Worsdell . — The Structure and Origin of the Cycadaceae . 131 
of two from each leaf, are peculiar from the fact that they describe a girdle- 
like course part way around the stem, gradually passing inwards to join 
the central cylinder, having to traverse the medullary rays in order to 
reach the latter in those cases where two or more cylinders occur. Some- 
times, however, as in Macrozamia , the leaf-traces pass directly inwards. 
In all species of the genus Encephalartos , and in certain species only of 
Macrozamia there occurs a system of medullary bundles scattered in great 
numbers throughout the pith. Each is always accompanied on the side of 
the phloem by a mucilage-canal. These bundles are orientated in every 
conceivable direction as seen in a transverse section, and are collateral in 
structure. They are cauline in origin, uniting at various points with the 
primary vascular cylinder, but having no connexion with the leaf-traces. 
The roots, both primary and adventitious, vary as regards the number 
of bundles or rays composing the stele ; often the root is pent- or tetrarch 
at the base, i. e. near the cotyledonary node, becoming successively tri- 
and diarch as one proceeds towards the apex, but it is often diarch 
close to the node. A peculiar feature of Cycads is the possession of an 
apogeotropic root-system ; each root branches in a dichotomous coralloid 
manner above the surface of the soil, and contains in the intercellular spaces 
of a particular zone of the cortex colonies of Anaboena , an Alga belonging 
to the Cyanophyceae. 
The leaf ] in most genera, is pinnately branched. But in Bowenia , 
Macrozamia heteromera , and Cycas Micholitzii it is further subdivided. 
This gives to them a more graceful Fern-like character. This Fern-like 
character is exhibited by Stangeria in another way. Each pinna of 
the frond possesses a thick midrib containing two or three vascular 
bundles ; the rest of the broad lamina is supplied by dichotomously- 
branching lateral veins running out to the margin. In Cycas the 
narrow pinna has also a midrib containing a single large bundle, but 
in this case there are no other bundles in the lamina. The vascular 
structure of the pinna here is particularly interesting. The bundle of 
the midrib is of collateral structure with a somewhat arched contour; 
the xylem is of the mesarch type, i. e. the protoxylem is situated in 
a more or less central position with, towards the dorsal or phloem side, 
a little secondary centrifugal, and towards the ventral side a well-developed 
primary centripetal xylem. Transfusion-tissue, consisting of isodiametric 
tracheides whose function is to supply with water the mesophyll of the leaf, 
are situated on either side of the xylem, as seen in transverse section. 
They really constitute part of the centripetal xylem with which they are 
connected by elements transitional, not only in space, but also as regards 
the shape and other characters of the tracheides. The structure of the 
bundle just described is common to the bundles of the lamina in all genera 
of Cycads. In Cycas the inadequate functions of the transfusion-tissue are 
K 2 
