3o6 
EDMUND W. SINNOTT 
a foliar trace of three rather distant strands, each causing a gap of its 
own in the stele. In certain genera and species, however, such as 
Sisymbrium leiocarpum {fig. 13), these three bundles approach each 
other very closely and are separated only by two very small segments 
of the cylinder. In all the other Cruciferae the departure of the foliar 
supply causes but a single gap in the wall of the cylinder, but in such 
cases the leaf trace itself almost always consists of three bundles 
{fig. 14). In the Cruciferae the three originally distant strands thus 
seem to have gradually become approximated until by the disappear- 
ance of the separating segments of the stele, they come off close together 
and cause but a single gap. In the Dilleniaceae a very similar transi- 
tion is also evident. 
In many of the groups of plants which are characterized by a 
unilacunar nodal structure the leaf-trace at its origin frequently 
consists, as in the Cruciferae, of three distinct strands or of a deeply 
three-lobed one {photo. 3). Such a structure is particularly common 
in those great unilacunar orders the Centrospermae and the Tubiflorae 
and indicates that the nodal condition in these and similar groups has 
arisen by reduction from the primitive three-bundled type. 
In certain instances even this triple division of the leaf-trace has 
been lost in the older parts of the plant, as is shown by figure 8, which 
is a section through the mature stem of Chenopodium album. In the 
young plant, however, where the whole stem structure is simpler, the 
leaf-trace consists of three quite distinct strands {fig. 7). Such 
"recapitulations" are of frequent occurrence. 
Among such families as the Amaranthaceae which are characterized 
by anomalous growth of the vascular tissue, the leaf- trace is often 
exceedingly complicated at maturity {fig. 10). In the simpler members 
of the family, however {fig. 9) , and in the seedlings of the complex forms, 
the primitive triple condition is retained. The ancestors of such plants 
probably possessed the nodal structure of primitive angiosperms, with 
three isolated bundles departing to the leaf. These first became 
approximated and the gaps fused; and later the foliar supply under- 
went various further modifications. 
The unilacunar condition seems to have been derived from the 
trilacunar, in certain families, by the abortion of the two lateral strands 
instead of their fusion with the central trace. In Ilex opaca {fig. 19), 
for example, the three typical bundles are usually present, but one or 
both of the lateral ones tend to be very small. In all other members 
