IN LEAVES OF GYMNOSPERMOUS PLANTS. 119 
dorsal (lower) surface of the lamina is placed the phloem; next 
comes the ordinary xylem, which is formed by the cambium in a 
centrifugal * manner; on the inner side of the secondary wood 
there may or may not be a few elements of primary centrifugal 
wood, and then comes the protoxylem, consisting of narrow, 
elongated, spirally- or reticnlately-thickened elements. Further, 
beyond the protoxylem, 2. e. between this tissue and the ventral 
(upper) surface of the leaf, occurs another strand of xylem, 
primary in origin, and of much greater development than that of 
the centrifugal wood: this is centripetal in development, i. e. its 
elements are formed successively from the protoxylem towards 
the ventral (upper) surface of the leaf ; it is characteristic of the 
Cycadee. In the petiole the structure of the bundles is the 
same though their orientation is different. 
In all other Gymnosperms and in Angiosperms this tissue is, 
so far as hitherto observed, completely absent from the vascular 
bundles. No trace of any such tissue has been found either in 
the leaves of the Conifere or of the Gnetacem. In all these 
cases the whole of the wood has been regarded as centrifugally 
developed. 
n investigating the structure of the cotyledons of some 
seedlings of Gingko biloba, grown in the Royal Gardens, Kew, 
I observed a most interesting point in regard to the minute 
“tucture of the vascular bundles, a pair of which traverse each 
M etn. In shap e, as seen in transverse section, each bundle 
was curved, having the form of an arc of a circle. The phloem 
side MAQMI developed. There was a cambium which, on the 
the inside. em, had eut off but a very few elements ; aud to 
ventral sd. mais lay the small group of protoxylem. On the 
the latter th of the protoxylem, however, and directly opposito 
position an N re were yet other tracheides present, which, by their 
other than t} relative development, I determined to be none 
e equivalent of the centripetal xylem as 1t occurs 
* The terms “ 
the parts of the 
such a way that 
(protoxylem), , 
Outside of the 
direction awa 
centrifugal” and “ centripetal,” applied to the development of 
bundle, are used with reference to the centre of the stem, in 
in the case of the xylem, starting from the first-formed trachea 
elements formed successively nearer the phloem (2. e. towards the 
stem) are called centrifugal, and those formed successively in the 
to be centrine M from the phloem (i. e, towards the centre of the stem) are said 
'Petally developed. ‘The same terms (“ centrifugal ” and “ centri- 
petal ” : 
) “re applied to the development of the bundles of the leaf whatever may 
&r orientation, 
