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The following points highlight the six main parts of monocot root in plants. The parts are: 1. Epiblema 2. Cortex 3. Endodermis 4. Pericycle 5. Vascular Strand 6. Pith.
Monocot Root: Part # 1. Epiblema or Piliferous Layer (Rhizodermis):
It is the outermost layer of young root which has thin-walled cells. Some of the cells give rise to root hairs. They have a gummy pectic layer on the outside for cementing with soil particles and retaining water.
Root hairs are tubular in outline and lie in contact with soil water. Both epiblema and root hairs are devoid of cuticle. They take part in the absorption of water and mineral salts. In older parts the epiblema is shed or becomes impervious.
Monocot Root: Part # 2. Cortex:
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It is very wide region of parenchyma cells that enclose intercellular spaces for the exchange of gases. The cells store food. In older roots the outer one (e.g., Smilax) or more (e.g., Maize) layers of the cortex become thick walled and suberized. They constitute the exodermis. It is protective and to some extent absorptive in function.
The cortex of a monocot root has, therefore, three functions:
(i) Conduction of water from the root hairs to the inner tissues
(ii) Storage of food
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(iii) The outermost layer or layers of the cortex produce protective exodermis in the older roots.
Monocot Root: Part # 3. Endodermis:
Endodermis or inner boundary of the cortex is single layered. It is made up of barrel-shaped cells which do not enclose intercellular spaces. The young endodermal cells possess an internal strip of suberin and lignin which is known as casparian strip. However, it soon becomes indistinguishable due to the additional thickening of the endodermal cells.
Endodermal cells lying opposite the protoxylem groups, however, remain in the primary stage with usual casparian strip (Zeiglar et al, 1963). These un-thickened cells are called passage or transfusion cells. The passage cells are meant for the conduction of fluids inwardly from the cortex and outwardly from the interior into the cortex.
The thickened cells can also allow transport through plasmodesmata of pits (Clarkson and others, 1968). The endodermis regulates the flow of fluid both inwardly as well as outwardly by functioning as biological check post.
Monocot Root: Part # 4. Pericycle:
Pericycle or outer boundary of vascular strand lies below the endodermis. Pericycle may be uniseriate (single layered, e.g., Maize) or multiseriate (multi-layered, e.g., Smilax). In monocots the pericycle does not form cambium. It only produces lateral roots. The pericycle is composed of thin-walled parenchymatous cells in the young root. But later on it becomes thick-walled in many monocots.
Monocot Root: Part # 5. Vascular Strand or Cylinder:
It is in the form of several (8 or more) alternate and radial xylem and phloem bundles. The number is 20-30 in Maize and 100 or more of each type in Pandanus and palms. In many cases the vascular bundles are embedded in a cylinder of sclerenchymatous conjunctive tissue (e.g., Maize). The vascular bundles are arranged in the form of ring around a central pith.
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The xylem bundles are exarch, i.e., protoxylem lies towards the outside while the meta-xylem faces inwards. Because of the presence of numerous xylem bundles and exarch condition, xylem of monocot root is poly-arch. Xylem is made up of rounded or oval vessels and xylem parenchyma.
The vessels of meta-xylem are larger than those of protoxylem. Protoxylem vessels are narrow. They have a spiral annular or reticular thickenings. Metaxylem vessels are broad. Usually they possess pitted thickenings. Xylem provides mechanical strength and helps in the conduction of water and mineral salts.
Phloem bundles alternate with the xylem bundles. These two are separated from each other by means of narrow strip of conjunctive tissue. The cells of this tissue store food if parenchymatous.
They provide mechanical strength on becoming sclerified. They do not take part in the formation of cambium. Phloem consists of sieve tubes and companion ceils. It helps in the conduction of organic food. A distinction between protophloem and metaphloem may or may not be present.
Monocot Root: Part # 6. Pith:
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The centre of monocot root is occupied by the pith. It consists of parenchymatous (thin-walled or thick-walled) cells which may be rounded or angular. Intercellular spaces are present amongst the pith cells. The pith cells store food.
