308 
EDMUND W. SINNOTT 
phery of the stem, much as in the multilacunar dicotyledons save that, 
of course, a continuous vascular ring with typical leaf gaps is absent. 
In families which are admitted to be very primitive, however, such as 
the Potamogetonaceae, a much simpler condition prevails. The 
anatomy of various members of this family has been investigated by 
Chrysler (2), who comes to the conclusion that the genus Potamogeton 
is the most primitive in the group; and that its more robust species, 
such as P. pulcher, represent the original type from which the others 
have been reduced. The vascular tissue in this form is arranged in a 
cylinder, as in the dicotyledons. The foliar traces are three in number. 
After running for a short distance in the pith, they leave the cylinder 
from three distinct points {fig. 2q), thus presenting a striking re- 
semblance to the trilacunar type which we have regarded as primitive 
for the dicotyledons, and suggesting the way in which the more com- 
plicated nodal structure of the higher monocotyledons ha§ arisen. 
The young plants of many of these higher types show decided 
anatomical resemblances to the primitive monocotyledons and to the 
dicotyledons. Chrysler (i) has investigated the anatomy of the 
seedling in several groups, particularly the Araceae and Liliaceae. 
From his description and figures it is evident that in a great many 
instances the first few leaves are supplied with three bundles, each of 
which arises separately and causes a gap of its own in the cylinder 
which in these earlier stages much resembles the woody ring of dicoty- 
ledons. Figure 30 shows the node of a seedling of Acorus calamus 
with the three traces departing to one of the earlier leaves. 
If such forms as the Potamogetonaceae are actually primitive among 
monocotyledons, and if the structure of the young plant is rightly 
regarded as displaying primitive characters — conclusions for both of 
which there seems to be ample evidence — then it would certainly appear 
that the monocotyledons have had their origin from plants possessing 
a medullated ring of vascular tissue and the trilacunar nodal condition 
which we have regarded as primitive for dicotyledons. 
We have already remarked that almost every family of the dicoty- 
ledons has a particular type of nodal structure which is extremely 
constant. That it is almost entirely independent of the size, shape or 
mode of attachment of the leaf is also apparent. The large pinnate 
leaves of Fraxinus, Juglans and Daucus, which are essentially similar 
in size and shape, are provided, respectively, with one, three and many 
traces, each of which arises independently from the vascular cylinder. 
