THE ANATOMY OF THE NODE IN ANGIOSPERMS 305 
particular number of bundles and gaps is extremely constant among 
related forms, and almost every family has a characteristic nodal 
formula which is subject to little variation. 
As to what was the primitive structure of the node among angio- 
sperms we cannot be sure, but in all probability the earliest condition 
was rather plastic and variable. It seems clear, however, that a 
foliar supply of three bundles, each causing a gap of its own in the 
stem cylinder, is certainly a very ancient type among the dicotyledons. 
We may designate this as the trilacunar condition (figs. 4, 5, 11, etc.). 
It characterizes most of the members of the Piperales, Salicales, Myri- 
cales, Juglandales, and Fagales — in short, of the former Amentiferae; 
and is present in the great majority of the Ranales and Rosales, as 
well. Since in all likelihood one of these great groups approaches the 
primitive angiosperms (or at any rate the primitive dicotyledons) in 
its character, we may feel reasonably sure that the trilacunar condition 
became fixed in the angiosperm line very far back. This is rendered 
still more probable by the fact that all other variations of nodal 
structure in the phylum seem to have been derived, as we shall see, 
from such a three-bundled type. This type is not confined to the 
lower Archichlamydeae but also characterizes in general the Rhoedales, 
Geraniales, Rhamnales, Malvales and many Sapindales and Parietales 
among the Archichlamydeae; and the Plantaginales, Cucurbitales, 
Caprifoliaceae and Compositae among the Metachlamydeae, thus 
occurring in the majority of dicotyledons. 
This ancient trilacunar condition has been modified both by re- 
duction and by amplification. In many orders we find that only a 
single gap is left by the foliar supply, whether the latter is single or 
multiple (figs. 14, 16, 20 etc.). Such a single-gapped or unilacunar 
type of nodal structure characterizes all the Centrospermae, most of 
the Myrtiflorae and numerous families of the Sapindales and Parietales 
among the Archichlamydeae; and the Ericales, Primulales, Ebenales, 
Contortae, Tubiflorae and families in the Rubiales and Campanulatae 
among the Metachlamydeae. That this simple condition is not a 
primitive one, however, but has been derived by reduction from the 
trilacunar type is indicated by a study of the node in those few families 
which are transitional from one type to the other. 
The Cruciferae are perhaps the best example of such a family. 
Most of the genera in the Thelypodieae, Sisymbriineae, Cochleariineae 
and Brassicineae, as well as. scattering genera in other tribes, possess 
