300 Scott and Brebner . — On Internal Phloem , &c . 
Fig. 10. Part of a transverse section of a young octarch root at the commence- 
ment of secondary thickening. The cambium is forming inside the isolated proto- 
xylem-elements. Primary in terxylary phloem-groups are in course of development. 
X 300. 
Fig. 11. Do. do. from a decarch root, at a stage very slightly later than 
Fig. 10. Four protoxylem-groups shown. To the left the cambial divisions take 
place inside the protoxylem. The cambium passes outside the next two groups, 
and on the right divisions are seen on both sides of the protoxylem-element. This 
root had a considerable pith containing a large group of internal phloem, x 300. 
Fig. 12. Do. do. from a root at nearly the same stage as Fig. 9, showing 
the centrifugal development of the phloem-islands on the inner side of the cam- 
bium. x 300. 
Figs. 13 and 14. Apocynum cannabinum. 
Fig. 13. Part of a transverse section of a stem in which the normal wood is 
70-90 cells thick. The development of centripetal wood from the internal cam- 
bium has just begun. It is separated from the protoxylem of one of the primary 
bundles by one or two layers of parenchyma, x 1 70. 
Fig. 14. Similar section from an older stem. The internal wood is now about 
15 cells thick. The internal phloem projects far into the pith, x 170. 
Fig. 15. Willughbeiafirma. 
Part of a transverse section from a thick stem, showing the centripetal wood 
and internal cambium and phloem, and their relation to the protoxylem of the 
normal wood, x 300. 
Figs. 16-20. Acantholimon glumaceum. 
Fig. 16. Semi-diagrammatic transverse section of an old stem, to show the 
distribution of the tissues. Red — protoxylem. Yellow — xylem generally. 
Blue — phloem. The cortex, rays, and pith are left white. 
Fig. 17. Part of a transverse section showing the inner part of the normal 
wood, the centripetal wood, and the internal cambium and phloem. The centri- 
petal wood is alone shaded, x 300. 
Fig. 18. Longitudinal section of the same. Notice specially the loosely coiled 
spirals of the protoxylem-vessels, on the inner side of which the centripetal wood 
is forming. Some of the internal sieve-tubes have callous plates, x 300. 
Fig. 19. Transverse section showing part of an interxylary band of cambium 
and phloem. The cambium lies on the inner side of the phloem, x 300. The 
external zone of wood {oca) begins with large vessels. 
Fig. 20. Longitudinal section of the same. The callous plates of the older 
interxylary sieve-tubes are shown. The outer part of the inner zone of wood (x 2 ) 
consists chiefly of narrow spirally thickened vessels, while the inner part of the 
outer zone {x 2 d) has large pitted vessels, x 300. 
All the figures were drawn under the camera lucida. Those x 300 were drawn 
with a power of 450 diam, and subsequently reduced to two-thirds. 
