in the Root and Stem of Dicotyledons. 291 
cases they occur in the parenchyma of the bundles themselves 
outside the protoxylem, so that the latter is cut off from the 
body of the wood 1 . The internal cambium at once begins to 
form centripetal wood on its outer surface and internal 
phloem towards the pith. If the cambium has cut off the 
protoxylem the latter is necessarily carried inwards towards 
the centre of the pith in front of the advancing phloem. Figs. 
17 and 18 show the more normal type, where the cambium 
arises within the protoxylem. The difference is of no impor- 
tance whatever, as regards the final result. The medullary 
formations are localized opposite the bundles ; sometimes 
there is one internal strand to each bundle, sometimes there 
are two. Both internal wood and phloem agree exactly in 
structure with the normal tissues. In the former, the rela- 
tively large vessels are conspicuous. The secondary forma- 
tions encroach very seriously on the pith. We have found the 
internal wood as much as twenty-five elements in radial 
thickness ; the phloem is generally less extensive. It is quite 
possible that the internal cambium may at first be continuous 
with the normal cambium at the nodes, as Robinson found to 
be the case in lodes tomentella' 2, , but we have no proof of 
this. The formations in Acantholimon bear a close resem- 
blance to those observed by Robinson. The similarity with 
Tecoma is more superficial, for in this latter plant a medullary 
phloem-strand is present before the cambium appears, which 
is not the case here. In Acantholimon the internal wood and 
phloem are entirely secondary, and it would be a forced view 
to regard them as forming part of the leaf-trace system. The 
curious structure described by Dangeard ( lx .) in Acanthophyl - 
him has some analogy, but differs in the fact that the cam- 
bium completely surrounds the wedges of wood, which we 
have never found in Acantholimon. We suspect that the 
anomaly discovered by Morot in the Basellaceae is more 
nearly related to the present case than to ordinary bicollateral 
structure. 
1 This process is comparable mutatis mutandis to the mode of origin of the 
normal cambium in the root of Chironia. 2 Loc. cit. 
