In this article we will discuss about the arrangements of primary tissues in roots.
A cross-section of root reveals the epidermis, cortex and vascular cylinder.
Epidermis of root is also referred to as epiblema or rhizodermis as it differs from that of shoot in origin, structure and function. Epidermal cells are compactly set, elongated, and thin-walled with thin cuticle that covers the cell wall. Sometimes, mucilage may occur superficially. Epidermal cells of the underground and exposed parts of root that persist for a long time may become thick-walled and contain lignin, cutin or siuberin.
Except some hydrophytes, the most notable feature of epidermis is the presence of root hairs that are the tubular prolongations from the single cells of epidermis. They function in anchorage and in the absorption of water and salts.
Usually all epidermal cells may give rise to root hairs. But in certain plants like Phleum, Hydrocharis, Sinapis Alba etc. root hairs are formed from some special epidermal cells that can be distinguished from other epidermal cells by their smaller size and dense cytoplasm.
These specialized cells are known trichoblast or piliferous cell. So, the root epidermis that bears root hairs is also termed as piliferous layer. Root hairs are usually short lived; long-lived root hairs are observed in some species of Asteraceae. They are unbranched and unicellular. Multicellular root hairs are observed in the aerial adventitious root of Kalanchoe fedtschenkoi.
Epidermis of root is typically single layered, i.e. uniseriate but exceptions are noted in the aerial roots of certain epiphytic orchids where the epidermis is several layered. This multilayered (Fig. 18.1) or multiple epidermis is specially termed as velamen that is derived from the initials of epidermis, i.e. dermatogen.
Velamen consists of several layers of dead cells that are devoid of any contents and forms a circular sheath around the root. The cells are compactly arranged and the cell walls are strengthened by spiral or reticulate bands of lignin. The walls also contain many primary pit fields.
The inner limiting layer of velamen is known as exodermis. This layer is uniseriate and forms a partition between the thick-walled cells of velamen and thin-walled cells of cortex. Exodermis is considered as the outermost layer of cortex as it is derived from the periblem and not from the dermatogen that gives rise to velamen.
Two types of cell are observed in the exodermis of orchid root-long and short, which are more or less alternately arranged. The small cells are thin walled and known as passage cells. The long cells are thickened on their tangential and radial walls only. Thus exodermis may be comparable with endodermis of root.
It is regarded that velamen is a protective tissue that prevents water loss from the inner cortical cells, thus the delicate thin-walled cortical cells are protected from desiccation. It was previously believed that velamen absorbs and conserves water from atmosphere, but later studies revealed that velamen and exodermis are almost impermeable to water and solutes.
Cortex of root is mainly composed of parenchyma cells. In many monocotyledons sclerenchyma develops in the cortex in addition to parenchyma. Schizogenous and lysigenous intercellular spaces are frequently formed in the root cortex of some members of Gramineae and Cyperaceae. In many hydrophytes the cells of root cortex are very regularly arranged to form aerenchyma.
Apart from epiphytic orchids, exodermis is also observed in Iris, Smilax, Phoenix, Citrus etc. In these genera this layer is either uniseriate or multiseriate and forms a kind of hypodermis, just beneath the epidermis. The outermost layer or layers of root cortex are differentiated into exodermis whose cell walls are thickened with suberin.
In some cases Iignin is also deposited on the cell walls and casparian strips are also formed. A mature cell of exodermis may contain living protoplast. It is regarded that exodermis protects the inner delicate cortical cells.
Collenchyma rarely occurs in roots, e.g. Monstem where it is present in the cortex. Root cortex is usually very wide and the cells often contain starch, crystals etc. Aerial roots of many epiphytes contain chloroplastids in the cortical cells. Several plants possess secretory cells, resin ducts and laticifers in cortex. Trichosclereids are found in the root cortex of Monstera.
The innermost layer of cortex is endodermis that delimits cortex from stele. Endodermis is uniseriate, compactly set and has casparian strips in young cells (Fig. 18.2). The band of suberin that runs around the radial and cross walls of endodermal cell is the casparian strip. The strip may be narrow, thread like or as wide as the wall itself.
