Anatomy of dicotyledonous and monocotyledonous plants
- To study the transverse sections of the mature zones of roots, stems and leaves, it is important to study their tissue organization.
- The outermost layer is epidermis.
- The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces.
- The innermost layer of the cortex is called
- The tangential as well as radial walls of the endodermal cells have a deposition of water-impermeable, waxy material suberin in the form of casparian strips.
- Next to endodermis lies a few layers of thick-walled parenchyomatous cells referred to as pericycle.
- The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.
- There are usually two to four xylem and phloem patches, later a cambium ring develops between the xylem and phloem.
- All tissues on the inner side of the endodermis such as pericycle, vascular bundles and pith constitute the stele.
Fig. Dicot root
- It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
- As compared to the dicot root, which has fewer xylem bundles, there are usually more than six (polyarch) xylem bundles in the monocot root.
- Pith is large and well developed.
- Monocotyledonous roots do not undergo any secondary growth.
Fig. Monocot root
- Epidermis is the outermost protective layer of the stem.
- The cells arranged in multiple layers between epidermis and pericycle constitute the cortex.
- It consists of three sub-zones-
- Cortical layers
- The outer hypodermis, consists of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem.
- Cortical layers below hypodermis consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces.
- The innermost layer of the cortex is called the endodermis.
- The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath.
- Pericycle is present on the inner side of the endodermis and above the phloem.
- In between the vascular bundles, there are a few layers of radially placed parenchymatous cells, which constitute medullary rays.
- A large number of vascular bundles are arranged in a ring.
- A large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith.
Fig. Dicot stem
- The monocot stem has a sclerenchymatous hypodermis, a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue.
- Vascular bundles are conjoint and closed and the phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles.
Fig. Monocot stem
- The vertical section of a dorsiventral leaf through the lamina shows three main parts, namely, epidermis, mesophyll and vascular system.
- The epidermis which covers both the upper surface is called adaxial epidermis and which covers the lower surface is called abaxial epidermis.
- The tissue between the upper and the lower epidermis is called the mesophyll.
- Mesophyll, which possesses chloroplasts and carry out photosynthesis, is made up of parenchyma, which has two types of cells – the palisade parenchyma and the spongy parenchyma.
- Vascular system includes vascular bundles, which can be seen in the veins and the midrib.
- The vascular bundles are surrounded by a layer of thick walled bundle sheath cells.
Fig. Dorsiventral leaf
Isobilateral leaf (monocotyledonous leaf)
- The stomata are present on both the surfaces of the epidermis and the mesophyll is not differentiated into palisade and spongy parenchyma.
- Certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells, which are called bulliform cells. Example- grass.
- When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed and when they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.
Fig. Isobilateral leaf
- The growth of the roots and stems in length with the help of apical meristem is called the primary growth.
- Most dicotyledonous plants exhibit an increase in girth, which is called the secondary growth.
- The tissues involved in secondary growth are the two lateral meristems: vascular cambium and cork cambium.