286 Scott and Brebner . — On Internal Phloem 
with it, chiefly by extension of their cells without division. 
The internal sieve-tubes are usually about *025 mm. in 
diameter, and have very abundant proteid contents. Besides 
the typical phloem-elements, the internal groups, like the pith, 
contain septate crystal-sacs, such as are also found in the 
normal external phloem. The two phloem-systems are about 
equal in extent. The internal ring has the advantage in 
radial thickness (which amounts to ten elements), but this 
is balanced by the greater periphery of the external 
phloem. 
In an older stem, in which the normal w r ood has about 140 
elements on the radius, the centripetal xylem has appeared. 
It is formed in irregular groups, one of which is shown in Fig. 
15* The elements of this internal wood are (1) vessels with 
bordered pits, composed of long joints, and reaching *04 mm. 
in diameter (about half that of the largest vessels of the 
normal wood) ; (2) xylem-parenchyma, and (3) radially elon- 
gated cells which may be regarded as forming xylem-rays. 
The centripetal wood, which is entirely produced by the 
internal cambium, is therefore of perfectly typical structure. 
The parenchyma, lying between the outer margin of the 
centripetal wood and the protoxylem, becomes partially 
lignified. In one case we found reticulated elements, probably 
vessels, in this position. 
In the older stems the greater development of the internal 
vascular tissues has necessarily produced increased obliteration 
towards the pith. Many of the older sieve-tubes have very 
callous plates. 
3. Periploca graeca , L. (Asclepiadeae). In this plant there 
is a very distinct internal cambium, which produces a large 
amount of secondary phloem towards the pith. It also 
occasionally forms a few lignified elements in centripetal 
direction, but in the stems investigated the amount of internal 
wood was insignificant. 
The cases of Apocynum and Willughbeia are sufficient to 
invalidate the generalization of Vesque and Weiss that the 
inner cambium of bicollateral bundles is a ‘false’ cambium 
