6 
EDMUND W. SINNOTT AND IRVING W. BAILEY 
the vascular supply to the leaf causes either a single gap (unilacunar 
type, fig. 85), three gaps (trilacunar type, figs. 83 and 84), or more than 
three gaps ; that these types are very constant within large groups of 
plants, and that the trilacunar condition is probably the most ancient 
of all, the other two having been derived from it by amplification or 
reduction. It is pointed out that the main distinction lies really be- 
tween the tri- and multilacunar, on the one hand (which in this paper 
we shall designate together as multilacunar) , and the unilacunar on the 
other. That the former type is almost certainly more ancient than the 
latter is made evident by the fact that it characterizes the vast majority 
of all those orders (Amentiferae, Ranales, Rosales and Malvales) 
which are recognized as being relatively ancient and among which 
are presumably to be found the most primitive living Dicotyledons. 
The following table, based on the genera of Dicotyledons enumerated 
by Engler, also shows the distribution of these two nodal types among 
the trees, shrubs and herbs of the Archichlamydeae and Metachla- 
mydeae.^ 
Table III 
Genera. Multilacunar. Unilacunar. 
A rchichlamydeae 
Trees 972, 76% 909, 24% 
Shrubs 794, 71% 322, 29% 
Herbs 688, 59% 466, 41% 
Metachlamydeae 
Trees 19, 6% 269, 94% 
Shrubs 199, 20% 767, 80% 
Herbs 752, 43% 994, 57% 
Metachlamydeae, excl. Compositae 
Trees 4, 1% 269, 99% 
Shrubs 26, 3% 767, 97% 
Herbs 134, 10% 994, 90% 
It is evident that the great majority of the Metachlamydeae 
(especially if we exclude the Compositae) are unilacunar and that 
the multilacunar forms are massed in the Archichlamydeae. The 
contrast is much more striking among woody plants (which are 
really the only ones significant in our problem) than among herbs. 
This predominance of the multilacunar node among the older orders 
and in the more primitive of the two great subclasses; and the con- 
^ Analyses of nodal type are based on a study of a very large number of species 
in about 700 genera, from practically all the families. 