Suberinization occurs across the middle lamella of radial and cross walls, but not on the tangential walls. In roots where there is no secondary thickening suberin deposits as lamellae over the whole inner primary wall of endodermal cells including casparian strips. Later, cellulose is deposited centripetally on the inside of suberin lamella (ex. Iris).
Thus the endodermal cells attain considerable thickness on radial walls. These thickened cell walls often become lignified on the inner radial and tangential walls in addition to cellulose and suberin. Later study with electron microscope confirms the thick-walled nature of endodermis and reveals the presence of thick wall in the region of casparian strips.
The deposition of suberized material initially begins to those endodermal cells, which are adjacent to phloem strands. Later deposition spreads radially to other adjacent endodermal cells. The cells that are situated opposite the protoxylem are delayed in wall thickening, occasionally remain un-thickened and have casparian strips only.
These cells are known as passage cells, which may remain thin walled throughout the entire life or later may become thickened like other endodermal cells. It is regarded that the casparian strip regulates the movement of materials in the root and the passage cells provide passages for the movement of substances between vascular cylinder and cortex.
Plasmodesmata exist between cortical and endodermal cells and it is absent from the region of casparian strips. The main functions of endodermis are more or less the same as stems that were mentioned previously.
The vascular cylinder consists of vascular tissues (i.e. xylem and phloem) and pith. Peripheral to the vascular tissue and lying immediately inside the endodermis, there is a cylindrical region of cells – termed pericycle. The pericycle is usually uniseriate and sometimes may be multiseriate (e.g. Agave, Smilax, Salix etc.). It consists of thin walled parenchyma and sometimes fibres.
This layer is in direct contact with protoxylem and protophloem and can be distinguished prior to the differentiation of them.
The pericycle retains its meristematic activity and gives rise to:
(i) The primordia of lateral roots in spermatophytes,
(ii) In dicotyledons the portions of vascular cambium that give rise to secondary vasGular tissues, and
(iii) The phellogen — the meristem of cork.
The pericycle is sometimes referred to as pericambium which is an older term. In German literature pericambium means pericycle. Pericycle is absent in some parasites and hydrophytes. In the pericycle laticifers and secretory ducts are sometimes observed. In monocotyledons where there is usually no secondary growth, sclerification occurs in the cells of pericycle.
Below the pericycle in roots there lie the primary xylem and phloem. In contrast to stem these two vascular tissues lie separately and compose the radial vascular bundle. In cross section xylem appears as triangular rays consisting of thick walled, lignified tracheary elements that mature centripetally.
So protoxylem lies on the peripheral side of metaxylem and therefore the xylem is exarch. Centripetal differentiation is also observed in phloem. The vertex of the triangular xylem rays is free near the pericycle. The rest of the xylem strands, in many dicotyledons, extend towards the centre and form a star-shaped solid central core of xylem.
In most monocotyledons the xylem strands do not approach towards the centre of the vascular cylinder where large pith is formed made up of either parenchyma or sclerenchyma. The number of xylem rays with protoxylem groups varies.
It may be one, two, three, four, five or many and accordingly expressed by the terms monarch (e.g. Trapa nutans), diarch (e.g. Beta, Daucus, Raphanus etc.), triarch (e.g. Pisum), tetrarch (e.g. Vicia, Ranunculus etc.), pentarch and polyarch (e.g. monocotyledonous root).
Usually the number of xylem arches is constant for a plant but exceptions are noted in Libocedrus decurrens where the root may be diarch, triarch, pentarch and hexarch. The aerial root of Tinospora cordifolia also has tri-, tetra-, or pentarch xylem. In polyarch roots the number of xylem arches may be as high as 100 or more (e.g. Palmae).
In the root of Triticum there is a single large metaxylem vessel situated at the centre of vascular cylinder and the peripheral protoxylem are separated from the metaxylem by parenchyma. In Iris, Zea etc. a number of metaxylem vessels are arranged in a circle around the pith.