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 
probably 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 
may 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 system, 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 (Prillieux, 1864, p. 182) 
with two or sometimes more, borne on alternate nodes, most resemble 
it. The roots of Cymodocea (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 (Grénland, 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 peculiar sheath-like structure, termed by 
. Irmisch (1858) the “coleorrhiza” or root sheath (Pl. VII, fig. 38; 
Pl. VUI, fig. 45, Text-fig. 21). Beyond mentioning the fact that it 
occurs in Potamogeton crispus, Zannichellia palustris 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 
