NODAL ANATOMY AND THE MORPHOLOGY OF STIPULES 44/ 
of the nodal anatomy suggests an explanation for this fact, for the 
stipules are invariably inserted directly opposite the points of origin 
of the two lateral bundles (figs, i, 4 and 5), really as continuations of 
the two swellings on the surface of the stem caused by the exit of the 
traces. If we go back to the primordium of such a leaf at the growing 
point we find that it usually begins as a three-lobed structure, the 
central lobe giving rise to the petiole and blade and the lateral ones 
to the stipules. These three lobes mark the future position of the 
three leaf -traces. In subsequent development the stipules usually 
grow considerably, often equalling or exceeding the young blade for 
a time {fig. 2), and still showing their relation to the three swellings 
which mark the position of the procambial strands that are to give 
rise to the leaf-traces. The traces, however, when they are finally 
formed, do not enter the three primordial lobes of the leaf but all 
three converge into the central one, which is to form the petiole and 
blade. Each stipule obtains its vascular supply, if it has one, from 
branches derived from the corresponding lateral traces (figs. 1 and 5). 
These facts as to the topography of the vascular system at the base 
of the petiole and as to the innervation of the stipules have been 
worked out by numerous investigators, particularly by Colomb (i). 
He makes the origin of the vascular supply to the stipules their most 
distinguishing character and defines a stipule as "an appendage 
inserted on the stem at the base of the leaf, all the bundles of which 
are derived entirely from the corresponding foliar bundles." 
The vsignificant fact, however, seems to be the intimate connection 
between the stipules and the swellings opposite the lateral leaf-traces 
This is even nianifest in the case of stipules which are adnate to the 
petiole for a considerable distance, as in certain of the Rosaceae and 
many other plants ; for here the base of the free portion of each stipule 
(its oldest part) is directly opposite the point of origin of one of the 
lateral traces {fig. 3), although the subsequent growth of the basal 
portion of the leaf {a-b) carries the stipules up and renders their con- 
nection with the lateral bundles, except in very young leaves, less 
obvious. In a unilacunar node there is of course only one swelling 
{fig. 6) and this developes directly into the petiole and blade without 
an accompanying formation of stipules. All these facts suggest the 
conclusion that the early growth of the tissue which is to form the 
lateral traces exerts a stimulating influence which results in the forma- 
tion of a considerable body of tissue, the stipule. It is almost a^if a 
