Foliar Venation — Pray 
7 
Fig. 6. Transverse section of mature lamina showing 
general leaf histology, X 200. Tracheary elements 
indicated with black walls; sclerenchyma with stippled 
walls. 
ONTOGENY OF THE VENATION 
Throughout blade ontogeny a basipetal 
pattern of maturation prevails, both in the 
initiation of secondary veins and in the dif- 
ferentiation of the minor venation between 
the secondaries. When the intersecondary 
panels are first delimited the cells of the plate 
meristem which will produce the minor vena- 
tion are essentially isodiametric (Figs. 8, 17). 
The insertion of new cell walls anticlinal to 
the surface during this phase is apparently 
random. In a panel of such isodiametric cells, 
localized, oriented divisions in a continuous 
series of cells produce the first procambial 
strands (tertiaries) (Fig. 9) in a manner simi- 
lar to that described for Liriodendron (Pray, 
1955^.” 21). Concurrent with the differentia- 
tion of the tertiary procambial strands the 
nature of the intervening ground meristem 
becomes noticeably altered with the establish- 
ment of a general tendency for the cells to 
become markedly longer than wide (Figs. 7, 
10, 13). This condition is due largely to re- 
peated cytokinesis of a given meristem cell in 
the same plane producing small packets of 
similarly elongated cells (Figs. 10-14). Simi- 
lar divisions in two or possibly three contigu- 
ous cells can produce the same effect. The 
latter apparently happens infrequently be- 
cause there is a decided tendency for the 
planes of cell division in adjacent cells to be 
completely unrelated and, in fact, they are 
rather frequently more or less perpendicular 
to one another. The tendency for small paral- 
lel groups of ground meristem cells to be 
formed, each independently oriented with re- 
spect to their neighbors, is characteristic of 
the ground meristem during the phases of 
leaf development concurrent with the forma- 
tion of the minor venation as illustrated by 
examples in Figures 10-13. In a given section 
(Figs. 10-13) elongate cells are not evenly 
distributed. Sometimes small areas will dis- 
play considerable regularity while others of 
the same leaf will have a rather sporadic ex- 
pression of this tendency. It is from such 
parallel groups of cells that series of similarly 
oriented procambial strands are delimited 
(Fig. 16). Thus the essential nature of the 
minor venation of Bobea is determined by 
planes of cell division in the ground meristem 
immediately preceding procambial differenti- 
ation. 
In the development of the minor venation 
(exclusive of obvious tertiaries) the delimita- 
tion of the procambium from the ground 
meristem appears to follow a rather orderly 
procedure when studied in paradermal sec- 
tion. A series of elongate cells derivable from 
subdivisions of a single cell or several ad- 
jacent cells is formed in this process. Most of 
these will subsequently redivide perpendicu- 
larly to the previously predominant plane of 
division, while one or sometimes several re- 
main undivided. The elongate cells thus de- 
limited are precursors of procambial strands. 
This series of steps can be illustrated by the 
following figures. In Figure 11 (top) there is 
a group of cells elongated perpendicularly to 
the course of the two procambial strands 
delimiting the areole. If, then, two or more of 
these cells remain undivided while the inter- 
vening ones further subdivide by a series of 
divisions at right angles to their long axes, 
the initials of procambial strands separated by 
a group of nearly isodiametric cells which are 
potentially ground tissue will be delimited. 
This apparently has occurred in Figures 12 
(upper left) and 14 (upper right). This same 
