CONSERVATISM AND VARIABILITY IN SEEDLING OF DICOTYLEDONS 12 7 
A second feature of the structure of the seedling which is constant 
throughout large groups is the venation of the cotyledon. In the great 
majority of the families this is palmate and three-veined (3, Plate III), 
a condition which we have reason to believe is primitive for the 
Angiosperms. In a few cases, however, it seems to be constantly 
pinnate, with a strong midrib. This is characteristic, so far as the 
writer's observations have gone, of the Amaranthaceae, Anacardiaceae, 
Boraginaceae, Capparidaceae, Celastraceae, Ebenaceae, Moraceae, 
Myoporaceae, Nyssaceae, Periplocaceae, Pittosporaceae, Polemoni- 
aceae, Rutaceae, Simarubaceae, Solanaceae, and of portions of other 
families. In all these cases the cotyledonary node is unilacunar, so 
that there is in the seedling a similar relation between nodal topography 
and leaf venation which has been found to occur in the mature 
plant (3). 
The most constant and invariable character of the seedling, how- 
ever, is the double nature of the cotyledonary trace, a fact emphasized 
by the work of Miss Thomas (4). In the ferns and gymnosperms the 
trace of the mature leaf, where it leaves the vascular ring, is at least 
primitively either a double bundle or an arc with an even number of 
bundles. A radical change brought about at the origin of the Angio- 
sperms was the conversion of this double bundle into a single one or 
into an arc with an odd number of strands. This change is made 
evident externally by the development of the strong midrib so char- 
acteristic of the leaf of Dicotyledons. In the seedling of the Dicoty- 
ledons, however, we find the original condition persisting. The vena- 
tion of the cotyledon, to be sure, is angiospermous, with a midrib and 
lateral veins, which distinguish it from the cotyledons of the gymno- 
sperms.^ At the cotyledonary node, however, the ancient double 
trace still persists unchanged. In its essential topography the node of 
the seedling is the same throughout all seed plants. In figure 4, a, is 
shown the node of Ephedra, presenting the typical gymnospermous 
condition, and in the other figures are some of the types found among 
dicotyledons. There may simply be two traces to each cotyledon or 
there may be two pairs of traces. In the trilacunar type it will be 
noted that an even number of bundles is given off, due to the fact 
that the central gap provides two strands. 
It is usually in the petiole of the cotyledon that the transition 
^ In many conifers, where the seed-leaves are numerous and needle-like, a single 
bundle is sometimes all they possess. 
