104 A. H. Graves, 



1. The line connecting the points of insertion of the roots is 

 straight, provided the stem has undergone no torsion (Text- 

 fig. 22). 



2. On account of the alternate arrangement of the leaves, the 

 roots appear at successive nodes on opposite sides of the median 

 longitudinal plane of the leaf, — i. e., if at one side of the leaf at any 

 node, then at the other side of the leaf at the next higher or lower 

 node (Text-fig. 22). 



These simple rules of arrangement are, however, not often apparent. 

 In a great many instances the stems to a considerable extent, due 

 probabty to currents of water, light conditions, &c., become 

 twisted. 



In such cases the orientat ionof the roots with regard to the 

 leaves remains nevertheless constant, in accordance with the rule 

 stated in (2). In other words, no matter what position the leaves 

 ma}' assume on the stem, the roots always emerge, in any two 

 successive nodes, at one side of the leaf at one node, and at the 

 other side of the leaf at the next node. 



Compared with the other Potamogetonaceae. Ruppia has perhaps 

 the simplest root sj-stem, if one considers the number of roots alone. 

 Zannichellia (Campbell, 1897, p. 40), with generally two slender, un- 

 branched rootlets at each node, and Althenia (Prillieiox, 1864, p. 182) 

 with two or sometimes more, borne on alternate nodes, most resemble 

 it. The roots of Cjmodocea (Bornet, 1864) are often borne singly, 

 but differ in being branched. The roots of many Potamogetons 

 (Irmisch, 1858) are more numerous, arising often in a circular line 

 of insertion at the nodes. In Zostera (Gronland, 1851) and Phyllo- 

 spadix (Dudley, 1893) a cluster of roots occurs at each node. 



C. Anatomical Structure 

 1. The Coleorrhiza. 



Extending for a distance of often 5 mm. on the root from its 

 junction with the stem is a pecuhar sheath-like structure, termed by 

 Irmisch (1858) the " coleorrhiza " or root sheath (PL VII, fig. 38 ; 

 PI. VIII, fig. 45, Text-fig. 21). Be3'ond mentioning the fact that it 

 occurs in Potamogeton crispus, Zannichellia paliistris and Ruppia 

 rostellata, Irmisch gives no description of it. In the related plants 

 which Prillieux (1864), Magnus (1870), Schenck, (1886), Campbell 

 (1897), and others describe, no mention is made of such a structure. 

 Sauvageau (1889) indeed merely mentions a coleorrhiza as appearing 

 in Zostera. Bornet (1864) notes in Cymodocea the formation of a 



