*4 
A. S. Marsh. 
of cells cut off by the cambium. The portion which I have 
designated primary persists to different extents in different bundles. 
In some, even in sections £ and ?/, there is a connecting hand of 
tracheids between the two kinds of xylem, but in all cases this 
disappears sooner or later and at a level of about 2 cm. from the 
base leaves a group of bundles with centripetal and centrifugal 
xylems quite distinct from one another. It might be mentioned that 
the section depicted in Scott’s “Studies in Fossil Botany” (2nd ed.), 
Fig. 133, is probably fairly near the base, as one of the products of 
the bifurcating bundle there shown has retained the connection 
between the two kinds of wood. The centrifugal wood does not 
consist of scattered elements, as in Cycas, but forms a compact 
mass in the median line of the bundle (see Figs. 8 and 9). 
Fig. 6.—Tip of terminal leaflet to show position of sections “ a ” and “b” 
described in the text. 
Before leaving the basal region of the petiole, one may 
mention that Le Goc reports in the Cycads which he investigated, 
the occurrence at the base of the petiole of various types of bundle, 
collateral and concentric. In Stnngeria all bundles were strictly 
collateral: the only apparent exceptions were bundles undergoing 
division in which the centrifugal xylem split and the resultant 
portions came to lie at diametrically opposite sides of the bundle 
before the centripetal xylem mass bifurcated. We may represent 
this by * Wm * where the asterisks are centrifugal xylems, and 
the dark mass centripetal xylem. In all higher parts of the petiole 
bifurcation of a bundle takes place about a plane at right angles to 
this, which we may represent so :— giving ^ ^ Here 
we still see that the centrifugal is divided before the centripetal 
(see Fig. 8). 
Turning again to our consideration of the changes in the 
bundle as we pass from the base upwards, we find that at the level 
of rj we have very little centrifugal xylem. Usually there are only 
