10 
PACIFIC SCIENCE, Voi. XIII, January 1959 
Figs. 14-16. Portions of paradermal sections in which ultimate areoles are probably being delimited (or have 
been delimited), X 500. Figure 14 ^ {left): Secondary vein at left margin; other large veins in these figures are 
tertiaries and quaternaries. (Fig. 15, top right'. Fig. 16, lower right.) 
ent random spatial relationship among various 
small groups of minor veins is a result of the 
apparent random planes of cytokinesis in each 
original ground meristem cell as compared 
with its neighbors. 
A comparison of transverse sections in suc- 
cessive stages of leaf ontogeny (Figs. 17, 18) 
demonstrates that the sclerenchyma and vas- 
cular tissues of a vein have a common origin 
from an original procambial initial by a series 
of longitudinal divisions periclinal to the leaf 
surface. The lowermost cell or several cells 
then subdivide in various longitudinal planes 
to produce a strand of narrow procambial 
cells. During the earlier phases of differentia- 
tion these two components of the fibro- 
vascular system of the leaf are not readily 
distinguishable in paradermal section. There- 
fore, it is only in the later phases of leaf de- 
velopment that we can distinguish clearly the 
future sclerenchyma from the actual con- 
ductive tissues. Figures 19 and 20 show the 
same area of a lamina at two levels; Figure 20 
shows a section 12 yt closer to the upper sur- 
face than Figure 19. In Figure 19 there are 
shown a number of probable ultimate areoles 
delimited by obvious procambial strands, 
forming a pattern characteristic of the mature 
venation of the species. At this level in the 
relatively immature lamina (15 mm. long) 
the plate meristem has apparently been fully 
segregated into procambium and funda- 
mental tissue with a reversion to a com- 
pletely random insertion of anticlinal walls 
producing groups of more or less isodiamet- 
ric cells (viz., Fig. 8). Apparently no more 
procambial strands are to be formed at this 
level. Just 12 above the latter section we 
find the pattern much more intricate and ap- 
parently still in the process of differentiation. 
The cells delimited from the ground tissue in 
Figure 20 are the initials of the sclerenchyma. 
Careful examination of the figures under dis- 
cussion reveals that in addition to a series of 
