Scott and Hill.—Since hire of Isoetes Hystrix . 453 
xylem ; x 2 , secondary xylem, in course of differentiation ; ph, phloem, showing 
clathrate cell-walls; cb, cambium, x 442. 
Fig. 8. Similar section, from another stem, at the commencement of secondary 
growth, x, primary xylem; cb, cambium; ph , phloem, lying outside cambium. 
X 300. 
Fig. 9. Secondary tracheide, x 2 , in longitudinal section, isolated among cells of 
the secondary parenchyma. The primary wood is on the side X. x 360. 
Fig. 10. Secondary tissues in longitudinal section, from an old part of stem. 
X, direction of primary wood ; ph 1 , obliterated secondary phloem ; x 2 , secondary 
xylem; cb, cambium; c, secondary cortex. X 180. 
Fig. 11. Transverse section, showing well-developed secondary xylem, x 2 . 
cb, cambium ; X, direction of primary wood, x 325. 
Fig. 12. Secondary xylem (x 2 ) and phloem (ph 2 ) in transverse section. The 
latticed structure of the phloem cell-walls is well shown. Z. T, phloem of leaf-trace, 
continuous with that of stem ; X, direction of primary wood, x 340. 
Fig. 13. Diagrammatic transverse section of the upper, cylindrical part of the 
stele, x, primary wood; ph 2 , region in which secondary phloem ; x 2 , that in which 
secondary xylem is developed ; cb, cambium ; c 1 , secondary cortex ; l.t, leaf-traces. 
X 68. 
Fig. 14. Diagrammatic transverse section of the lower, triquetrous part of the 
stele, rt , adventitious roots; other letters as before, but no secondary xylem 
present, x 22. 
Fig. 15. Transverse section, showing one arm of the triquetrous part of stele. 
x, primary xylem, much disorganized; cb, cambium; rt, root-traces, of which 
several are shown; ph, phloem, and x, xylem, of a root-trace, x 213. 
PLATE XXIV. 
Fig. 16. Phloem (ph 2 ) from stem, showing pitted cell-walls, and beginning of 
formation of callus (?). x 576. 
Fig. 17. Obliterated cells of phloem (ph 2 ). x 576. 
Fig. 18. Diagrammatic transverse section of lamina of leaf, showing intercellular 
cavities, stomata, fibrous strands, and vascular bundle, x, xylem ; ph, phloem; 
Gc, stoma, x 17. 
Fig. 19. Transverse section showing structure of foliar bundle in the lamina, x, 
xylem, five tracheides shown; px, canal, marking position of primitive tracheide; 
a fragment of its lignified wall is shown; to the right and left are the other two 
xylem-canals ; ph, phloem, x 310. 
Fig. 20. Transverse section (cut just above sporangium) of a very young foliar 
bundle, px, the primitive tracheide, from which the central canal will be formed ; 
this is at present the only lignified element; pph, protophloem, x about 430. 
Fig. 21. Transverse section from a partly developed leaf, showing a persistent 
transverse wall of the protoxylem, forming a perforated diaphragm across the canal, 
x 680. 
Fig. 22. Foliar bundle of lamina, in radial section, x, a tracheide of the xylem ; 
px, protoxylem, now forming a canal, crossed by a persistent transverse wall; ph, 
phloem, x 296. 
Fig. 23. Corner of lamina in transverse section, showing a fibrous strand, 
x 280. 
