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 infolee immer weiter gehender Anpassung der Structur an 
die Lebensweise der Pflanze unter Wasser diese Leitbiindel bei 
eleichzeitiger Reduction des Xylems nach der Axe zusammen zu 
einem gemeinsamen Strang, in welchem die Xylemteile nach und 
nach zu einem einzigen axilen Kérper verschmolzen, wahrend die 
Phloemteile, ihre normale Lage nach aufsen 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—fibrovascular 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 (PI. 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 
