ADULT ANATOMY OF WELWITSCHIA MIRABILIS, 349 
considerable age, where they terminate in the outer ridge. The four collateral cotyle- 
donary bundles are joined in the upper portion of the hypocotyl by the bundles which 
enter from the buds, ridges, and later from the leaves. The rotation of all the bundles 
takes place almost as soon as they enter the hypocotyl, and there is therefore not at any 
time a characteristic stem-like structure. The associated bundles form at once four 
concentric groups, but many of the more newly intercalated bundles of the leaves, &c., 
may be separately prolonged downwards for a greater or lesser distance before they 
become connected with one of these groups. 
The bundles entering from a bud are joined on to both the constituents of the double 
trace of the cotyledon subtending the bud; but each ridge receives bundles which are 
connected with one member of each pair of cotyledonary bundles, and the same is true 
of the leaf, one half of each leaf-trace being associated with a member of each of 
the double cotyledonary traces. 
4. The method of transition in Welwitschia is found to be most closely comparable 
with that in Araucaria. Welwitschia, Araucaria, Podocarpus, and some other forms 
make up a series, in all of which two bundles enter from each cotyledon and form one 
pole of the diarch root; in each case the collateral cotyledonary bundles rotate as 
a whole, and after a time the xylems of a pair of bundles fuse together, while the 
phloems leave them and fuse with the phloems of the other pair. The members of the 
series differ in the distance over which this process extends, according to the length 
of the hypocotyl. In other Gymnosperms the transition is still more rapid and the 
cotyledonary bundles do not rotate as a whole. It is thought probable that in this 
series the resemblance in the form of the transition is thus dependent oa habit and 
not on relationship. 
5. One of the most remarkable characteristics of the anatomical structure of Wel- 
witschia is the small amount of primary vascular tissue in this plant. The development 
of primary xylem and phloem is limited to the poles of the root and the four main 
hypocotyledonary bundles, which are derived from the two poles, and which supply the 
cotyledons. The bundles supplying the two ridges and the inflorescences are clearly 
entirely derived from meristematic divisions of the parenchyma, and it is only later that 
they are connected by cambial divisions with the primary bundles. ; 
The small size of the bundles in the cotyledonary buds makes it impossible to be 
certain whether or no there is any primary xylem present, but it appears most probable 
mat there is none. The possibility remains that there is primary tissue 1n the two 
original pairs of bundles which first supplied the leaves. I have no material young 
E in which to investigate this, as the first-formed xylem elements Ru qfi 
e Een get disrupted and separated from one another in aigu de e 
by the rapid growth going on at the base of the leat. But H eA ; y i 
m eames all the xylem elements are arranged a zee agree 
connect th downwards into the hypocotyl by secondarily dev ‘th ounger seedlings 
"lem with the cotyledonary bundles (m, text-fig. 3, C). Inthey "s td 
connection is established by means of a meristem only, no xylem elements having 
Yet been lignified, 
