The following points highlight the top eleven anatomical characteristics of monocotyledonous roots. The characteristics are: 1. Xylem 2. Cambium 3. Pith 4. Epidermis 5. Cortex 6. Endodermis 7. Pericycle 8. Vascular Tissue 9. Formation of Lateral Roots 10. Mycorrhiza 11. Formation of Adventitious Roots.
Monocotyledonous Roots: Anatomical Characteristics # 1.
Xylem:
The xylem groups are numerous (polyarch condition) and generally vary from twelve to twenty.
Monocotyledonous Roots: Anatomical Characteristics # 2.
Cambium:
The cambium is altogether absent even in later stages, as there is no secondary thickening in such roots.
Monocotyledonous Roots: Anatomical Characteristics # 3.
Pith:
The pith is large and well developed. In certain cases (e.g., in Canna), the pith becomes sclerenchymatous.
Monocotyledonous Roots: Anatomical Characteristics # 4.
Epidermis:
The epidermis or outermost layer of the root is commonly known as rhizodermis, epiblema or piliferous layer. It is uniseriate and composed of compact tabular cells having no intercellular spaces and stomata. The tubular unicellular root hairs are also present on this layer.
A well known example of a multiseriate epidermis is the velamen of aerial roots of orchids and epiphytic aroids (Guttenberg, 1940). The velamen is a parchment-like sheath consisting of compactly arranged non-living cells with thickened walls. The cells of velamen are quite big in size and contain air and water in them.
The cell walls develop fibrous thickenings. Generally, beneath the epidermis there are present one or more layers of exodermis. Usually the exodermis consists of a single row of cells with thickened outer and lateral walls except certain passage cells which remain thin-walled.
Monocotyledonous Roots: Anatomical Characteristics # 5.
Cortex:
Immediately beneath the epidermis a massive cortex lies consisting of thin-walled parenchyma cells having sufficiently developed intercellular spaces among them. Usually in an old root of Zea may a few layers of cortex immediately beneath the epidermis undergo suberization and give rise to a simple or multi-layered zone—the exodermis.
This is protective layer which protects internal tissues from injurious agencies! The starch grains are abundantly present in the cortical cells. The sclerenchyma cells are commonly found in the cortex of monocotyledons.
Monocotyledonous Roots: Anatomical Characteristics # 6.
Endodermis:
The innermost layer of the cortex is called the endodermis. It is composed of barrel-shaped compact cells having no intercellular spaces among them. The endodermal cells possess Casparian strips on their anticlinal walls. The Casparian strip is the part of primary cell wall. The strip is typically located close to the inner tangential wall.
In most of monocotyledons the endodermis commonly undergoes certain wall modifications. There are two developmental states, sometimes very distinct, in addition to the primary state when only the Casparian strip is present. In the secondary state a suberin lamella covers the entire wall on the inside of the cell.
At a later stage of development this suberin lamella is covered by a layer of cellulose which in some monocot roots attains a considerable thickness. Thus the walls of the endodermal cells become sufficiently thickened and the thick-walled passage cells are formed opposite the protoxylem poles. The passage cells are meant for diffusion and are also called the transfusion cells.
Monocotyledonous Roots: Anatomical Characteristics # 7.
Pericycle:
It is usually uniseriate and composed of thin-walled parenchymatous cells. In the monocotyledons, the pericycle often undergoes sclerification in older roots, partly or entirely. In many monocotyledons (e.g., some Gramineae, Smilax, Agave, Dracaena, palms) the pericycle consists of several layers.
The pericycle may be interrupted by the differentiation of xylem (many Gramineae and Cyperaceae) or phloem elements (Potamogetonaceae) next to the endodermis (Guttenberg, 1943). Here, the pericycle gives rise to lateral roots only.
Monocotyledonous Roots: Anatomical Characteristics # 8.
Vascular Tissue:
The vascular tissue consists of alternating strands of xylem and phloem. The phloem occurs in the form of strands near the periphery of the vascular cylinder, beneath the pericycle. The xylem forms discrete strands, alternating with the phloem strands. The centre is occupied by large pith which may be parenchymatous or sclerenchymatous.
Bundles are numerous and referred as polyarch. The adventitious roots of Palmae and Pandanaceae have considerably higher number of vascular bundles, as many as 100 or more. In some roots (e.g., Hydrilla, Triticum), a single vessel occupies the centre and is separated by non-tracheary elements from the peripheral strands. In other variable numbers of large metaxylem vessels are arranged in circle around the pith (e.g., Zea mays).
