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 elaborated 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, through a 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 node of the creeping stern 

 — a simplicity of form and development which 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 alread}^ been pointed out, the development of a coleorrhiza 

 or root sheath may have some bearing on the environment, protecting 



