Read this essay to learn about the arrangements of primary tissues in the roots of plants.
1. Epiblema:
The dermal tissue system is called epiblema or rhizodermis as it differs from that of the shoot in origin, structure and function. The cells of this layer are elongated, thin-walled with a thin cuticle on the outer cell wall and devoid of intercellular spaces. Sometimes mucilage may occur superficially. In the long lasting parts of the root system the cells of the epiblema may become thick-walled due to the deposition of lignin, cutin or suberin.
The most notable feature of the epiblema is the presence of unicellular and unbranched root hairs that are projections of certain cells of this layer. They perform the function of anchorage as well as absorption of water and mineral salts from the soil solution. The root hairs are formed from some special epidermal cells that can be distinguished from the rest by their smaller size and dense cytoplasm.
These specialised cells are known as trichoblasts or piliferous cells. For that reason the epiblema is also termed as piliferous layer. Root hairs are usually short-lived except in some species of Asteraceae. Multicellular root hairs are observed in the aerial adventitious root of Kalancho fedtschenkoi.
The epiblema is typically single-layered with exception in the aerial roots of certain epiphytes where it is multi-layered. This multiple epidermis is specially termed as velamen, the cells of which are derived from the dermatogen.
This layer contains several layers of dead cells externally forming a sheath around the root. The cell walls of the compactly arranged cells are strengthened by spiral or reticulate bands of lignin. The walls also contain many primary pit fields.
The innermost layer of the velamen is known as exodermis. This layer demarcates the thick-walled cells of velamen from thin-walled cells of cortex. The exodermis is considered as the outermost layer of the cortex as it is derived from the periblem and not from the dermatogen that gives rise to velamen.
In exodermis long and short cells remain arranged alternately of them the smaller ones are thin-walled and are known as passage cells. The longer cells are thickened on their tangential and radial walls only. Thus exodermis may be comparable with endodermis of root Uniseriate or multiseriate exodermis is also observed in Iris, Smilax, Phoenix, and Citrus etc. just beneath the epidermis.
Velamen protects the delicate thin-walled cortical cells from desiccation. The velamen and exodermis are almost impermeable to water and solutes.
2. Cortex:
The root cortex is mainly parenchymatous with schizogenous and lysigenous intercellular spaces and in many monocotyledons sclerenchyma may develop in addition to parenchyma. In hydrophytes the root cortical cells are very regularly arranged to form aerenchyma.
The root cortex is usually very wide and the cells often contains starch, crystals etc. Collenchyma rarely occurs in roots cortex (e.g., Monstera) and the cortical cells of many epiphytes contain chloroplastids. Severa plants possess secretory cells, resin ducts and laticifers in cortex. Trichosclereids are found in the root cortex of Monstera.
3. Endodermis:
The innermost layer of the cortex is the endodermis that encircles the stele. The cells of this uniseriate layer are compact and contain casparian strips in their radial and cross walls when young. The strip may be narrow, thread-like or may be as wide as the wall itself.
In roots devoid of secondary growth suberin deposits as lamellae over the whole inner primary wall of the endodermal cells including the casparian strips and afterwards cellulose is deposited centripetally on the inside of suberin lamella (e g., Iris). Thus the endodermal cells attain considerable thickness on their radial walls
The inner radial and tangential walls become further thickened due to lignification. Suberisation starts in the endodermal cells adjacent to the phloem followed by the deposition on adjacent endodermal cells. In the protoxylem adjacent endodermal cells thickening is either delayed or there is no thickening except the casparian strip.
These cells are known as passage cells which may remain thin-walled throughout or may become thick- walled later. Through the passage cells water and mineral matters get entry into the xylem from the cortical cells. The main functions of the root endodermis are more or less same as that of stem.
4. Stele:
The stele consists of radial vascular bundles and pith. Just beneath the endodermis and external to the vascular tissues there is a cell layer called pericycle. This layer is usually uni- seriate but sometimes may be multiseriate (e.g., Agave, Smilax, Salix etc.) containing thin-walled parenchyma and occasionally fibres.
The pericycle is sometimes termed as pericambium as it retains its meristematic activity. It can give rise to the lateral root primordia, vascular cambium that gives rise to secondary vascular tissues in dicots and the phellogen – the cork meristem. Laticifers and secretory ducts may sometimes be present in pericycle. Sclerified pericycle cells may be found in monocots.
The primary xylem and phloem remain alternately and radially arranged beneath the pericycle. The xylem contains thick-walled, lignified tracheary elements that mature centripetally Protoxylem lies on the periphery and metaxylem towards the center and, therefore, the xylem is exarch. Centripetal differentiation is also observed in phloem.
In many dicots the xylem strands extend towards the centre to form a star-shaped solid central core. In most monocotyledons, however, there is large parenchymatous or sclerenchymatous pith in the center.
The number of xylem strands with protoxylem groups varies from 1 to many (Fig. 5.82) and termed as monarch (e.g., Trapa natans), diarch (e.g., Beta, Daucus Raphanus etc.), triarch (e.g., Pisum), tetrach (e.g., Vicia, Ranunculus etc.), pentarch and polyarch (e.g., monocotyledonous root) respectively.
Usually the number of xylem arcs 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 strands may be as high as 100 or more (e.g., Palmae).
In Triticum a single large metaxylem vessel occupies the centre and remains separated from the peripheral protoxylem by parenchyma A number of meta-xylem vessels remain arranged in a circle around the pith as in Iris, Zea etc.