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 ft-om 

 some form similar to the present submerged Potamogetons, and 

 also that Zannichellia and Althenia are still further reduced. 



