Definition of Monocot Root
Monocot roots are fibrous or adventitious roots consisting of a wide network of thin roots and root fibers that originate from the stem.
- Monocot roots are highly variable depending on the plant species and the age of the plant. But most of the monocot plants are herbaceous with weak cambium that cannot hold woody tissues.
- The monocot root system consists of various roots characterized by their growth and complexity. The root system has a primary or taproot with associated lateral roots.
- Besides, seminal roots are present that are preformed in an ungerminated seed. Shoot-borne roots or adventitious roots are characteristic of monocot plants as well. The adventitious roots develop other regions of the seed than the radical.
- The adventitious roots in monocots are of two types; roots that originate at nodes present on the germinating seedling axis below the soil and roots that originate at nodes that are present above the soil. The second type of adventitious roots usually are observed at the lowermost 2-3 nodes and are often referred to as prop or brace roots.
- The roots of monocot plants lack cambium which prevents the formation of strong woody plants and limits the sufficient growth of the plant.
- The lack of cambium in the roots is replaced by the formation of adventitious roots or shoot-borne roots that provide stability and strength to the plant.
- The formation of primary roots of monocot plants begins during early embryogenesis and forms a distinct region within 10-15 days. It is then followed by vascular development during which the primary root is enclosed in a protective sheathing structure.
Image Source: Byjus.
Definition of Dicot Root
Dicot roots are taproots consisting of a single primary root from which secondary and tertiary roots develop and grow vertically downwards through the soil.
- The roots are a non-green part of the plan that is present below the soil and do not have any nodes or internodes.
- The roots in dicot plants are mostly similar in structure but the length, thickness complexity of the root system might differ.
- Some dicot plants might have modified roots for different purposes like respiration, food storage, and mechanical support.
- A typical root consists of different parts; root cap, meristematic zone, the zone of elongation, and zone of maturation.
- The primary root of the dicot root system is the taproot which grows vertically downwards to great depth. From the taproot, secondary roots arise which might grow sidewards as well as downwards.
- Tertiary roots might arise from the secondary roots in order to reach greater depth and enable absorption of water and minerals. Tiny root hairs are also present in the dicot root system.
- Dicot roots can be both herbaceous and woody depending on the plant species. The woody root system has cambium which enables the growth of large plants with thick stems.
Structure of Monocot and Dicot Root
The anatomy or internal structure of both monocot and dicot plants consist of the following parts;
1. Piliferous Layer or Epiblema or Epidermis
- Epidermis or Epiblema is the outermost layer of roots which is composed of a compact layer of thin-walled, polygonal parenchymatous cells with no intercellular spaces.
- The epidermis of the root doesn’t have a cuticle or stomata. Some of the cells of the epidermis give rise to specialized root hairs that are present in the maturation zone of both monocot and dicot roots.
- Due to the absence of intercellular spaces and the presence of hair cells, the layer is also called the piliferous layer.
- In monocots, the epidermis, also called rhizodermis, consists of hair-forming trichoblasts and non-root hair forming atrichoblasts.
- A specialized multilayered epidermis is present in monocot plants like orchid, which is called velamen. The velamen is involved in gaseous exchange.
- The epidermis is short-lived and in older roots, replaced by a lignified, suberinized exodermis. The exodermis develops from the outermost cortical cells that form an additional Casparian band.
- In dicot roots, the epidermis along with other tissues of the roots is peeled and later replaced by cork cambium.
- In shoot-borne roots of the monocot plant, the epidermis persists and forms a protective cuticula.
- The epidermis in both types of roots is involved in providing protection to the internal tissues. The minute root hairs enable the absorption of water and minerals from the soil by providing a larger surface area.
- Cortex is the tissue present underneath the epidermis which is composed of many layers of cortical cells.
- The cortical cells in monocots are thin-walled multilayered parenchymatous cells that have sufficiently large intercellular spaces between them.
- The cortex of dicot roots, however, consists of sclerenchyma in addition to the parenchyma.
- As the epidermal cells, the cells of the cortex are also non-photosynthetic as they do not have chlorophyll. Some dicot plants like Tinospora and Trapa have chlorophyll in the cells of the cortex and are photosynthetic.
- In some plants, the cells of the outer layer of the cortex undergo suberization and form a single-layered or multi-layered exodermis.
