FORMATION OF NODULES. 
273 
connections are frequently found completely overlaid by 
layers of wood tissue produced in the subsequent growth of 
the globular shoot. Exactly the same feature is found in some 
typical nodules, where aborted vascular connections occur 
completely sunk in the tissue of the nodule. In globular 
shoots, under suitable change of conditions, growth may be 
resumed, and disconnected vascular connections again become 
connected with the stem wood. The vascular connections of 
nodules arise under conditions of activity in the cortex, and 
their whole appearance and occurrence suggest a close analogy 
with those of globular shoots. 
(c) Nodule Cambium. 
The nodule cambium arises from cortical cells, which become 
active and begin to divide. It cuts off wood cells internally 
and cortical cells externally, the latter only to a small extent. 
Latex vessels are produced externally by the nodule cambium 
only after a long period of division, and then only sparingly. 
In the cortex overlying old nodules the latex vessels are scanty ; 
this is very noticeable in sections ; and in the field, in tapping 
such cortex, a poor yield of latex is obtained. It will be seen 
from the following that such cortex may with good reason be 
considered as entirely the product of the nodule cambium. 
In normal cortex two zones can be quite clearly distin- 
guished, Le., the inner cortex and the outer cortex. The inner 
cortex in longitudinal section exhibits well-developed latex 
vessels, sieve tubes, cortical cells mostly prosenchymatous, 
comparatively few cells with tannin content, no stone cells ; 
and the medullary rays are quite distinct. The outer cortex 
is marked by the presence of abundant stone cells, fragmentary 
remains of latex vessels, and abundant tannin cells ; the 
medullary rays are indistinct, and the cortical cells are hexa- 
gonal or isodiametric. 
In the cortex overlying old nodules the inner cortex is 
normal. The outer cortex differs strongly from the normal 
in the almost entire absence of stone cells and the much 
greater abundance of tannin cells. The absence of stone 
cells is very noticeable in cutting sections ; it is extremely 
difficult to obtain sections of normal cortex, whereas nodular 
6 ( 14)17 ( 38 ) 
