Read this essay to learn about the anatomical features of monocotyledonous roots:- 1. Arum Root 2. Maize Root 3. Smilax Root 4. Orchid Root.
Essay # 1. Anatomical Features of Arum Root (Monocotyledonous Root):
A transverse section through the root of arum (Colocasia sp. of family Araceae) shows the following anatomical features (Fig. 5.106):
The uniseriate epiblema is composed of a row of compactly arranged tubular cells without intercellular spaces. Some cells of the layer protrude to form the root hairs.
The massive cortex mainly consists of unspecialised parenchyma with profuse schizogenously formed spaces. In older roots the outer zone of the cortex next to epiblema forms the exodermis the cells of which become suberized when the epiblema is decayed and thus protects the internal tissues in absence of epiblema.
The innermost layer of cortex forms the endodermis consisting of barrel-shaped closely set cells with conspicuous Casparian strips. The radial walls of the endodermal cells become secondarily thickened except some cells against the protoxylem groups to form passage cells or transfusion cells for ready diffusion of absorbed sap into the xylem stream.
The central cylindrical core of tissues encircled by the endodermis forms the stele. It consists of radially, arranged vascular strands and intrastelar ground tissues. Internal to the endodermis there is a uniseriate parenchymatous cell-layer called the pericycle. There is alternate orientation of a good number of exarch xylem and phloem strands.
This type of stele is referred to as polyarch. Phloem is composed of sieve tubes, companion cells and phloem parenchyma. The outer protophloem and inner metaphloem can hardly be distinguished. Small amount of parenchymatous conjunctive tissue separates the xylem and phloem patches. The central portion of the stele is occupied by large pith.
Essay # 2. Anatomical Features of Maize Root (Monocotyledonous Root):
The anatomical features of the maize root (Zea mays of family Poaceae) are almost similar to that of arum, so far as the epiblema and cortex are concerned. Formation of exodermis in the outer zone of cortex is common in almost all monocotyledonous roots.
The innermost layer of the cortex forms the endodermis composed of thick-walled cells. As the secondary and tertiary layers are deposited on the walls of the endodermal cells the Casparian strips are not recognised.
Internal to the endodermis uniseriate pericycle is present but it is partly sclerenchymatous. The stele is polyarch with large number of exarch xylem and phloem strands. Xylem parenchyma cells undergo sclerosis and thus become thick- walled. A large pith made of loosely-arranged parenchyma cells containing abundant starch grains occupies the center (Fig. 5.107).
Essay # 3. Anatomical Features of Smilax Root (Monocotyledonous Root):
The root of Smilax sp. of family Liliaceae shows the same plan of tissue arrangement in t.s. as found in other monocot roots but differs in some details as follows (Fig. 5.108):
It is as usual uniseriate made of parenchyma cells with rounded outer walls.
The outer single row of heavily thick-walled cells of the cortex forms the exodermis just internal to epidermis. The rest of the cortex is composed of thin-walled parenchyma cells with distinct intercellular spaces. Starch grains are abundantly present in the cortical cells. The innermost layer of the cortex forms the endodermis the cells of which are thick-walled due to secondary deposition on their radial and inner walls.
The stele is polyarch consisting of a large number of xylem and phloem strands with alternate arrangement. The pericycle, unlike that of other roots, is multiseriate and sclerenchymatous. The outer protophloem elements are smaller than the inner metaphloem elements. Pith at the centre is fairly large and is made of thick-walled parenchyma.
Essay # 4. Anatomical Features of Orchid Root (Monocotyledonous Root):
The epiphytic orchid (Vanda spp. of family Orchidaceae) roots have a spongy outer tissue to absorb moisture from the atmosphere, known as velamen, instead of uniseriate epidermis (Fig. 5.109). The tissue consists of a few layers of compactly arranged dead and empty cells which often form a silvery outer coat. The variously thickened porous cell walls of the tissue work like a sponge.
The spirally or reticulately arranged fibres form the supporting ribs. In dry weather the cells remain filled with air, and in moist weather they can absorb moisture. Under water-saturated condition gaseous exchange takes place through some special structures, called pneumatodes, consisting of groups of cells with dense spiral wall thickenings.
Velamen originates from protoderm, and hence may be interpreted as a typical multiseriate epidermis, specially adapted to epiphytic condition. The outermost layer of velamen is known as limiting layer.
The outermost layer of cortex consists of a row of thick-walled cells forms the exo- dermis. The cells of this layer deposit suberin on their outer and lateral walls except certain of them remain unthickened to form passage cells. Through these cells water, absorbed by the velamen, moves inside.
The main bulk of the cortex occurs in-between exodermis and endodermis and composed of a few layers of chloroplast containing parenchyma cells with intercellular spaces. The root looks green due to these cells. The innermost layer of cortex is the endodermis with suberised radial and inner walls. Passage cells also occur in the endodermis opposite the protoxylem vessels.
The stele is polyarch consisting of a large number of xylem and phloem strands with alternate arrangement. Pericycle is uniseriate and made of thick-walled cells except the cells just internal to passage cells of the endodermis, which are thin-walled.
A good number of exarch xylem and phloem strands occur alternately in the stele. Sclerenchymatous conjunctive tissues surround the phloem groups. Sclerotic parenchymatous pith is present at the centre.