- The cells of this region store starch in the form of starch grains and contain leucoplasts. The most important function of the cortex is the movement of water from the epidermis to the inner tissues.
- The region of the cortex is wide in monocot roots when compared to the cortex of dicot roots as monocot roots have as many as eighteen layers of parenchymatous cells.
- The endodermis is another layer of dermis tissues present between the cortex and the central vascular tissues of the root. Endodermis acts as a barrier between the two layers.
- It is composed of tightly packed barrel-shaped cells that lack any intercellular spaces. The endodermis often has a single layer of cells.
- The young cells of the endodermis possess an internal strip of suberin and lignin, resulting in a layer of Casparian strip. As the cells mature, the strip becomes indistinguishable due to the thickening of the cells.
- The internal layer of cells of the endodermis gives rise to the adventitious roots in monocot plants.
- The young cells lie opposite to the protoxylem groups of the vascular bundles. These cells are also termed passage cells or transfusion cells as these are involved in the conduction of fluids inwardly from the cortex and outwardly from the vascular bundles. The number of passage cells is equal to the number of protoxylem cells.
- The thickened or older cells of the endodermis are also involved in the passage of fluid with the help of their plasmodesmata.
- The endodermis regulates the flow of fluid between the cortex and the vascular tissue acting as a biological checkpoint.
- The pericycle is a single-layered structure present underneath the endodermis which is the most distinct layer of cells between dicot and monocot roots.
- The pericycle of monocots is composed of a single layer of sclerenchymatous cells with few parenchymatous cells. The young cells of this layer are thin-walled but later become thick-walled due to the deposition of various substances.
- The pericycle in dicot roots is composed of a type of parenchyma called prosenchyma which is defined by the abundance of protoplasm.
- In monocots, pericycle can be either uniseriate or single-layered (Maize) or multiseriate or multi-layered (Smilax).
- In dicots, lateral roots originate from a part of the pericycle present opposite to the protoxylem. Thus, the lateral roots of dicots are endogenous in nature.
- Pericycle in dicot roots is involved in the formation of the vascular cambium. Cork cambium is also formed in dicot plants during secondary growth. The cambium is not formed in monocot roots.
- The pericycle is an important part of the root tissue as it is involved in the formation of lateral roots, cambium and provides support to the vascular tissue present beneath it.
5. Vascular Bundles
- The vascular bundles form the innermost tissues of the plant root consisting of alternate xylem and phloem units. The number of vascular bundles differs in dicots and monocot roots.
- In dicots, the vascular bundles are radial and exarch and the number of such bundles varies between two to six (diarch to hexarch). In some plants like Ficus, polyarch condition might be present with more than six vascular bundles.
- In monocots, the vascular bundles are also radial and exarch but the number of vascular bundles is always greater than six. In maize roots, 20-30 vascular bundles are present whereas more than 100 bundles can be observed in Pandanus and palms.
- In both types of roots, the vascular bundles are arranged in the form of a ring around a central pith. The vascular bundles are also termed radial bundles due to the alternate arrangement of xylem and phloem is
- The xylem bundles in both monocot and dicot roots are exarch; the protoxylem lies towards the outside (pericycle) and the metaxylem lies towards the center (pith).
- The xylem in the monocot is composed of oval vessels and xylem parenchyma whereas that in the dicot is composed of polygonal and thick-walled cells.
- Tin dicots, the protoxylem vessels bear annular thickenings whereas the metaxylem vessels have reticulate thickenings. Xylem parenchyma and fibers are also absent in dicot roots.
- The phloem bundles in dicots are present close to the pericycle and consist of sieve tubes, companion cells, and phloem parenchyma. The phloem fibers are absent.
- The phloem is also divided into metaphloem and protophloem but they are not easily distinguishable.
- The phloem bundles in monocots are present closer to the pith and consist of similar structures and cells.
- The xylem tissue in roots is involved in the conduction of water through the roots whereas the phloem bundles are involved in the conduction and storage of food.
6. Conjunctive tissues
- Conjunctive tissues are masses of parenchymatous or sclerenchymatous cells that are present between the xylem and phloem bundles in the vascular tissue.
- The amount of conjunctive tissue is more in monocot roots due to the larger number of vascular bundles when compared to the dicot root.
