The root cap is a cup-shaped, loosely cemented mass of parenchyma cells that covers the tip of the root. As cells are lost among the soil particles, new ones are added from the meristem behind the cap. The cap is a unique feature of roots; the tip of the stem has no such structure. From its shape, structure, and location, its primary function seems clear: It protects the cells under it from abrasion and assists the root in penetrating the soil. Phenomenal numbers of cap cells are produced to replace those worn off and lost as root tips push through the soil.
The movement is assisted by a slimy substance, mucigel, which is produced by cells of the root cap and epidermis. The mucigel
Lubricates the roots.
Contains materials that are inhibitory to roots of other species.
Influences ion uptake.
Attracts beneficial soil microorganisms.
Glues soil particles to the roots thereby improving the soil-plant contact and facilitating water movement from the soil into the plant.
Protects the root cells from drying out.
Root cap cells sense light in some as yet unexplained way and direct root growth away from light. The root cap also senses gravity to which roots respond by growing downward, bringing them into contact with the soil, the reservoir of nutrients and water used by plants. The root cap also responds to pressures exerted by the soil particles.
Zone of cell division
An apical meristem lies under and behind the root cap and, like the stem apical meristem, it produces the cells that give rise to the primary body of the plant. Unlike the stem meristem, it is not at the very tip of the root; it lies behind the root cap. Between the area of active division and the cap is an area where cells divide more slowly, the quiescent center. Most cell divisions occur along the edges of this center and give rise to columns of cells arranged parallel to the root axis. The parenchyma cells of the meristem are small, cuboidal, with dense protoplasts devoid of vacuoles and with relatively large nuclei.
The apical meristem of the root organizes to form the three primary meristems:protoderm, which gives rise to the epidermis; procambium, which produces xylem and phloem; and the ground meri-stem, which produces the cortex. Pith, present in most stems and produced from the ground meristem, is absent in most dicot (eudicot) roots, but is found in many monocot roots.