Foliar Venation — Pray 
11 
initials being present above each procambial 
strand there are also at the higher level nu- 
merous additional anastomoses. The latter 
mature into sclerenchyma. Also noteworthy 
in Figure 20 are the several initials which end 
freely in the ground tissue. These are the 
precursors of the abundant fibers which ex- 
tend into the mesophyll in the mature lamina. 
Quite infrequently idioblastic cells (Fig. 20) 
are encountered which are presumably the 
initials of the occasional idioblastic fibers or 
ftbro-sclereids which occur in the leaf of 
Bobea, Finally it may be noted that the ground 
tissue at the level of Figure 20 appears to re- 
tain a greater predominance of rectangular 
cells, suggesting that cell division is still active 
with the possibility of further differentiation 
of sclerenchyma continuing later than at the 
level of the clearly distinguishable procam- 
bium. The continued meristematic activity of 
this region is understandable in view of the 
late maturation of the palisade typical of 
dicotyledonous leaves in general. 
DISCUSSION 
The older literature pertaining to the onto- 
genetic aspects of foliar venation and the 
modern histogenetic accounts have been re- 
viewed and discussed by Foster (1952: 752- 
755) and Pray (1955c: 701-706). The present 
account of the leaf of Bohea appears to sub- 
stantiate the ontogenetic pattern suggested in 
the latter paper. Namely, there is a definite 
correlation , between the shape and arrange- 
ment of the plate meristem cells which gives 
rise to the vein system and the venation pat- 
tern of the mature leaf. In fact, a remarkable 
similarity will be found when the pattern of 
cell shapes in the ground meristem (Fig. 13) 
is compared carefully with the pattern of 
areoles in the mature leaf (Figs. 2, 3). The 
pattern of polygonal areoles constituting the 
minor venation of the leaf, as exemplified by 
Liriodendron, is of very widespread distribu- 
tion in angiosperms. It is assumed as a work- 
ing hypothesis that such a pattern is a basic 
type from which the distinctive venation 
Figs. 17, 18. Transverse sections of laminae at two 
stages of development, X 500. Figure 17 {above): 
Original 6-layered condition (between secondary veins) 
with differentiation of a tertiary in the third layer from 
the adaxial surface. Figure 18 {below): Several stages in 
the development of minor veins and the occurrence of 
periclinal divisions in the various subprotodermal lay- 
ers. Stippling indicates provascular initials as distin- 
guished from those cells directly above which will 
become the sclerenchymatous portion of the bundles. 
types displayed by the Quiinaceae and certain 
members of the Rubiaceae have evolved by 
parallel trends toward a lineolate disposition 
culminating in a lineolate orientation of the 
entire intercostal venation. The venation of 
Bobea may be considered to be typologically 
intermediate between that of Liriodendron and 
Quiina. It is therefore interesting to note that 
the ontogeny of the foliar venation presented 
in this paper also may be regarded as inter- 
mediate between the two known extremes. 
In Bobea the first intercostal veins are delim- 
ited at a time when the insertion of anticlinal 
cell walls in the plate meristem which pro- 
duces procambium for the minor venation is 
apparently random. The tertiaries and quater- 
naries thus delimited display a pattern in the 