- In dicot plants, the conjunctive tissues together with the pericycle give rise to the vascular cambium during secondary growth. No cambium formation occurs in monocot roots.
- These tissues are involved in the storage of food and also provide mechanical support to the root.
- Pith is the central mass of tissues composed of thin-walled parenchymatous cells in the vascular system of the root.
- In dicots, the pith is less prominent or less developed. In some cases, it might be completely absent as well.
- In monocots, however, the pith is prominent with many cells that are either rounded or polygonal.
- The cells of the pith are loosely attached with large intercellular spaces in between. The cells of the pith store food and help in the dispersal of air between the vascular bundles.
8. Passage Cells
- Passage cells are distinguished cells of the endodermis that are involved in the conduction of water and other materials between the cortex and the vascular bundles.
- Passage cells are prominent in monocot roots but are completely absent in dicots. These cells are also called transfusion cells.
- The passage cells of the endodermis are younger cells of the layer that do not have deposits of suberin or lignin called Casparian strips.
- The passage cells are often present near the protoxylem and enable the radial flow of material through the root system.
Functions of Monocot and Dicot Root
The basic function of the root is to provide support to the plant, which is the same in both monocot and dicot plants. Besides, there are other several functions of roots that are also more or less similar in both types of plants. The following are some of the functions of monocot and dicot roots;
- The most important function of roots is to anchor the plant to the soil or land in order to provide support.
- Roots are essential for the absorption of water and minerals dissolved in the soil. The vascular system in the root then functions to conduct the water and mineral to other parts of the plant.
- The root system also stores a large number of food particles in different tissues like conjunctive tissue, pith, and cortex. Roots of plants like radish and carrots are modified to store a large amount of food.
- Plants growing in marshy areas have roots that come out to the soil surface in order to obtain oxygen. These roots are termed pneumatophores that have tiny pores called pneumathodes involved in gaseous exchange.
- Many dicot roots exist in a symbiotic relationship with microorganisms like fungi that play essential roles in nitrogen fixation.
- The roots of some plants are involved in the propagation and dispersal of the plants.
Monocot vs Dicot Root (18 Key Differences)
|Characteristics||Monocot root||Dicot root|
|Definition||Monocotyledonous roots are fibrous or adventitious roots consisting of a wide network of thin roots and root fibers that originate from the stem.||Dicotyledonous roots are taproots consisting of a single primary root from which secondary and tertiary roots develop and grow vertically downwards through the soil.|
|Root system||Monocot plants have a fibrous or adventitious root system.||Dicot plants have a tap root system.|
|Primary root||The development of the primary root stops during the postembryonic development of the roots.||The primary root continues to grow throughout the life of the plant in the form of the taproot.|
|Epidermal covering||The monocot roots are covered by a cork cambium after the peeling of the epidermis.||The dicot roots are covered by exodermis which is a modified epidermis.|
|Cortex||The cortex in monocot roots is wide.||The cortex in dicot roots is narrow.|
|The cortex in monocot roots is composed of only parenchymatous cells.||The cortex of dicot roots is composed of both parenchymatous and sclerenchymatous cells.|
|Endodermis||The endodermis of monocot roots is thicker.||The endodermis of dicot roots is less thick.|
|Casparian strips are less prominent in monocot roots as these are only observed in young cells.||Casparian strips are more prominent in dicot roots.|
|Passage cells||Passage cells are found in the endodermis of monocot roots||Passage cells are absent in the endodermis of dicot roots.|
|Pericycle||The pericycle of monocot roots only forms the lateral roots.||Pericycle of dicot roots forms the cork cambium and the lateral roots.|
|The pericycle of monocots can either be single-layered or double-layered.||The pericycle of dicots is always single layered.|
|Cambium||Both cork cambium and vascular cambium are absent in monocot roots.||Both cork cambium and vascular cambium are found in dicot roots.|
|Vascular bundles||The number of vascular bundles is greater than six (polyarch).||The number of vascular bundles is usually between two and six (diarch to hexarch).|
|The xylem vessels in monocot roots are oval in shape.||The xylem vessels in dicot roots are polygonal in shape.|
|Xylem parenchyma is present.||Xylem parenchyma is absent.|
|Conjunctive tissue||The conjunctive tissue of monocot roots is parenchymatous.||The conjunctive tissue of dicot roots is both parenchymatous and sclerenchymatous.|
|Pith||The pith in monocot roots is developed and prominent.||The pith in dicot roots is less developed or reduced.|
|Secondary growth||Secondary growth doesn’t take place.||Secondary growth takes place.|
Examples of Monocot Root
1. Maize root
- The root system in maize is the fibrous or adventitious root system consisting of numerous short-borne roots that are present above the soil surface.
