ADULT ANATOMY OF WELWITSCHIA MIRABILIS. 345 
eedling, but still retained its distinctive features. In the figures * A indicates the upper 
level of the normal diarch root, at Bx the two poles of the root have diverged, C is the 
level of the formation of four separate groups, and E is the level of the feeder. Above 
the feeder the four groups are rapidly separated by the increase in amount of parenchy- 
matous tissue, at H they begin to assume their concentric character, 
It has been seen that the main features of the transition-region are the rotation of 
each cotyledonary bundle as a whole, and the fact that, while the exarch structure 
characteristic of roots is thus at once established, the collateral structure characteristic 
of stem-bundles is stil maintained througbout the hypocotyl and the upper part of 
the root. 
The transition phenomena in Araucaria present the strongest resemblance to those 
in Welwitschia. Here also two bundles enter from each cotyledon and, finally, form one 
pole of the diarch root. The four cotyledonary bundles rotate as a whole, but it is only 
fora very short time that the four exarch bundles thus formed retain their collateral 
structure. The four xylem groups rapidly fuse in pairs, and the four phloem groups, 
becoming detached from the xylem, also fuse in pairs, thus establishing a normal diarch 
root-structure. 
In Podocarpus f, the process is similar in essentials, but is still more rapid. Here 
again two bundles enter from each of the two cotyledons, and begin to rotate as a whole. 
Almost immediately, however, the xylem groups fuse in pairs and the phloem groups 
separate from them, and also fusing form two bands alternating with the xylem groups. 
In Sequoia $ the general plan of the transition is somewhat different, but in this case 
also the cotyledonary bundles rotate as a whole, retaining for a short time their collateral 
structure. 
A few other isolated cases occur in which the bundles rotate as a whole, so that they 
are for a short part of their course exarch and collateral |, but in most Gymnosperms 
the transition is more rapid €$/; the xylem usually rotates alone, while the phloem group 
associated with it is separated from it before the bundle becomes exarch. : 
These few examples appear to suggest that the character of the transition phenomena 
is partly governed by the length of the hypocotyl, a factor probably to a large extent 
dependent on the habit of the plant; the Jong stage in Welwitschia in which the four 
collateral bundles have their xylems turned outwards is probably a special modification, 
and it does not appear that in this series a resemblance in such characters can be taken 
to denote phylogenetic relationship. d D 
In the SA features of Wo transition the Cycads do not resemble VW elwitschia, 
but there are some remarkable points of comparison which appear to be of considerabl " 
interest. It will be remembered that the four collateral cotyledonary bundles in 
Welwitschia each form in the hypocotyl one constituent of dimopeinteind ig ; vies 
concentric groups are unique in character, but some parallel is afforded by the three 
* Compare the relative levels in Pl. 34. fig. 4 5, described on p. " 
T Hill, T. G., and De Fraine, II. 1909, p. 213 
i 3 . ^5 3 . , p. i r 5 3. 
t Hill, T. G., and De Fraine, I. 1908. $ He. * T eum jr del 
l|. E. g. Pinus contorta; Ibid. I. 1908, p. 205. € Ibid, I., IL, IIL, 1908, << 
