Roots
Roots are plant organs with a number of functions to perform. One of the major functions of roots is to anchor the plant into the soil on which it grows; another is to efficiently absorb water and minerals from the soil and to pass these on to other plant organs. Roots may also serve as sites of food storage and hormone synthesis, and perform many other functions as well.
The structure of roots serves to facilitate these functions. Absorption of water and minerals is optimized by the constant formation of new lateral (branch) roots and by the presence of root hairs as part of the epidermis near all young root tips. In most dicots, the lateral roots are branches off a taproot derived from the primary root of the embryo. In monocots, the primary root ceases growth early in the plant's development and adventitious roots develop from the base of the stem. Lateral roots then branch off from these adventitious roots forming a fibrous root system.
The surface of roots is formed by a layer of epidermis, usually with little or no cuticle at its exterior. Present under the epidermis is a cortex of mostly parenchyma cells, which often provides a site for storage of starch and similar metabolic products. The innermost layer of the cortex, the endodermis, has cells with secondary cell walls impregnated with lignin and suberin which help to control water movement through roots. Directly next to the endodermis is a layer of pericycle, the cells of which undergo rapid division to produce lateral roots. In the center of roots is a vascular cylinder, consisting of a central core of xylem surrounded by patches of phloem.
Procedure:
1. Observe, and briefly sketch, the examples of a taproot system and a fibrous root system in the display provided by your instructor.
2. Examine a slide of an Allium root tip in longitudinal section. Draw this root tip and label the root cap, apical meristem, protoderm, ground meristem and procambium. The same tissues can be seen in a Zea root tip l.s.
3. Examine a slide of a root tip whole mount exhibiting root hairs. Draw one of these root hairs. At lower magnification your specimen will look like this, but at higher magnification you will be able to find individual root hairs attached to epidermal cells.
4. Obtain a slide of a mature Ranunculus root c.s. Draw a wedge shaped section of this root, labeling the epidermis, cortex, endodermis, pericycle, phloem, and xylem.
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