Again, the medium in which an organism lives--whether aërial or aqueous--has an important relation with its form. A delicate seaweed, the beautifully radiating form of which is a just object of admiration as long as it is supported by its denser natural medium (the sea water), collapses into an amorphous mass when withdrawn thence into the thin air. Obviously a much greater rigidity and strength of structure is needed to support an aërial organism than an aquatic one, unless the former can support itself on other solid structures, such as rocks or trees. In the latter case the form attained may be very elongated and slender, as in the many creeping and climbing plants, which are so often furnished with processes for grasping (tendrils) to aid them in their mode of life. An aërial fixed organism, if it does not rise from the surface of the earth, cannot spread itself very far without developing other points of support--without rooting again. This re-rooting is a familiar phenomenon in many plants, as, _e.g._, the strawberry. But even a shrub like the common bramble (which is not itself prostrate, but which sends out extraordinarily prolonged branches) is aided by such a process. The ends of its long branches apply themselves to the ground and begin to pierce its surface, the incipient leaves of its terminal bud becoming metamorphosed into roots. An aquatic fixed organism, however, may extend to a very great length, freely floating without effecting any such fresh attachment. Thus the seaweed _Laminaria digitata_[61] will spread over a circle 12 feet in diameter, while _L. longicornis_ grows in the form of an elongated riband, from 8 to 12 feet in length and 2 or 3 feet wide. The giant form _Macrocystis_ (with a much more subdivided outline) may extend to the extraordinary length of 700 feet. The conditions under which needful gaseous interchange can be effected and food obtained by different living creatures, govern in various other ways the forms of their bodies. Thus, if it is helpful to the life of a creature to submit as large a surface of its body as possible to the influence of light, or to the action of air or water, then for this purpose its body must be expanded and its expanded parts divided and subdivided as they extend in different directions. It is for this reason that trees branch, and that their branches and twigs divide and subdivide as they do. It is for this reason also that their branches do not grow out one above another in precisely the same direction, but, on the contrary, grow in such a manner that each one may overshadow those immediately beneath as little as may be. Similarly and for the same reason leaves are developed mostly in an alternating fashion, so that each may be able to expose its green surface to the light and air as much as possible.