Those plants whose seed contains only one cotyledon or embryonic leaf is known as monocotyledon or simply monocot. In this section, you will learn about characteristics and anatomy of monocot root.
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Primary Structure of Monocot roots
The typical monocot roots show following features:
- Epiblema is single layered, thin walled, colorless, polygonal without intercellular spaces, with presence of unicellular root hairs, hence also called as piliferous layer or rhizodermis.
- Cortex may be heterogeneous with outer dead exodermis.
- Endodermis consists of barrel shaped parenchyma without intercellular spaces. Casparian stripes are little present but passage cells are absent.
- Pericycle gives lateral roots only, secondary growth is absent.
- Vascular bundles are radial; Xylem is exarch, bundles more than six. Metaxylem elements are oval or circular.
- Conjunctive tissues are limited or even absent.
- Pith is large or rarely reduced.
Anatomy of Monocot root (Zea mays)
Monocot roots of maize shows following distinct regions:
- Vascular bundles
T.S of Monocot Root (Maize)
Features of Different Regions of Monocot Root
- Epiblema is the outermost single layer made from compactly arranged parenchymatous cells without intercellular space. Usually Epiblema has no stomata but bears unicellular epidermal root hairs and less amount of cutin. It contains more cuticle than dicot roots. The root hairs and thin walled epidermal cells take part in the absorption of water and minerals from the soil. The epiphytes have several layered hygroscopic epidermis, called velamen tissues. It is made from spongy dead cells which helps in absorption of water from atmosphere. It also checks excessive loss of water from cortex. Usually the wall of velamen has spiral or reticulate secondary thickening of cellulose, pectin and lignin.
- Cortex is a multi-layered well developed and made from oval parenchymatous cells with intercellular spaces. The intercellular spaces usually help in gaseous exchanges, storage of starch, etc. In monocots and several old roots, few layers of cortex just below epiblema give rise to a single or multilayered cuticularised sclerenchymatous region called exodermis. Cortex helps in mechanical support to the roots (like hypodermis to stem).
- Endodermis is innermost layer of cortex made from barrel shaped parenchyma. It forms a definite ring around the stele. These cells are characterized by the presence of casparian stripes. It is deposition of suberin and lignin, and their radial and tangential walls. Usually passage cells are absent in monocot roots. Due to presence of casparian stripes, endodermis forms water tight jacket around the vascular tissues, hence it is also called biological barrier. It regulates the inward and outward flow of water and minerals and prevents diffusion of air into xylem elements.
- Pericycle is uniseriate (multiseriate in Smilax) and made from thin walled parenchymatous cells. It is outermost layer of stellar system. Usually it is made from parenchymatous cells but it may become sclerenchymatous in older roots. Several lateral roots arise from this layer. Hence, lateral roots are endogenous in origin.
- Vascular bundle is radial, arranged in a ring (except mangrove, which also contains lenticels), polyarch (presence of many alternating xylem and phloem bundles). Xylem and phloem are found at different radii alternating with each other (radial). The number of xylem and phloem vary from, 8 to 46 (100 in pandanus). The xylem is exarch, i.e. the protoxylem lies towards periphery and metaxylem toward center. The protoxylem has smaller vessels with spiral or annular thickening, whereas the walls of metaxylem contains pitted thickening. Phleom consists of seive tubes, companion cells and phloem parenchyma. Usually phleom sclerenchyma or fibers are absent. The phloem is also exarch (protophloem towards the periphery and metaphloem towards the center). Secondary growth is absent in monocot roots due to lack of vascular and cork cambium. Conjunctive tissue is parenchymatous tissues which separates xylem and phloem bundles. It may become sclerenchymatous in older roots.
- Pith is large, well developed portion of monocot root. It occupies the central portion and made from thin walled parenchymatou tissue with intercellular spaces. It contains abundant amount of starch grains.
Monocot root of maize have bands of vascular bundles. Bundles are not separate and vessels are not found in linear rows, but arranged in V-shaped structure.