Root is the underground part of a vascular plant. And Leaf is the flattened outgrowth from stem of a vascular plant.
In this article, Primary structure of Root and Leaf will be discussed in detail.
Primary Structure of Dicot Root
The root of Dicot plants usually consists of Epidermis, Cortex, Endodermis, Pericycle, Vascular bundle etc. These are described as follows:
• The xylem bundles vary from 2-6 in number. They may be diarch, triarch, tetrarch, pentarch or hexarch.
• Pericycle gives rise to lateral roots and secondary meristem (cambium and phellogen).
• The cambium appears later as a secondary meristem.
• Pith is scanty or altogether absent.
- Consists of closely packed elongated cells with thin walls that usually lack of cuticle and stomata.
- In some dicot thickened outer walls occur in root parts growing in air.
- The root epidermis (Piliferours layer) rhizodermis or epiblema is typically uniseriate.
- Most of the epidermal cells extend out in the form of tubular unicellular root hairs.
- Some roots also develop a specialized layer of exodermis beneath the epidermis. The exodermis arises from one or several of the sub-epidermal layers of the cortex. The cell
walls of exodermal became suberized. The exodermis is present in few dicot.
- Massive and thin-walled rounded or polygonal parenchyma cells in space.
- The parenchyma of cortex contain starch grains.
- The dicot root possess secondary growth parenchyma.
- Cortex cell arranged in radial row or they may be alternate with one another in the concentric way.
- The innermost distinct layer of the cortex is endodermis.
- Uniseriate universally present in roots.
- Cells are living.
- Endodermis is characterized by the presence of Casparian strips or Casparian band on the anticlinal cells.
- The thin-walled passage cells are also found in the endodermal layer which lie
against the protoxylem poles.
- Next to the endodermis.
- Thin walled parenchyma.
- It makes the outer boundary of the primary vascular cylinder of the dicot root.
- Uniseriate or multiseriate.
- Lateral root in dicots arise in this tissue.
- The phellogen and part of the vascular cambium originate in the pericycle.
- Root without pericycle is rare but found in water plant.
The vascular System
- The phloem of the root occurs in the form of strands distributed near the
periphery of the vascular cylinder beneath the pericycle.
- Generally xylem forms discrete strands, alternating wit phloem.
- Sometimes the xylem occupies the center with strand-like parts projecting
from the center core-like ridges.
- If xylem is not differentiated in the center, the center is occupied by a pith.
- Exarch xylem: protoxylem is located near the periphery.
Primary Structure of Monocot Root
In the structure of Monocot Root, following parts are found:
- It consists of thin walled closely packed elongated cells. This wall bears a cuticle (Exception).
- A well known example of a multiseriate epidermis is the velamen of air roots of Orchidaceae, epiphytic Araceae and some of the terrestrial monocotyledons.
- Velamen is one to many layered parenchyma like sheath consisting of compactly arranged non-living cells with thickened walls. The thickening may be spiral, reticulate or pitted.
- An exodermis occurs beneath the velamen.
- A typical characteristics of the root epidermis is the development of root hair.
- Root cortex may be homogenous and simple in structure. Or it may contain various types of cell.
- Parenchyma, collenchyma may develop in root cortex.
- It is characterized by Casparian strip on its anticlinal wall.
- Pericycle consists of thin walled parenchymatous cells. Lateral root and phellogen originate from pericycle.
- It is uniseriate except in many monocot.
- Root without pericycle rare but may be found in water plant.
- The phloem of root occurs in the form of strand near periphery of vascular cylinder beneath the pericycle.
- Xylem form discrete alternate with the phloem occupied the center.
- Root has an exarch xylem.
Internal Structure of Leaf
The components that are found in the internal structure of Leaf, are given below:
- Dorsiventral leaves usually grow in a horizontal direction with distinct lower and upper surface. The upper surface strongly illuminated than the lower surface.
- There is difference in the internal structure between upper and lower surface of the dorsiventral leaf due to unequal illumination. More stomata present on the lower surface.
- Single layered epidermis present on both surface.
- Outer wall of epidermis are usually thick, waxy surface may be present, cuticle may be thick or thin.
- In xerophytic leaves, the epidermal cells are elongated and lignified.
- The tissue of leaf lies between the upper and lower epidermis and between the
veins consist of thin walled parenchyma in mesophyll tissue. Mesophyll tissue always contain chloroplast.
- Mesophyll tissue includes Palisade parenchyma and Spongy parenchyma.
- The palisade parenchyma is composed of elongated and cylindrical cells which are close together with long axis.
- In transverse section, arrangement of palisade parenchyma is quite compact. Intercellular spaces present.
- Single or more layers.
- Compactness of palisade parenchyma depends on light intensity.
- Photosynthesis occurs here.
- Spongy parenchyma is composed of round, oval shaped cells. Presence of large air spaces in comparison of palisade parenchyma.
- Less chloroplasts are developed in spongy parenchyma.
- Exchange of gases between the cells and the atmosphere.
- Tissue of conducting system situated near or at the center of the midrib. The
system have various shape, the formation of a ring, crescent, scattered and patches.
- The inner part of the ring is composed of xylem (towards upper surface) and
phloem (towards lower surface)
- Vascular bundle remain surrounded by parenchyma cell with small number of
- Isobilateral leaf hang vertically. So both surface are equal.
- Uniform structure are found on both the surface. A few dicot and most of the
monocots are example of isobilateral leaf.
- Single layered upper and lower epidermis are found on both surface.
- Compact arrangement of cell.
- Thin cuticle on the upper and thick cuticle on the lower surface.
- Bulliform cells are found on the upper epidermis.
- Homogenous tissues.
- Compactly arranged thin walled parenchymatous cell which are isodiametric in shape.
- Intercellular spaces are present
- Chlorophylls are present.
- Small and large vascular bundle are present at the same row. Bundle sheath
- Sclerenchymatous cell patch are present on both the surface of bundle.
- Xylem towards the upper and phloem towards the lower epidermis.
1. Class Lecture of Parveen Rashid, PhD
Professor, Department of Botany, University of Dhaka.