The other cases occur where circulation is arrested or checked in a different way; namely, in plants that are wholly submerged. These are the _Podostemaceæ_. Clothing as they do the submerged rocks, their roots play the part of rhizomes, being attached to the substratum by hairs and other processes, and having the leaf-bearing and flower-bearing shoots on their surfaces. The latter spread out more or less horizontally and are also fixed to the substratum in the same manner as the roots. Observe then the connexion of facts. One of these _Podostemaceæ_ needs no internal stiffening substance, for it exists in a medium of its own specific gravity; and being in a position to absorb water over its entire surface, it has no need for a circulation of crude sap--nor, indeed, in the absence of evaporation from any part of its surface, could any active circulation take place. Here, accordingly, the tracheal and mechanical elements are undeveloped. Though spiral vessels are not entirely absent, yet they are so rare as to do no more than verify the inference of phænogamic relationship drawn from the flowers. The method of agreement, the method of difference, and the method of concomitant variations, thus unite in proving a direct relation between the demand for support and circulation, and the existence of these vascular woody bundles which the higher plants habitually possess. The question which we have to consider is--Under what influences are these structures, answering to these requirements, developed? How are these internal differentiations caused? The inquiry may be conveniently divided. Though the supporting tissues and the tissues concerned in the circulation of liquids are closely connected, and indeed entangled, with one another, we may fitly deal with them apart. Let us take first the supporting tissue.