=790.= (6) =The renewal of the delicate structures for absorption.=—The delicate root hairs are easily injured. The thin cell-walls through which food solutions flow become more or less choked by the gradual deposit of substances in solution in the water, and continued growth of the root in diameter forms a firmer epidermis and cortex through which the solutions taken up by the root hairs would pass with difficulty. For this reason new root hairs are constantly being formed on the growing root-tip throughout the growing season, and in the case of perennial plants, through each season of their growth. =791.= (7) =Aid in preparation of food from raw materials.=—For most plants the food obtained from the soil is already in solution in the soil water. But there are certain substances (examples, some of the chemical compounds of potash, phosphoric acid, etc.) which are insoluble in water. Certain acids excreted by the roots aid in making these substances soluble (see Chapter III). In a number of plants the roots have become associated with fungus or bacterial organisms which assist in the manufacture of nitrogenous food substances, or even in the absorption of ordinary food solution from the soil, or in making use of the decaying humus of the forest (see Chapter IX). =792.= (8) =The maintenance of the required balance between the environment and the increasing or changing requirements of the plant.=—In this matter the entire plant participates. Mention is made here only of the general relation which the root sustains to its own environment and the increased burden placed upon it by the shoot. The increase in the root system keeps pace with the increasing size of the stem system. The roots become stronger, their ramifications wider, and the number of absorbing rootlets more numerous. The observation is sometimes offered that the correlation between the root system of a plant, and the form of the stem system and position of the leaves, is of such a nature that plants with a tap-root system have their leaves so arranged as to shed the water to the center of the system, while plants with a fibrous-root system have their leaves so arranged as to shed the water outward. In support of this attention is called to the radiate type of the leaf system of the dandelion, beet, etc. In the second place the imbricate type as manifested in broad-leaved trees, and in the overlapping branch systems of many pines, etc. One should note, however, that in the former class the leaves are often arranged to shed as much water outward as inward. As to the latter class, there is need of experiment to determine whether these empirical observations are correct, for the following reasons: 1st, Root and leaf distribution are governed by other and more important laws, the root being influenced by the location of food in the soil which usually forms a very thin stratum while the shoot and leaf is mainly influenced