82 A. H. Graves, 



bundle, but the product of the gradual centripetal union of many 

 stem bundles, and is therefore not comparable to the concentric 

 bundle of the majority of the Gleicheniaceae, for example. This 

 is true both of mono- and dicotyledonous water plants. 



In support of this theory Schenck shows how there exists at the 

 present time in different aquatic and amphibious species, every 

 gradation from the typical bundle arrangement common to land 

 plants to the simplest structure as exemplified in the submerged 

 aquatics. He says, " Im Laufe der phylogenetischen Entwicklung 

 riickten infolge immer welter gehender Anpassung der Structur an 

 die Lebensweise der Pflanze unter Wasser diese Leitbtindel bei 

 gleichzeitiger Reduction des Xylems nach der Axe zusammen zu 

 einem gemeinsamen Strang, in welchem die Xylemteile nach und 

 nach zu einem einzigen axilen Korper verschmolzen, wahrend die 

 Phloemteile, ihre normale Lage nach aufien beibehaltend, zu einer 

 Ringzone sich vereinigten." 



According to this interpretation therefore, Ruppia is descended 

 from a form with four distinct main vascular bundles, traces of which 

 are still apparent in the four phloem regions at the nodes; for 

 here, as is generally admitted, ancestral characters are wont to 

 appear. 



Summary. The stem is composed of a thin walled epidermis; 

 a loosely constructed cortex, containing as its principal feature a ring 

 of radially arranged lacunae, and also two small bundles ending in 

 the cortex — fibro vascular connections with the leaf sheaths ; and 

 a well defined endodermis, surrounding a very simple axial vascular 

 region of a concentric structure, the appearance of which at the 

 nodes indicates its reduction from four vascular bundles. 



b. Rootstock. 



The branching of the rootstock, or horizontal axis, as already 

 stated, conforms strictly to the monopodial type, although torsion 

 and the decay of the leaves often renders this obscure. 



In its internal structure, as would be expected, it agrees with 

 the stem (PL III, fig. 9). No points of difference were noted, except 

 that sometimes the cells are somewhat larger than is customary in 

 the upright axis, and often contain a large amount of starch. 



The main differences between this and its morphological equivalent, 

 the upright axis, are its horizontal position and the occurrence of 

 roots at the nodes 



