Plants are fixed to the earth, and receive nourishment from it by imbibing its liquids, which circulating upwards through the porous structure of the plant itself, and becoming exposed to the air in the leaves, attract to themselves nourishment from that source also. The part of a plant which grows into the earth is called the root; this has a variety of forms, in some it is branched like the upper part, and these branches divide into rootlets or fibres penetrating deep into the ground, and absorbing nourishment in all directions, but this absorption does not take place from the whole surface of the root but from spots covering it, and from the slightly expanded ends of the fibres, these portions are formed of new and porous cellular tissue, and are called "spongioles." The part of plants which springs upwards from the earth is called the stem, if large the trunk, this divides into branches and twigs; stems in some cases continue for a distance more or less underground. The part of a potato plant, usually called the root, and from which the tubers or potatoes grow, is in reality an underground stem, and the fibres which spring from it are the roots; the underground suckers of mint are also portions of stem, and in some cases these are greatly expanded, they then obtain the names of "tubers" (as the potato), or "corm," as in the crocus and meadow saffron; when the stem is thin and runs along the ground, sending in roots at intervals (as in the strawberry), it is called a "runner," when thicker and running horizontally under the ground, it is called a "rhizome."