162 A. H. Graves, 
Seed and Fruit. The fruit is an achene, the hard coat being 
derived from the inner portion of the ovary wall. 
Seedling. At germination the cotyledon develops chlorophyll, 
and the adventitious root of the embryo is the first functioning 
root. 
At first the young seedling is upright, but very soon a horizontal 
root-stock is developed. 
SUMMARY. OF RELATIONSHIPS TO OTHER 
POTAMOGETONACEAE. 
On account of the incomplete knowledge we have at present of 
the members of the Potamogetonaceae, a thorough comparison with 
them in all points of gross morphology and of anatomy is of 
course impossible. In the course of this paper I have from time 
to time alluded to the points of likeness and difference with re- 
spect to the remaining Potamogetonaceae, as far as I have been 
able to glean such from the various articles cited. It is my pur- 
pose now to briefly review these here, touching upon the various 
characters in the order pursued in my paper. 
As regards the methods of branching, Cymodocea, Phyllospadix, 
and Zostera most resemble Ruppia, with a monopodial system in 
the rootstock and its branches, and a sympodial inflorescent system. 
In stem anatomy, perhaps Potamogeton pectinatus and Zostera 
marina resemble Ruppia most closely, while Zannichellia and Althenia 
are also very similar in all respects except that they lack the cor- 
tical bundles. The evidence here seems to point to the conclu- 
sion that Zostera and Potamogeton are more primitive genera, 
while, on the other hand, Zannichellia and Althenia are more re- 
duced, with Ruppia somewhere between. 
The leaf of Ruppia shows a great similarity in external form 
to certain species of Potamogeton and to Zannichellia and Althenia. 
I do not find the secretion cells of the leaf—or indeed of any 
part of the plant—reported for any but Cymodocea. Posidonia, and 
Halodule, besides Ruppia. In other points of anatomy, however, 
the leaf of Potamogeton pectinatus is very similar, while the leaves 
of Zannichellia and Althenia show a strong resemblance, but with 
absence of the lateral bundles. The natural conclusion based on 
this evidence is, as stated before, that Ruppia is descended from 
some form similar to the present submerged Potamogetons, and 
also that Zannichellia and Althenia are still further reduced. 