- The necessity of shoot-borne roots in monocots arises from the lack of cambium in the root. These roots provide support to the plant required to reach sufficient height. The adventitious roots of maize can be seen in 4-5 nodes of the stem above the soil surface.
- During growth, a single maize plant can exploit about 200 cubic feet of soil and absorb about 30-35 gallons of water.
- Depending on the soil type, the lateral roots of the plant can reach up to 3-4 feet on all sides of the plant and penetrate depths of 5-6 feet under the soil.
- The primary root of the maize plant is enclosed within a protective sheath called coleorhizae which enables the primary root to push through the seed coat.
- Maize seed like most monocot seeds contains 3-7 seminal root primordial that begin growth both laterally and vertically.
2. Orchid roots
- Orchids are monocots usually used as ornamental plants for decorative purposes. Orchids can be either terrestrial or epiphytic.
- Terrestrial orchids have ground-dwelling, thick and fleshy roots that serve the function of storage.
- The epiphytic orchids, in turn, have modified aerial roots that are long and consist of a special structure called velamen.
- Velamen is a layer on orchid roots composed of dead cells that functions in absorbing moisture and nutrients from the surrounding environment.
- Healthy orchid roots are firm and green to white in color. The roots are mostly only green right before they need to be watered. Roots that appear green all the time indicate excess water.
- The emerging new roots in epiphytic orchid often indicate the best time to re-pot the orchid in the next pot for its propagation.
Examples of Dicot Root
1. Banyan tree roots
- Banyan trees have characteristic tap root systems often with aerial prop roots that mature into thick and woody trunks.
- As the trees become old, the roots become indistinguishable from the primary root. The lateral roots begin spreading laterally in all directions and cover a wide area.
- The complexity of the banyan root is due to the increasing weight of the tree trunk as the tree continues to grow.
- The roots continue to grow throughout the life of the plant where additional cells are added to the meristematic part of the root.
- The root tip is composed of dead cells which is the strongest part of the root. The root tip thus enables the penetration of hard rocks during growth.
2. Roots of garden peas
- Garden peas have a simple tap root system consisting of highly branched primary root that reaches only about 6 inches under the soil.
- The secondary roots of peas exist in a symbiotic relationship with different bacteria. These bacteria exist in the root nodules of the root system where they are involved in nitrogen fixation.
- The root system of such plants is essential for the biogeochemical cycling of nitrogen as the bacteria can fix atmospheric nitrogen into usable forms for the plants.
- The root nodules can be observed on the secondary and tertiary roots of the root system.
References and Sources
- Hochholdinger F. (2009) The Maize Root System: Morphology, Anatomy, and Genetics. In: Bennetzen J.L., Hake S.C. (eds) Handbook of Maize: Its Biology. Springer, New York, NY. https://doi.org/10.1007/978-0-387-79418-1_8
- Feldman L. (1994) The Maize Root. In: Freeling M., Walbot V. (eds) The Maize Handbook. Springer Lab Manuals. Springer, New York, NY. https://doi.org/10.1007/978-1-4612-2694-9_4
- Yadegari R., Goldberg R.B. (1997) Embryogenesis in Dicotyledonous Plants. In: Larkins B.A., Vasil I.K. (eds) Cellular and Molecular Biology of Plant Seed Development. Advances in Cellular and Molecular Biology of Plants, vol 4. Springer, Dordrecht. https://doi.org/10.1007/978-94-015-8909-3_1
- Hochholdinger Frank, Marcon Caroline, Baldauf Jutta A., Yu Peng, Frey Felix P. Proteomics of Maize Root Development. Frontiers in Plant Science. VOL 9, 2018; pg 143. DOI:10.3389/fpls.2018.00143. https://www.frontiersin.org/article/10.3389/fpls.2018.00143.
- Zhang, Shibao et al. “Physiological diversity of orchids.” Plant diversity vol. 40,4 196-208. 25 Jun. 2018, doi:10.1016/j.pld.2018.06.003
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