782. Compass plants.—These plants with vertical leaf arrangement, and exposure of both surfaces to the lateral rays of light have been mentioned in other sections (Lactuca scariola).
783. Open patterns.—Open patterns are presented by divided or “branched” leaves. Where the leaves are very finely dissected, they may be clustered in great profusion and yet admit sufficient light for some depth below. Where the leaflets are broader, the leaves are likely to be fewer in number and so arranged as to admit light to a great depth so that successive leaves below on the same or adjacent stems may not be too much shaded. On such plants, often the leaves lying next the ground are entire or less divided.
[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.