There are two principal kinds of roots, one of which, the tap-root (Fig. 26), goes deep into the soil, growing straight down and sending out lateral roots at intervals. The other spreads out near the surface of the soil (Fig. 27) and consists of a mass of fine rootlets. It has the advantage of estab lishing itself quickly and absorbing moisture vigor ously from the start, thus inducing a rapid growth of the plant. But it cannot utilize the deeper soil food nor withstand drought. On the other hand, tap-roots many endure long periods of drought: the long-rooted Peruvian cotton is said to survive a rainless period of six years.
A well-developed root system forms a mass of finely interlacing filaments that thoroughly ex plore the soil. The total length of these has been estimated at a quarter of a mile for a vigorous corn plant, while measurements on a squash vine proved the root to be over fifteen miles in length, the greater part of this being produced at the rate of a thousand feet per day.
Because of need of air, most roots are unable to thrive in wet soil, and their best work is done in soil in which the water is held in a thin film around the soil-particles. Each particle constitutes a minute water reservoir. To reach and tap these reservoirs is the work of the root-hairs, which ap pear just back of the root-tip as outgrowths from the surface cells of the root (Figs. 28 and 29). They force themselves energetically between the soil-particles and attach themselves so closely that they often break off rather than loosen their hold when the root is pulled up. Thus they come into contact with the water-films that surround the particles, and by means of water - attracting sub stances within the root-hair they pull the water away from the particles. As each tiny reservoir is emptied of its supply, water flows in from surrounding ones and these also yield up their stores.
The water passes from the root-hair through the soft outer tissue (cortex) to the wood-cells, in which it passes directly to the leaves.
These thick-walled wood cells form groups that al ternate with groups of thin walled tissue or bast which conveys proteids and other food from the leaves to the root and to other parts of the plant. The wood and bast are surrounded by a row of small cells (endoder mis), whose closely joined walls prevent the entrance of air, which would im pede the progress of water in the wood-cells.
The absorptive surface of the root may be in creased from seven to seventy-five times by the root hairs. The fine roots, on which the root-hairs are principally produced, are known as " feeding roots," and all tillage should be practiced with special reference to them. Tillage aids the work of the root by increasing the air and water-supply, and by loosening the soil. Roots will penetrate hard soil, or even hard substances like sealing-wax, but they grow very slowly under such conditions. They may develop a pressure of 50 to 100 pounds per square inch. The root-cap protects the delicate tip as it is forced into the soil.
The water absorbed by the root contains mineral substances. If the plant is burned, these will remain in the form of ashes. By growing plants in distilled water, to which has been added chemi cally pure salts in various combinations, it has been found that certain substances are indis pensable to the plant while others are not. The indispensable substances comprise four bases and four acids. The bases are potash, lime, magnesium and iron ; the acids are nitric, phosphoric, sul furic and carbonic—the carbonic acid absorbed from the air by the leaf. If all these substances, with the exception of carbonic acid, be dissolved in distilled water, plants can be grown in the solution and will produce mature seed ; but if any of the sub stances be lacking in the solution, except carbonic acid, growth will soon cease. All these substances are present in the soil, together with others of little or no value, as alumina, silica and others, but in order that the plant may absorb them they must be dissolved in the soil-water. Most of them exist in the soil in compounds that are but little soluble in water. The soil-water contains carbonic acid, derived principally from decaying organic matter, which has a decidedly solvent action. In addition, the root constantly excretes carbonic acid, which dissolves the plant-food within its reach. By the excretion of acid, roots may etch polished marble surfaces ; and they impart to distilled water an acid reaction.
Roots of many members of the pea family supply themselves with nitrogen from the air by means of the bacteria which inhabit tubercles on their roots.