The following points highlight the four tissues of monocot stem of plants. The tissues are: 1. Epidermis 2. Hypodermis 3. Ground Tissue 4. Vascular Strand.
Monocot Stem: Tissue # 1. Epidermis:
It is the outermost layer of the stem which is made up of compactly arranged transparent, elongated and rectangular—barrel-shaped living parenchyma cells. The outer walls of epidermal cells possess deposition of silica and cutin. A separate layer of cuticle also occurs on the outside.
The cuticle and cutinised epidermal cells prevent the evaporation of water from the stem. Silica provides stiffness. Hair are usually absent. At places the epidermis possesses stomata for gaseous exchange. Each stomate has two dumbbell shaped guard cells.
Monocot Stem: Tissue # 2. Hypodermis:
It is 2-3 layered thick and lies below the epidermis. Hypodermis is made up of thick walled lignified sclerenchyma fibres. It acts as heat screen and provides rigidity and mechanical strength to the stem.
Monocot Stem: Tissue # 3. Ground Tissue:
It does not show distinction into cortex, endodermis, pericycle, pith and pith rays. The ground tissue is parenchymatous and occupies the whole stem interior. In the stem of Maize the cells are small and angular towards the hypodermis but become large and oval in the inner region.
The ground tissue stores food. Some of the outer cells may also synthesize food due to the presence of chloroplasts in them (chlorenchymatous cells). Abundant intercellular spaces are present in the ground tissue. These spaces communicate with exterior through the stomata present in the epidermis.
Monocot Stem: Tissue # 4. Vascular Strand:
The vascular strand is in the form of atactostele where a large number of vascular bundles lie scattered throughout the ground tissue. Vascular bundles are smaller but more numerous towards the outside than towards the centre.
The vascular bundles (Fig. 6.27 B, C) are oval or rounded in outline. They contain both phloem and xylem. Phloem lies towards the outside and the xylem on the inner side. Cambium is absent as the whole pro-cambium is consumed in the formation of vascular tissues.
The vascular bundles are, therefore, conjoint, collateral but closed. Each vascular bundle is surrounded by a sheath of sclerenchyma known as bundle sheath. The bundle sheath is more developed on the outer and the inner sides. Hypodermis and bundle sheaths coalesce in some of the outer vascular bundles. A bundle sheath is absent in Asparagus.
Phloem consists of sieve tubes, companion cells and a few phloem fibres. Phloem parenchyma is absent. The sieve tubes conduct organic food. In Maize, phloem is distinguished into outer protophloem and inner metaphloem. The protophloem gets crushed in the later stages.
Xylem is in the form of letter Y. It is endarch, i.e., protoxylem lies towards the centre of the stem. Xylem is made up of vessels, tracheids, xylem parenchyma and a few xylem fibres. Metaxylem generally consists of two large oval or rounded vessels lying at the upper two angles of xylem. The meta-xylem vessels have pitted walls. The two vessels are connected with each other by polygonal tracheids having pitted thickenings.
Protoxylem has a few (2-3) small oval vessels. They lie at lower angle of xylem. The vessels of protoxylem show spiral and annular thickenings. Xylem parenchyma and a few fibres are found just outside them. Some of the protoxylem vessels and xylem parenchyma cells dissolve or separate during the rapid growth of the stem to form a cavity called protoxylem cavity or lacuna (absent in Asparagus).
The protoxylem cavity of Maize is schizolysigenous in origin. It generally stores water. The tracheids and vessels help in the conduction of sap as well as mechanical support. In Maize the protoxylem cavity and protophloem can be absent in the smaller vascular bundles.