How Plants Feed: The Root System
The root system is usually the larger part of the plant. It not only anchors the plant in thebut seeks and absorbs the nutrients needed for growth. In some plants the root is also a storage organ, which enables the plant to survive a winter and grow again the next season. Although to fulfil their tasks roots invariably grow downward, this tendency may be somewhat modified by obstacles in the soil and by available space and food materials.
When a plant is uprooted, what is seen is the tough, brownish older parts of the root system that served to hold the plant firm in the soil. The working and growing part of a root is the fragile, whitish flexible tip, which is usually torn off when a plant is lifted from the soil. The cells at this tip form a protective thimble-like root cap, and are constantly being rubbed off at the outside as the root works its way through the soil, thus making the tip slimy and helping it to slip easily between the soil particles. At the same time the cells are renewed from within, so that the root cap remains constant in size.
Just behind the tip is a smooth and glistening part of the root, where cells elongate, and, as they lengthen, extend the root through the soil to exploit fresh feeding grounds. Behind this part is a velvet-like zone, where fine whitish hairs grow out from all sides of the root. These hairs vary in length from Tao to I in., growing longer as they age and eventually withering. New hairs are formed constantly near the root cap as the root grows; the shorter hairs are therefore nearer to the root cap, and the hair zone remains constant in length. At no time is the active hair zone more than a fraction of an inch long.
Collectively, the root hairs form the ‘mouth’ of the plant. It is their job to take in moisture and nutrients. To do this they penetrate the minute spaces that separate the soil particles, and tap the microscopic film of water solution adhering to them. The absorption of water solution to maintain a constant stream of sap through the plant is a her-culean task. A single wheat plant may need half a gallon a day, while a fairly mature maple tree in active growth can use as much in two minutes.
To absorb this quantity of water solution, the root system branches into tremendous complexity and length. The measurement of root systems is very difficult but it has been estimated that a single rye plant can produce root hairs with a total length of over 6,000 miles, and that a sunflower plant had a total root length of 1,448 ft., and that a single grass plant, occupying a cubic inch of soil, had between 16,000 and 20,000 root hairs with a total length of 3,000 to 4,000 ft.
The water solution is a thin ‘soup’ of soil moisture and dissolved mineral salts which the roots absorb by various complex processes, some of which are not yet fully understood. The most important is osmosis, a process whereby water and the dissolved substances needed by the plant filter through the permeable membrane walls of the root cells and root hair cells and pass into the cells of the plant itself. As a result, the cells become swollen, and a pressure is built up within their walls. Under this pres-sure, the watery solution or sap begins to move from cell to cell and is driven upward through the plant.
Behind the root cap and hair zone the root develops an outer skin of cork-like tissue, almost impervious to water, and also increases in girth. This increase exerts a pressure like a wedge on surrounding soil, displacing it and tending to make it more friable.
The root systems of individual plants vary considerably according to their kind and to the texture and structure of the soil and the plant nutrients it contains. For instance, a dense, heavy clay offers more resistance to root development than a light loam or sandy soil. Roots branch more freely and extensively in the latter in their search for food. Most soils are better aerated and more fertile near the surface, so that the feeding roots of plants tend to be more numerous and active in these areas. Nevertheless, roots penetrate deep into the soil, and a plant’s root system is never static.
Apart from their functions as feeding and anchoring mechanisms, the roots of many plants keep them at a constant level in the soil, moving the plants slightly up or down by means of root growth and contraction. By this faculty many bulbous and perennial plants, when left undisturbed, are kept at the same level year after year. As a plant ages, some roots die, while others may be injured, and the root system will bud and form new roots to take their place.
The ability to form new roots is inherent throughout the plant, particularly so at points where there are numerous living and identical cells, such as nodes. Roots that do not originate from the seed or root system are called adventitious roots. Such roots form naturally on strawberry and blackberry shoots and at the back of ivy stems, though in this latter case the roots serve only as an anchor. The faculty to produce adventitious roots is exploited when plants are propagated by means ofor layers. Once the cut surface is brought into contact with moist soil or humid conditions, the roots begin to form.