The following points highlight the six anatomical characteristics of the root. The characteristics are: 1. Root Cap 2. Epidermis 3. Cortex 4. Endodermis 5. Pericycle 6. Vascular System.
Root: Anatomical Characteristic # 1. Root Cap:
The root cap consists of parenchymatous cells in various stages of differentiation. It is protective in function. Recent experiments have indicated that the root cap has another function that is physiologically of great importance.
The root cap is apparently the site of the perception of gravity; thus it appears to be capable of controlling the production in the meristem of the growth- regulating substances involved in geotropism or their movement.
However, the root cap itself is not the site of synthesis of growth substances. The experiments show that the root cap can control the movement, if not also the synthesis of endogenous auxin in the root apex.
Root: Anatomical Characteristic # 2. Epidermis:
The epidermis is also known as epiblema or piliferous layer. In most of roots, root hairs develop from some of the epidermal cells at a little distance from the apical meristem. The cells, giving rise to root hairs are known as trichoblasts. In the roots of the grass (Phleum), trichoblasts are formed by unequal division, of a cell of the immature epidermis, the protoderm.
The trichoblast is the distally situated cell of this division. Root hairs are outgrowths of single cells, and function both in the absorption and in anchorage. Root hairs are usually eventually sloughed off, but occasionally they are persistent.
Electron microscopy reveals (Leech, Mollenhauer and Whaley, 1963) that the wall of emerging root hair is a continuation of only the inner component of the wall of the epidermal cell which gives rise to it.
In some species, root hairs are formed from special cells which are distinct in size and metabolism from the neighbouring epidermal cells. These cells are known as trichoblasts. In other species, root hairs may develop from a whole row of cells, which stain more densely than their neighbours.
In the aerial roots of certain epiphytic orchids a multiple epidermis or velamen is present. This tissue is derived from the epidermal initials and may be several cells thick. The cells are dead and devoid of contents, the cell walls are strengthened with bands of lignin.
On the inner side of the velamen there is a specialized layer of cells which is derived from the periblem and not the dermatogen, and may therefore be considered as the outermost layer of the cortex, the exodermis.
This layer is composed of alternating long and short cells; the long cells become thick-walled on their radial and outer tangential surface, but the small cells remain thin-walled and are called passage cells. The velamen is thought to function as a protective tissue, preventing undue water loss from the delicate cortical cells of the exposed aerial root.
Formally this was believed that the cells of the velamen also absorbed and conserved water drawn from the atmosphere, but recent experiments show that the mature velamen and exodermis are nearly impermeable to water and solutes.
Root: Anatomical Characteristic # 3. Cortex:
In most roots the cortex is parenchymatous. In some roots, the cells of the cortex are very regularly arranged, both radially and in concentric circles. Conspicuous intercellular spaces may be present, and especially evident in aquatic species, where they form a type of aerenchyma.
The cortical cells often contain starch, and sometimes crystals. Sclerenchyma is more common in the roots of monocotyledons than those of dicotyledons.
The characteristic trichosclereids are found in the roots of Monstera. Collenchyma is occasionally present in roots (e.g., Monstera). The outermost layer or layers of the cortex, just beneath the epidermis may be differentiated as an exodermis, a kind of hypodermis, with suberized walls. The innermost layer of the cortex is usually differentiated as an endodermis.
Root: Anatomical Characteristic # 4. Endodermis:
The endodermis comprises a single layer of cells differing physiologically and in structure and function from those on either side of it. In the young endodermal cells a band of suberin, Casparian strip, runs radially around the cell and is thus seen in the radial walls in transverse sections of roots.
This suberin deposit, to which the protoplast of the cell is attached, is continuous across the middle lamella of the radial walls, but is absent from the tangential walls.
A study of the endodermis with the electron microscope (Bonnett, 1968) reveals a thickening of the wall in the region of the Casparian strip. The plasma-lemma is thicker here and adheres strongly to the cell wall. The thin-walled passage cells often remain in the endodermis in positions opposite the protoxylem.
Root: Anatomical Characteristic # 5. Pericycle:
The pericycle is usually a single layer of parenchymatous cells lying just within the endodermis and peripheral to the vascular tissues. The pericycle has a capacity for meristematic growth, and gives rise to lateral root primordia, parts of the vascular cambium, and usually the meristem which produces cork, the phellogen. The pericycle is sometimes called pericambium.
Root: Anatomical Characteristic # 6. Vascular System:
The vascular system of the root as seen in transverse section consists of a variable number of triangular rays of thick-walled, lignified tracheary elements, alternating with arcs of thin-walled phloem. In the root, the xylem and phloem do not lie on the same radius. The xylem may form a solid central core, or there may be a parenchymatous or sclerenchymatous pith, as in the roots of many monocotyledons.
Roots with 1, 2, 3, 4, 5 and many arcs of xylem are respectively called monarch, diarch, triarch, tetrarch, pentarch and polyarch. The xylem is exarch, i.e., protoxylem lies towards periphery and metaxylem towards the centre.
The xylem is always centripetal in its development. The phloem bundle consists of sieve tubes, companion cells and phloem parenchyma. The protoxylem consists of annular and spiral vessels and meta-xylem of reticulate and pitted vessels.
The parenchyma found in between xylem and phloem bundles is known as conjunctive tissue. The pith may be large, small or altogether absent.