[46] Seedlings are usually very sensitive to light and are good objects to study. CHAPTER XLI. THE ROOT. I. Function of Roots. =784.= The most obvious function of the roots of ordinary plants are two: 1st, To furnish anchorage and partial support, and 2d, absorption of liquid nutriment from the soil. The environmental relation of such roots, then, in broad terms, is with the soil. It is very clear that in some plants the root serves both functions, while in other plants the root may fulfil only one of these requirements. The problems which the plant has to solve in working out these relations are: (1) Permeation of the soil or substratum. (2) Grappling the substratum. (3) A congenial moisture or water relation. (4) Distribution of roots for the purpose of reaching food-laden soil. (5) Exposure of surface for absorption. (6) The renewal of the delicate structures for absorption. (7) Aid in preparation of food from raw material. (8) The maintenance of the required balance between the environment as a whole and the increasing or changing requirements of the plant. =785.= (1) =Permeation of the soil or substratum.=—The fundamental divergence of character in the environmental relations of root and stem are manifest as soon as they emerge from the germinating seed. Under the influence of the same stimulus (_gravity_) the root shows its geotropic character by growing downward, while the geotropic character of the stem is shown in its upward growth. The medium which the root has to penetrate offers considerable resistance, and the form of the root as well as its manner of growth is adapted to overcome this difficulty. The slender, conical, penetrating root-tip wedges its way between the minute particles of soil or into the minute crevices of the rock, while the nutation of the root enables it to search for the points of least resistance. The root-tips having penetrated the soil, the older portions of the root continue this wedge action by growth in diameter, though, of course, elongation of the old parts of the root does not take place. It is the widening growth of the tapering root that produces the wedge-like action. The crevices of the rock are sometimes broadened, but the resistance here is so great, the root is often greatly flattened out.