284 Scott and Brebner . — On Internal Phloem 
parenchymatous cells intervening between the primitive 
tracheae and the nearest internal sieve-tubes. It is not until 
the stem has attained a considerable thickness that any 
departure from ordinary bicollateral structure begins. At 
the stage shown in Fig. 13, the normal wood has a radial 
thickness of from seventy to ninety elements. The very 
characteristic internal phloem forms a ring, which is not how- 
ever continuous but is broken up into groups by narrow 
radial strands of parenchyma from one to three cells wide. 
Often, but not always, these rays correspond to the primary 
medullary rays of the normal wood. Not only is the internal 
greater in amount than the external phloem, but its individual 
elements are also larger. The internal sieve-tubes often 
reach *03 mm. in diameter, while the normal ones scarcely 
exceed *02 mm. In some places an internal cambium has 
appeared, which at one or two points, opposite the protoxylem 
of the larger bundles, has begun to form tissue on its outer 
side. This tissue is the internal centripetal wood. The 
cambial divisions begin in the parenchymatous cells im- 
mediately bordering on the outer edge of the phloem. Thus 
the internal is separated from the normal wood by one or two 
layers of thin-walled cells. The first xylem-elements formed 
are lignified parenchyma. 
The internal cambium soon extends all round the pith, but 
its production of wood is for a long time limited to the 
regions where it first started. Elsewhere its activity is limited 
to the formation of additional phloem on its inner face. 
When the normal wood is from 130 to 150 cells in radial 
thickness the internal woody masses are about fifteen cells 
thick (Fig. 14). 
At a still later stage the internal wood becomes much more 
extensive. In the oldest stem at our disposal, about 1*4 cm. 
in diameter, it forms eight masses, which together occupy the 
greater part of the circumference of the pith. These masses 
lie opposite those parts of the normal wood which are richest 
in vessels, or, in other words, the internal wood still shows a 
relation to the primary bundles. Its maximum thickness now 
