350 MISS M. G. SYKES ON THE SEEDLING AND 
In the young plant the bundles are all better developed and the xylem more strongly 
lignified in the upper part of their course through the ridges, inflorescences, and leaves 
than in their downward extensions into the hypocotyl. It would appear that vascular 
bundles are differentiated in the earlier stages of these organs, and are later continued 
downwards and become associated with the four original cotyledonary bundles, thus giving 
rise to the four concentric groups so characteristic of the hypocotyl of Welwitschia. 
Such a series of facts appears to me to indicate that in this plant no distinction 
between primarily and secondarily developed vascular tissue is possible for purposes of 
morphology *. 
6. Centripetal xylem is developed in connection with the four bundles in the bases of 
the cotyledons, and after the bundles have rotated a small isolated group of primary 
tracheids accompanies the four exarch bundles down the hypocotyl, and is also 
associated with the bundles of the root during the greater part of their course. It is 
obvious that this separate peripheral mass of primary xylem occupies the same position 
relative to the protoxylem as did the centripetal xylem in the cotyledonary bundles 
before they rotated, although its position relatively to the centre of the hypocotyl is now 
exarch. 
In many of the outer series of bundles in the outer ridge, although these are entirely 
secondary, the first-formed elements are not the innermost ones, and some xylem is 
formed in both centrifugal and centripetal directions. In longitudinal section trans- 
fusion tracheids can also be seen on the centripetal side of many of the ridge-bundles : 
these merge gradually into the transfusion-tissue which is profusely scattered among 
the parenchyma. 
7. It is difficult to make any close comparison with other plants. The retention of the 
seedling characters in the adult plent makes comparison with other seedlings of most 
interest, but since the vascular structure of seedlings probably depends to a remark- 
able extent on their habitat and environment, such comparison is probably not the most 
profitable for indicating relationships. The most remarkable character in Welwitschia 
is its four concentric groups of vascular tissue, with primary xylem tracheids scattered 
in their pith. Concentric steles are found in the hypocotyls of the seedling of some 
Cycads, and have been described in an abnormal seedling of Araucaria Bidwillii. 
‘Such concentric groups are, however, most comparable with the several central steles 
of the Medulloseze, and the peripheral series of secondary vascular bundles also recalls 
the outer rings of secondary vascular tissue in these plants. 
8. The method of branching of the root when injured by stones or other obstacles has 
been described, and its investigation has afforded fresh evidence of the remarkable 
vitality of this plant. A seedling grown under conditions damper than the natural was 
found to develop a number of minute lateral rootlets. | | 
9. Certain interesting abnormalities have been recorded in which (1) extra leaves 
have been produced in cases where the apex has not been completely buried, (2) one 
of the two normal leaves has been abnormally developed, or (3) further lobing of the 
cotyledonary buds has taken place. These have not thrown much light on the 
morphology of the plant. 
+ 
* Worsdell, V. p. 141, 
