The following points highlight the top eight anatomical characteristics of dicotyledonous roots. The characteristics are: 1. Xylem Bundles 2. Cambium 3. Pith 4. Epidermis 5. Cortex 6. Endodermis 7. Pericycle 8. The Vascular System.
Dicotyledonous Roots: Anatomical Characteristic # 1.
The xylem bundles vary from two to six numbers, i.e., they may be diarch, triarch, tetrarch, pentarch or hexarch.
Dicotyledonous Roots: Anatomical Characteristic # 2.
The cambium appears later as a secondary meristem.
Dicotyledonous Roots: Anatomical Characteristic # 3.
The pith is scanty or altogether absent.
Dicotyledonous Roots: Anatomical Characteristic # 4.
The epidermis consists of closely packed elongated cells with thin walls that usually lack a cuticle and stomata. In some dicotyledons thickened outer walls occur in root parts growing in air and also in roots that retain their epidermis for a long time. The root epidermis (also known as piliferous layer, rhizodermis or epiblema) is typically uniseriate.
Most of the epidermal cells extend out in the form of tubular unicellular root hairs. Normally, the root hairs are confined to a region between one and several centimetres in length near the tip. They are absent in the nearest proximity of the apical meristem, and they die off in the older root parts.
Some roots also develop a specialized layer—the exodermis—beneath the epidermis. The exodermis arises from one or several of the sub-epidermal layers of the cortex. The cell walls of exodermis become suberized. The exodermis is found to be present in few dicots.
Dicotyledonous Roots: Anatomical Characteristic # 5.
The cortex is massive and consists of thin-walled rounded or polygonal parenchyma ceils having sufficiently developed intercellular spaces among them. The parenchyma cells of the cortex contain abundant starch grains in them. In the roots of dicotyledons which possess secondary growth and shed their cortex early, the cortex consists mainly of parenchyma.
As seen in transverse sections, the cortical cells may be arranged in radial rows, or they may alternate with one another in the successive concentric layers. The presence of schizogenous intercellular spaces is typical of the root cortex. In the water plants the intercellular spaces are large and form distinct air spaces.
The cortex of roots is usually devoid of chlorophyll. Exceptions are roots of some water plants and aerial roots of many epiphytes (e.g., Tinospora spp). Various idioblasts and secretory structures are found in the root cortex. Some dicotyledons (e.g., Brassica, Pyrus, Prunus, Spiraea, etc.) may develop prominent reticulate or band-like thickenings in cortical cells outside the endodermis.
Dicotyledonous Roots: Anatomical Characteristic # 6.
The innermost distinct layer of the cortex is known as endodermis. The endodermis is uniseriate and almost universally present in the roots. The cells of endodermis are living and characterized by the presence of Casparian strips or Casparian bands on their anticlinal walls.
The strip is formed during the early ontogeny of the cell and is a part of the primary wall. The strip is typically located close to the inner tangential wall. Guttenberg (1943) says, that the suberin-like materials are found in the strips. The cytoplasm of an endodermal cell remains firmly attached to the Casparian strip.
This firm attachment controls the movement of the materials in the root and their passage into xylem cells. The thin-walled passage-cells are also found in the endodermal layer which lie against the protoxylem poles. The passage cells either remain unmodified as long as the root lives or develop thick walls like the rest of the endodermis.
Dicotyledonous Roots: Anatomical Characteristic # 7.
The layer next to the endodermis is commonly known as pericycle. The pericycle of relatively young roots consists of thin-walled parenchyma. It makes the outer boundary of the primary vascular cylinder of the dicotyledonous roots.
It may be uniseriate or multiseriate (e.g., Moms, Salix, Ficus benghalensis, etc.). The lateral roots in dicots arise in this tissue. The phellogen and part of vascular cambium originate in the pericycle. Roots without pericycle are rare but may be found among water plants and parasites.
Dicotyledonous Roots: Anatomical Characteristic # 8.
The Vascular System:
The phloem of the root occurs in the form of strands distributed near the periphery of the vascular cylinder, beneath the pericycle. Generally the xylem forms discrete strands, alternating with the phloem strands. Sometimes the xylem occupies the centre, with the strand-like parts projecting from the central core like ridges.
If xylem is not differentiated in the centre, the centre is occupied by a pith. The root typically shows an exarch xylem, i.e., the protoxylem is located near the periphery of the vascular cylinder, the metaxylem farther inward. The phloem is also centripetally differentiated, i.e., the protophloem occurring closer to the periphery than the metaphloem.
Most dicotyledons have few xylem strands. The taproot is frequently di-, tri-, or tetrarch, but it may have five to six and even more poles, (e.g., many Amentiferae, Castanea). Only one xylem strand occurs in the slender root of the hydrophyte Trapa natans. In Raphanus, Daucus, Linum, Lycopersicon and Nicotiana the roots are diarch. In Pisum the root is triarch.
In Cicer, Vicia, Helianthus, Gossypium and Ranunculus the roots are tetrarch. In certain dicots the root of the same plant may show di-, tri-, and tetrarch xylem. For example, tetrarch and polyarch roots have been reported from Nymphaea chilensis. (Wardlaw, 1928). Banerji (1932) reported tri-, tetra-, and pentarch roots in Enhydra fluctuans.
Such roots are known as heteroarchic roots. The protoxylem consists of annular and spiral vessels whereas metaxylem of reticulate and pitted vessels. The phloem strand consists of sieve tubes, companion cells and phloem parenchyma. The parenchymatous conjunctive tissue occurs in between xylem and phloem strands. The pith is scanty or altogether absent.