in the Root and Stem of Dicotyledons « 293 
petal (cf. Figs. 19 and 20). The only satisfactory explanation 
of the whole structure is that we have here to do with 
successive layers of extrafascicular cambium. For a time the 
normal cambium continues its activity as in ordinary Dicoty- 
ledons. Then a new cambial layer is formed by division of 
the cells of the pericycle or of the outer phloem-parenchyma. 
This layer may be continuous (except in the rays) all round 
the stem, or it may be partial, abutting at either end on the 
normal cambium. The new layer forms wood internally and 
phloem externally, so that the original cambium and phloem 
become enclosed, and so the first interxylary zone is formed. 
After an interval the process is repeated. The first extra- 
fascicular cambium is replaced by a more external layer, and 
so another complete or partial phloem zone is left behind and 
becomes imbedded in the wood. This process may take 
place over and over again, until we have the complicated 
structure of an old stem, as shown in the diagram (Fig. 16). 
As we have already seen, each enclosed cambium retains 
something of its activity and does not itself pass over into 
permanent tissue. 
These are conclusions, which we have drawn from the 
careful comparison of many stems, but we have not yet been 
so fortunate as to observe the first origin of a new cambial 
layer. Possibly the first extrafascicular cambium may arise 
in the pericycle, certainly the later ones are formed from the 
phloem-parenchyma. In the exceptional cases, when the 
interxylary zones have no sieve-tubes, we may assume that 
this new cambium arose very near the old, so that the 
differentiation of the products of the latter remained incomplete. 
Occasionally small round bundles are seen in transverse 
sections imbedded in the wood, just outside an interxylary 
zone. These are leaf-trace bundles on their very oblique out- 
ward course, which have become enclosed by extrafascicular 
cambium. 
The adventitious roots have a normal structure. 
Our conclusions as to Acantholimon are as follows: — 
1. An internal cambium is formed at a late stage, either 
