120 A. H. Graves, 
and on the other hand, the transpiration current, unaided in its work 
by stomata or even by apical leaf pores, and, moreover, rendered un- 
necessary by the presence of water on all sides, must inevitably be 
diminished. 
That the sieve tubes and their companion cells should remain 
intact simultaneously with a complete degeneration of the xylem, 
seems reasonable when we consider that the function of the phloem 
is the transportation of e/aborated food. 
In another respect the reduction of the vascular system is of 
ecological importance ; for not only are the vascular elements reduced 
quantitatively, but also the fact that practically all of the vascular 
system is concentrated into a single bundle, which is axial, deserves 
especial note. 
That this axial position of the bundle in the stem is considered 
to have been attained phylogenetically, througha gradual displacement 
of the more peripheral bundles toward the center of the stem and 
fusion there into a single concentrically arranged bundle, has al- 
ready been touched upon in the account of the morphology of the 
stem (p. 81-82). 
Ecologically considered, this axial arrangement enables the plants 
to bend about easily and accommodate themselves to the move- 
ments and currents of water. For the axial bundle of the stem, 
with its slightly thickened endodermis enclosing the long cells of 
the vascular tissue, is naturally the region most resistant to ben- 
ding movements. Now, it may easily be seen that a plant with 
its vascular area in such a position is capable of bending much 
more readily than one in which this area is more or less peri- 
pheral, as in the typical land plant. Such an arrangement is ana- 
logous to the axial strand of roots, and there subserves a similar 
purpose. 
B. The Root 
1. Gross Morphological Adaptations. 
Another result of the absorption of nutrient solutions by the shoot 
system directly from the surrounding medium is the great reduction 
of the root system. This consists entirely of slender, unbranched, 
adventitious roots arising singly at each gode of the creeping stem 
—a simplicity of form and developme hich is correlated with 
the function of the root, and which has been already discussed in 
detail under the morphology of the root (pp. 103-114). 
As has already been pointed out, the development of a coleorrhiza 
or root sheath may have some bearing on the environment, protecting 
