192 Le Goc. — Observations on the Centripetal and Centrifugal 
Also these wound-induced tissues imitate structures often met with at the 
base of a petiole. It would be reasonable to suppose that all the phases 
in the evolution of the vascular bundle can be traced in a living petiole of 
a Cycad. Unfortunately the principle at the base of this argument is not 
proved in its generality, and to quote the Cycas wounds as an instance 
would obviously be a vicious circle. 
It is, perhaps, more likely that such structures are developed in response 
to a physiological need. As a general outlook no one can help seeing that 
the anatomy of plants corresponds to a physiological function or purpose. 
Through some mechanism of life they must have been developed in relation 
to such a need. In this instance the increased activity of the cambium in 
the bundles tends to answer such a purpose by increasing the area of the 
bundles as their number is reduced. Also the new bundles of transfusion 
cells are evidently adapted to fulfil a function, a need. Why this is so 
could not, perhaps, be answered by Experimental Botany, which is more 
concerned with the question how it is done. 
Thus, reduced to a physiological .interpretation, these structures 
artificially induced are without any direct bearing on the phylogenetic 
interpretation of the bundle. Our final conclusions are therefore : 
1. At the very base of the petiole the structure of the vascular bundles 
of Cycads is entirely centrifugal, and assumes different forms, concentric, 
collateral, or the combination of both. 
2. The centrifugal xylem at the base is, at least in its main bulk, 
a secondary growth. This is indicated by the regular rows of xylem, 
cambium, and phloem, separated by medullary rays, and proved by the 
study of petioles at different ages. 
3. The centripetal xylem is a primary structure laid down at an early 
age, but only gradually lignified. 
4. The centrifugal xylem and centripetal xylem are probably distinct 
in origin, juxtaposed in response to physiological demands but morpho- 
logically discontinuous. 
5. During most of the course along the petiole the two xylems remain 
distinct, therefore the bundle is more properly called pseudo-mesarch 1 or 
diploxylic. 
6. The two xylems overlap at their ends. The remains of centripetal 
xylem scattered at the base might point to a time when it ran further 
down, perhaps into the stem. 
1 A word suggested by Prof. A. C. Seward. 