In the woody monocotyledons the inner metaxylem elements may form two to three circles (e.g., Latana), or they may be widely separated from each other (e.g., Phoenix dactylifera), or scattered throughout the centre (e.g., Raphia hookeri). In some monocotyledons (e.g., Cordyline, Musa, Pandanaceae) phloem strands are scattered among the tracheary elements in the centre of the root.
The xylem is exarch, i.e., the protoxylem lies towards periphery and the metaxylem towards the centre. The vessels of protoxylem are narrow and the walls have annular and spiral thickenings whereas that of metaxylem the vessels are broad and they possess reticulate and pitted thickenings.
The phloem strands consist of sieve tubes, companion cells and phloem parenchyma. The phloem strands are also exarch having protophloem towards the periphery and metapholem towards the centre. The parenchymatous or sclerenchymatous conjunctive tissue is found in between and around the xylem and phloem strands.
The central part of the stele is occupied by a well developed pith. In Canna, Oryza sativa, Avena sativa, the pith is sclerenchymatous.
Monocotyledonous Roots: Anatomical Characteristics # 9.
Formation of Lateral Roots:
In flowering plants, the lateral roots are endogenous in origin, that is, they originate in the inner tissue of the mother root, and appear externally only after their growth is well begun. The root meristems arise in the pericycle found immediately beneath the endodermis. (In ferns, and other pteridophytes the lateral roots originate in the endodermis).
Usually the lateral roots are restricted to the regions opposite the xylem and come out in vertical rows, the number being equal to that of xylem strands present. In the formation of a lateral root, the cells of the pericycle lying against the protoxylem become meristematic and begin to divide first tangentially and then periclinally and anticlinally, thus a few layers of cells are cut off.
This way, the endodermis is pushed outwards and a protrusion is being formed. Very soon this protrusion comes out of the cortex and three regions of root apex, that is, dermatogen, periblem and plerome, become quite distinct.
The endodermis and some of the cortical cells form a part of the root cap. The lateral root forces its way out through the cortex, endodermis and epidermis, and passes into the soil. Very soon the root cap is sloughed off and renewed by the calyptrogen.
The lateral root primodria are formed in distinctive positions in relation to the xylem and phloem of the parent root. In diarch roots they usually occur between the xylem and the phloem, in triarch and tetrarch roots in positions opposite to the protoxylem, and in many polyarch roots opposite to the protophloem. However, in some polyarch roots, the lateral roots are developed in sites opposite to the protoxylem.
Monocotyledonous Roots: Anatomical Characteristics # 10.
Mycorrhiza:
This is also known as ‘fungus root’. This is an association of fungus with root of a higher plant. Mycorrhizas are of common occurrence.
There are two main types:
(1) Endotrophic, in which fungus is within cortex cells of root, e.g., orchids, and
(2) Ectotrophic, in which it is external, forming a mantle that completely invests the smaller roots, e.g., Pinus. The mycorrhizas are believed to constitute an example of a mutually beneficial symbiotic association, probably evolved from an original host parasite relationship.
It has been clearly shown (a) that mycorrhizal plants benefit from the association, e.g., under natural conditions presence of fungus partner is vital for establishment and growth of seedling trees of a number of different species, e.g., pines, and (b) that association of fungus with tree is necessary for development and reproduction by fungus.
Monocotyledonous Roots: Anatomical Characteristics # 11.
Formation of Adventitious Roots:
The adventitious roots may occur on the hypocotyl of a seedling, at nodes and internodes of stems, and in roots. They may be formed in young organs or in older tissues which are still meristematic. Most adventitious roots arise endogenously.
Stem-borne adventitious roots make the main vascular system in vascular cryptogms (pteridophytes), in most monocotyledons, in dicotyledons propagating by means of rhizomes or runners, in water plants, in saprophytes and in parasites. The roots which are developed on cuttings, directly from the stem or from the callus tissue, are also adventitious.
Usually the adventitious roots are initiated in the vicinity of differentiating vascular tissues of the organ which gives rise to them (Datta and Majumdar, 1943). In young organ, the adventitious primordium is initiated by a group of cells near the periphery of the vascular system.
In older organ, it is located deeper, near the vascular cambium. In young stems, the cells that form the root primordium are derived from the inter-fascicular parenchyma, while in older stems from a vascular ray. In certain cases the adventitious roots are initiated by divisions in the cambial zone (Smith, 1936).
Usually the seat of the root primordium in the case of stems is known as pericycle. The origin of the adventitious roots in the vascular ray, or in the cambium places the young root close to both the xylem and the phloem of the mother axis and makes the vascular connection between the two organs (see fig. 42.30).
