346 MISS M. G. SYKES ON THE SEEDLING AND 
concentric steles found in the hypocotyls of Encephalartos Barteri* and of Cycas 
siamensis 1. 
An abnormal seedling of Araucaria Bidwillii t, in which Shaw found four concentric 
steles, should also be mentioned in this connection. 
The smaller concentric bundies scattered in the parenchyma of the hypocotyl of 
Welwitschia may be compared with the anomalous concentric cortical strands which are 
a characteristic feature of the roots of some Cycads §. 
Section VIT.—CENTRIPETAL XYLEM IN THE SEEDLING. 
Cotyledon and Ridge Bundles.—At the base of the cotyledons there are differentiated 
on the centripetal side of the cotyledonary bundles a small number of rather short 
broad tracheids with reticulately pitted walls (cf. L, M, text-fiz. 3), which are often 
transversely connected with the bundles supplying the outer ridges. This centripetal 
wood dies out almost at once, before the bundles branch to form the parallel series 
which traverses the cotyledon: there is then no trace of centripetal elements. 
In the 18 months old seedling no transfusion tissue was yet developed, and only 
a small amount of this tissue is present in connection with the bundles traversing the 
withering cotyledons of seedlings two to three years old. The transfusion tissue here 
first arises and is always best developed on the phloem side of the bundle, and it is not 
possible to trace any continuity between it and the centripetal xylem |. 
A varying number of transfusion tracheids accompany the ridge-bundles during the 
upper part of their course ; these occur mainly on the centripetal side of the xylem. The 
centripetally-developed tracheids both here and in the cotyledonary bundles are 
somewhat intermediate in character between the centrifugal xylem and the short wide 
transfusion-tissue elements scattered throughout the parenchyma of the ridges]. "There 
were no unmistakable indications of centripetal xylem in the young bundles of the leat. 
- Hypocotyl.—It is not easy to follow the exact behaviour of the centripetal xylem 
when the cotyledonary bundles rotate and become exarch in the upper part of the 
hypocotyl. Scattered tracheids are present throughout the parenchyma in the centre 
of each of the four concentric groups of vascular tissue which are there formed, and a 
small group of primary xylem is also present (Pl. 35, fig. 22) on the outer edge of the 
protoxylem **. The primary xylem is indeed often separated into two distinct portions, 
both of which are more or less triangular in cross-section, the apices of the triangles 
being directed towards one another. 
In the lower unswollen portion of the hypocotyl, after the concentric character of the 
* Worsdell, V. pp. 136-137. T Matte, p. 189, fig. 248. 
i Shaw, F. J. F. ; see also Seward & Ford, p. 334, fig. 16, and p- 944, fig. 19. 
.$ Cf. Hill, T. G., and De Fraine, III. p. 442, for concentric strand in young root of Macrozamia. 
= e Scheit, who considers that the origin of the transfusion-tissue is always in the xylem (ev Worsdell, us 
p. 905). 
*| Cf. Worsdell, V. p. 768. 
i Er remarks that the protoxylem in the hypocotyl is much esongated in a lateral direction, I. pl. 4. 
g. » «ec, j 
