Plant-like animals which grow up in an arborescent manner from a fixed base do not generally branch in so regularly alternating a mode as do plants, and in some cases their successive branches may even be regularly superimposed. This is due to their not requiring, as plants do, that their surface should be very extensively exposed to light, neither their gaseous interchange nor their nutrition being impaired by such superposition. The water which carries to them both the nutritious particles on which they feed and the gases they respire, will act with nearly or quite the same efficiency in either arrangement of their parts. If the exigences of life require any organism to retain much fluid within it, this circumstance may lead to its assumption of a dilated more or less globular form, as in the melon cactus, and, to a less degree, in the leaves of the common stonecrop. But the conditions under which alone certain fixed organisms can obtain their food may govern also their internal structure. Thus, we shall see that in plants which feed by absorbing matters through their roots, an internal arrangement has to be effected for distributing material thus obtained, and conveying it upwards through the stem. So, again, many fixed animals need a greater supply of food and gases than they can obtain from the water which bathes or may reach them without effort on their parts. Such animals may be provided with special internal structures, which cause currents of water to flow towards them, and very often to penetrate within them, as in the shell _Mya_ or the razor shell.[62] Fixed subterranean creatures are rare, but such do exist, as, for example, the truffle (_Tuber cibarium_). Surrounding influences must in such instances be alike on all sides, while the imbedded position of such organisms render superfluous the development of any elongated process for the purpose of fixing them. Such creatures, then, have a spheroidal figure, and neither internally nor externally are their structures developed in special directions.[63] The fixed organisms which are the most aërial in their habits are attached to elevated objects, such as trees, and necessarily have a portion of their frame set apart to fix them to the object which supports them. The most conspicuous creatures of this kind are, perhaps, the plants termed "Epiphytes," on account of this habit. Amongst them may be mentioned the beautiful orchids called "air plants," and the familiar mistletoe. Other vegetable organisms--the multitude of creeping plants--rear themselves to great heights by the aid of their more robust brothers, but they can hardly be reckoned as aërial organisms.[64] The colours which plants display have sometimes a singular relation to the mountain elevations or geographical positions they inhabit, but these considerations will be aptly treated of in the relations borne by living creatures to physical conditions and to one another.