
But it is evident that a primary growth cannot be the continuation of a 
secondary structure, and therefore we are led to think that the cfx. and cpx. 
are two different structures, morphologically independent, though physiologi- 
cally connected for performing their conductive functions. This independence 
