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  MG Manual Reference
Ch. 1, pp. 16 - 19
[Plant Parts and Functions: stems | leaves | buds | roots | flowers | fruit | seeds]

Taproot of Carrot


A thorough knowledge of the root system of plants is essential if their growth, flowering, and fruiting responses are to be understood. The structure and growth habits of roots have a pronounced effect on the size and vigor of the plant, method of propagation, adaptation to certain soil types, and response to cultural practices and irrigation. The roots of certain vegetable crops are important as food. Roots typically originate from the lower portion of a plant or cutting. They possess a root cap, have no nodes and never bear leaves or flowers directly. The principal functions of roots are to absorb nutrients and moisture, to anchor the plant in the soil, to furnish physical support for the stem, and to serve as food storage organs. In some plants they may be used as a means of propagation.

Types of RootsTop
A primary (radicle) root originates at the lower end of the embryo of a seedling plant. A taproot is formed when the primary root continues to elongate downward. This makes them difficult to transplant and necessitates planting only in deep, well-drained soil. The taproot of carrot, parsnip, and salsify is the principal edible part of these crops.

Fibrous Root of Grass
A lateral, or secondary root is a side or branch root which arises from another root. A fibrous root system is one in which the primary root ceases to elongate, leading to the development of numerous lateral roots. These then branch repeatedly and form the feeding root system of the plant. A fibrous root is one which remains small in diameter because of a lack of significant cambial activity. One factor which causes shrubs and dwarf trees to remain smaller than standard trees is the lower activity rate of the cambium tissue which produces a smaller root system.
If plants that normally develop a taproot are undercut so that the taproot is severed early in the plant’s life, the root will lose its taproot characteristic and develop a fibrous root system. This is done commercially in nurseries so that trees, which naturally have tap roots, will develop a compact, fibrous root system. This allows a higher rate of transplanting success.
The quantity and distribution of plant roots is very important because these two factors have a major influence on the absorption of moisture and nutrients. The depth and spread of the roots is dependent on the inherent growth characteristics of the plant and the texture and structure of the soil. Roots will penetrate much deeper in a loose, well-drained soil than in a heavy, poorly-drained soil. A dense, compacted layer in the soil will restrict or stop root growth.
During early development, a seedling plant nutrients and moisture from the few inches of soil surrounding it. Therefore, the early growth of most horticultural crops which are seeded in rows benefits from band applications of fertilizer, placed several inches to each side and slightly below the seeds.
As plants become well-established, the root system develops laterally and usually extends far beyond the spread of the branches. For most cultivated crops roots meet and overlap between the rows. The greatest concentration of fibrous roots occurs in the top foot of soil but significant numbers of laterals may grow downward from these roots to provide an effective absorption system a couple of feet deep.
Root Structure
Parts of a RootTop
Internally, there are three major parts of a root. The meristem is at the tip and manufactures new cells. It is an area of cell division and growth. Behind it is the zone of elongation, in which cells increase in size through food and water absorption. These cells by increasing in size, push the root through the soil. The third major root part is the maturation zone, in which cells undergo changes in order to become specific tissues such as epidermis, cortex, or vascular tissue. The epidermis is the outermost layer of cells surrounding the root. These cells are responsible for the absorption of water and minerals dissolved in water. Cortex cells are involved in the movement of water from the epidermis and in food storage. A layer of suberized (a fatty material in some cells), known as the Casparian strips, has regulatory effect on the types of minerals absorbed and transported by the roots to stems and leaves.
Vascular tissues conduct food and water and are located in the center of the root. However, some monocots have the vascular system of their roots distributed around the root center.
Externally there are two areas of importance. Root hairs are found along the main root and perform much of the actual work of water and nutrient absorption. The root cap is the outermost tip of the root, and consists of cells that are sloughed off as the root grows through the soil. The root cap covers and protects the meristem and also senses gravity and directs in what direction the root grows.
Roots as FoodTop
The enlarged root is the edible portion of several vegetable crops. The sweet potato is a swollen root, called a tuberous root, which serves as a food storage area for the plant. Carrot, parsnip, salsify, and radish are elongated taproots.

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