172 Gibson. — Contributions towards a Knowledge 
protoxylem-strands can be distinguished on the outer surface 
of the xylem-mass (PL IX, Fig. 31). Lower down, however, 
no spiral elements are visible on the outside of the stele and 
the leaf-traces appear to plunge directly into the xylem. The 
meaning of this soon becomes apparent if serial sections be taken 
of the stem at different levels. At first the stele is seen to con- 
sist of a pericycle, phloem, and central xylem-mass, having on its 
outer margin seven prominent protoxylem-areas. In successive 
sections these gradually fuse amongst themselves until only 
three marginal protoxylems are visible. These are, however, 
not prominent ; on the contrary, they are more or less sur- 
rounded by metaxylem elements, save where a leaf-trace is 
inserted (PI. IX, Fig. 36). Finally, in the trailing portion of 
the axis, the protoxylems fuse into a central patch entirely 
surrounded by metaxylem (PI. IX, Fig. 37). The leaf-traces 
can be quite readily seen piercing the metaxylem to become 
united with this central protoxylem. 
The stele at different levels differs entirely in detailed 
structure. Near the apex one finds a one-layered pericycle 
covering a phloem which consists of (a) seven patches of sieve- 
tubes, one layer deep, and (b) one to two layers of parenchyma. 
Centrally there is a small-celled metaxylem of scalariform 
tracheides with seven patches of protoxylem alternate in 
position with the sieve-tube areas. In the trailing axis, on the 
other hand, the one-layered pericycle (which is very well 
marked) encloses a phloem-tissue one to two layers deep, 
whose elements have exactly the appearance of the proto- 
phloem-elements (PI. X, Fig. 39). They possess thick, brightly 
refractive cell-walls, and little or no contents. The metaxylem 
consists of fewer and larger scalariform tracheides enclosing a 
patch of a dozen or so spiral and annular tracheides. Frequently 
the metaxylem elements abut directly on the phloem, paren- 
chyma-cells being here and there entirely absent. As the 
protoxylem becomes external, the normal sieve-tubes appear 
and the semi-occluded protophloem-elements disappear. The 
trabeculae are normal in the erect stem, although in the creep- 
ing portion the cuticle does not separate as a distinct annulus. 
