Root

For other meanings of root, see Root (disambiguation).

Root structure

The outside surface of a root is the epidermis. Recently produced epidermal cells absorb water from the surrounding environment and produce outgrowths called root hairs that greatly increase the cell's absorptive surface. Root-hairs are very delicate and generally short-lived, remaining functional for only a few days. However, as the root grows, new epidermal cells emerge and these form new root hairs, replacing those that die. The process by which water is absorbed into the epidermal cells from the soil is known as osmosis. For this reason, water that is saline is more difficult for most plant species to absorb.

Beneath the epidermis is the cortex, which comprises the bulk of the root. Its main function is storage of starch. Intercellular spaces in the cortex aerate cells for respiration. An endodermis is a thin layer of small cells forming the innermost part of the cortex and surrounding the vascular tisues deeper in the root. The tightly packed cells of the endodermis contain a substance known as suberin and create an impermeable barrier of sorts. Water can only flow in one direction through the endodermis: in towards the center of the root, rather than outward from the stele to the cortex.

The vascular cylinder or stele consists of the cells inside the endodermis. The outer part, known as the pericycle, surrounds the actual vascular tissue. In monocotyledonous plants, the xylem and phloem cells are arranged in a circle around a pith or center, whereas in dicotyledons, the xylem cells form a central "hub" with lobes, and phloem cells fill in the spaces between the lobes.

plants

Root growth

Roots will generally grow in any direction where the correct environment of air, nutrients and water exists to meet the plant's needs. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks. At germination, roots grow downward due to gravitropism, the growth mechanism of plants that also causes the shoot to grow upward. In some plants (such as ivy), the "root" actually clings to walls and structures; this is known as thigmotropism, or response to touch.

Most plants experience growth only along the apical meristems; this is known as primary growth, which encompasses all vertical growth. On the other hand, secondary growth encompasses all lateral growth, a major component of woody plant tissues. Secondary growth occurs at the lateral meristems, namely the vascular cambium and cork cambium. The former forms secondary xylem and secondary phloem, while the latter forms the periderm, found only in woody plants.

In woody plants, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root. The cambium layer forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. As a result, tissues beyond the secondary phloem (including the epidermis and cortex, in many cases) tend to be pushed outward and are eventually "sloughed off" (shed).

At this point, the cork cambium (noting that this process only occurs in woody plants) begins to form the periderm, consisting of protective cork cells containing suberin. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.

The vascular cambium produces new layers of secondary xylem annually. This dead tissue is responsible for most water transport through the vascular tissue (systems and roots).

Types of roots

A true root system consists of a primary root and secondary roots (or lateral roots).

The primary root originates in the radicle of the seedling. During its growth it rebranches to form the lateral roots. Generally, two categories are recognized: