COMPARATIVE ANATOMY OF THE CYCADACER. 439 
the broadest part of the circular section, which are smaller, 
squarer, and more compactly joined together than those of the 
rest of the tissue. In one of the small seedlings one of the 
cotyledons is partly aborted; the other is considerably contorted 
and slightly folded round the smaller one. 
In the tip of the cotyledon, as seen in transverse section, 
are five bundles, one of which lies right on the edge of the endo- 
sperm. They are collateral in structure. The chief peculiarity 
is the development of a very conspicuous transfusion-tissue 
arising from the centripetal xylem, which I have already described 
ina former paper*. This transfusion-tissue is not seen in such 
great development in the larger seedling, only the smaller tra- 
cheides of this tissue which occur on the sides of the phloem are 
there seen. It disappears lower down in the cotyledon. The 
centrifugal xylem gradually predominates as the bundles ap- 
proach the base of the cotyledon. In each cotyledon the five 
bundles fuse into three, which pass into the stem to form part 
of the central cylinder. This course of the cotyledonary bundles 
into the stem is interesting inasmuch, as is also the case with 
those of the scale-leaves, it differs from the girdle-leaf-traccs 
of the foliage-leaves in being perfectly straight, and radial iu 
direction, whereas the girlles assume a tangential and circular 
direction through the cortex of the stem. ‘his latter course of 
the bundles is probably, therefore, a modern acquirement, the 
leaf-trace bundles originally having a straight course like those 
of the scale-leaves and cotyledons. As seen from the respective 
places of insertion of their bundles, the cotyledons are not both 
placed at the same level on the stem. Eventually, the central 
gtoup of bundles of the stem, which are entirely collateral in 
structure, and whose xylem is wholly centrifugal, form a cylinder 
Whose bundles, in one or two places, are united to form an arc. 
A transverse section of the young plumular leaf shows a 
group of eight or nine bundles forming an irregular Q; they 
‘ppear under a low power perfectly concentric structures; in 
reality, the phloem lies on one side only, though very much 
curved into an are of tissue around the minute group of 
“carcely thickened tracheides ; the phloem fibres are also already 
differentiated, 
Unfortunately, the exact transition from stem to root was not 
ts Transfusion-tissue : its Origin and Function in the Leaves of Gymno- 
‘Pertuous Plants" : Trans, Linn, Soc. ser. II. Bot. vol. v. 1897, pp. 301-319. 
