226 WorsdelL — The Vascular Structure of 
a second row of as many, but rather smaller and less clearly- 
defined tracheides, these two constituting the primary centri- 
fugal xylem ; these tracheides do not lie in the same radial 
rows as the cambium-cells further out, but are separated 
therefrom by the parenchymatous cells afore-mentioned, some 
of which overlap in position two of the cambium-cells. In 
the sterile portion the mesarch structure of the bundles is 
very pronounced. 
Female Sporophyll. In the stalk, besides the four bundles 
which exhibit the usual structure and no centripetal xylem, 
there is a very small one on the ventral side with inverted 
orientation and almost concentric structure, which has evi- 
dently sprung from one of the four bundles opposite to which 
it lies. 
Zamia pumila, Linn. 
Male Sporophyll. In the stalk is a row of about six bundles, 
quite small in size, one or two having only a single tracheide 
of centrifugal xylem. Of these small bundles, two occur 
on the dorsal side of the row, one of them lying sideways 
and being semi-concentric in shape. That there is a tendency 
for a concentric structure to appear in the bundles is shown 
also by one in which a rudimentary phloem occurs on the 
ventral side of the protoxylem. In one of the bundles two 
elements of centripetal xylem occur. 
In the sterile portion the bundles have the usual mesarch 
structure. 
Zamia Skinneri, Warsz. 
Male Sporophyll. In the stalk are three bundles with well- 
developed secondary tissue, of which the tracheides, especially 
in the median bundle, are of considerable size and very 
brightly coloured. To each bundle a single tracheide of 
centripetal xylem is seen to be attached. In the region 
just above where the bundles pass off to the sporangia, 
occur two large bundles with well-developed, typical mesarch 
