compared with other genera of Cycadeae . 605 
pith; but that, on the contrary, their course lies in almost 
every direction, owing to the fact that they form part of an 
anastomosing system whose different branches traverse the 
length and breadth of the pith. If one of the main branches 
of this system is transversely cut through, it will be seen to be 
a bundle similar in size and appearance to one of the girdles 
of the cortex, consisting of a thick band of xylem and phloem 
produced by the activity of a cambium. Unlike the bundles 
of the cortex, however, its xylem and phloem have no regular 
orientation with regard to the vascular cylinder or the exterior 
of the stem, but lie, on the contrary, in any direction. The 
fact is that each branch of this medullary system, whether 
large or small, follows closely and is accompanied by a branch 
of a similar anastomosing network of mucilage-canals which 
traverse the pith in every conceivable direction. It is the 
mucilage-canal which invariably determines the orientation of 
the bundle, so that the phloem is always directed towards, 
and the xylem away from, the canal (Figs. 3, 6-8). It may 
frequently happen that a canal is accompanied by two bundles, 
one on either side, which anastomose with each other here and 
there on their course. But as the mucilage-canal twists and 
bends about through the pith, the bundle accompanying it 
appears first on one side of it and then on the other — seeing 
that it takes a short cut and does not follow every twist and 
turn of the canal — and in order, therefore, to preserve its 
orientation, it must necessarily undergo a twisting on its axis, 
the phloem being always on that side of the bundle nearest 
the canal (Fig. 3). 
As showing how irregular and anomalous is the course 
of these medullary bundles. Fig. 5 represents a curious 
involution which very frequently takes place whereby the 
bundle or a portion of it assumes a perfectly circular course ; 
it may also sometimes be seen entwining a- mucilage-canal 
in this way. Renault 1 , in his investigations of fossil plants, 
has figured similar contortions amongst the tracheides of the 
wood in the fossil genus Cyccidoxylon. 
1 Structure comparee de quelques tiges de la Flore Carbonifere, Plate 14, 1879, 
