Anatomy is the study of the internal structure of organisms. While morphology deals with external form, anatomy reveals the internal organisation of tissues and their arrangement in plant organs. The internal structure of flowering plants is made up of tissues, which are groups of similar cells performing a common function. Meristematic tissues are composed of cells that divide continuously and give rise to new cells. Once these cells lose the power of division, they differentiate and mature into permanent tissues, which may be simple, complex or special.
The arrangement of tissues in the root, stem and leaf reveals how these organs perform their functions of conduction, support, protection and photosynthesis. The study of anatomy also helps us understand the monocot-dicot differences, secondary growth in dicot stems and roots, and the role of cork cambium in bark formation. This chapter systematically presents the tissue system of plants, the internal structure of the root, stem and leaf, and the process of secondary growth, providing the structural basis for all plant physiology studied in later chapters.
A group of tissues performing similar functions is called a tissue system. Based on their location and function, plants have three main tissue systems: the epidermal tissue system, the ground tissue system and the vascular tissue system.
The epidermal tissue system forms the outer protective covering of the plant body. It consists of the epidermis and its outgrowths. The epidermis is the outermost layer of the primary plant body, made of elongated, compactly arranged parenchymatous cells. This layer is usually single-layered and covers the entire outer surface of the plant. The cells are generally transparent with a thick waxy coating called the cuticle, which prevents water loss. In roots, the epidermis is called the epiblema or piliferous layer, and cuticle is absent because roots are involved in water absorption. The epidermis bears stomata, which are minute pores that regulate gaseous exchange and transpiration. Each stoma is bounded by two specialised epidermal cells called guard cells. In grasses, the guard cells are dumbbell-shaped, while in dicots they are bean-shaped. Trichomes, which are epidermal outgrowths, help in preventing water loss and in defence against grazing animals. In roots, epidermal outgrowths called root hairs absorb water and minerals.
The ground tissue system consists of all tissues between the epidermis and the vascular tissue. It is made of simple tissues such as parenchyma, collenchyma and sclerenchyma. This tissue includes the cortex, hypodermis, endodermis, pericycle and pith in roots and stems, and the mesophyll in leaves. In dicot stems, the ground tissue is differentiated into the cortex, endodermis, pericycle and pith, while in monocot stems it is not well differentiated.
The vascular tissue system consists of xylem and phloem, which form the vascular bundles. The xylem conducts water and minerals from the roots to other parts of the plant, while the phloem conducts the products of photosynthesis (food) from leaves to all parts of the plant. The complex tissues xylem and phloem together constitute the vascular bundles. In the xylem, tracheids and vessels are the conducting elements, while xylem fibres provide mechanical strength and xylem parenchyma stores food. In the phloem, sieve tubes and companion cells are the conducting elements, with phloem fibres and phloem parenchyma. Phloem is also called bast.
Meristems are tissues with the power of continuous cell division. Depending on their location in the plant body, meristems are of three types: - Apical meristems: Located at the tips of roots and shoots, responsible for the primary growth (increase in length) of the plant. - Intercalary meristems: Located between mature tissues in regions such as the bases of leaves (e.g., grasses), responsible for the elongation of leaves and internodes. - Lateral meristems: Located along the lateral sides of stems and roots, responsible for secondary growth (increase in girth). The cambium and cork cambium are lateral meristems.
Meristems are also classified as primary meristems (which occur directly in the embryo) and secondary meristems (which arise later from permanent tissues, e.g., cork cambium).
The transverse section of a dicot root (e.g., sunflower, gram) shows the following layers: - Epiblema (piliferous layer): The outermost single layer with root hairs. - Cortex: Large, thin-walled parenchymatous cells with intercellular spaces. This layer is broad. - Endodermis: The innermost layer of the cortex. Cells have casparian strips, which are lignified, waterproof thickenings on the radial walls, forming an impermeable barrier. - Pericycle: A layer of thin-walled cells just inside the endodermis, from which lateral roots originate. - Vascular tissues: The xylem and phloem are arranged in a radial manner on alternate radii (radial arrangement). Usually there are two, four or many xylem bundles (diarch, tetrarch, polyarch). In between the xylem bundles lie the phloem strands. There is no pith, or a very small pith may be present. The xylem is exarch, meaning the protoxylem lies towards the periphery and metaxylem towards the centre.
The monocot root (e.g., maize) resembles the dicot root in many ways but differs in important respects: - The epidermis is similar to the epiblema. - The cortex is broad, and the endodermis has casparian strips. - The pericycle gives rise to lateral roots and, in monocots, may also form the cambium. - The vascular bundles are many (polyarch), arranged in a ring. - The pith is large and well developed. - The xylem and phloem are arranged radially as in dicots, and the xylem is exarch.
The transverse section of a dicot stem (e.g., sunflower) shows: - Epidermis: The outermost layer covered with a cuticle and bearing multicellular hairs. - Cortex: Differentiated into the hypodermis (collenchymatous), the general cortex (parenchymatous) and the endodermis (starch sheath). - Pericycle: Present as patches of sclerenchyma. - Vascular bundles: Arranged in a ring, conjoint, collateral and open (cambium present between xylem and phloem). Xylem is endarch (protoxylem towards the centre, metaxylem towards the periphery). - Pith: Large, parenchymatous, present in the centre. Medullary rays connect the pith with the cortex through the vascular bundles.
The monocot stem (e.g., maize) differs from the dicot stem: - The hypodermis is sclerenchymatous. - The vascular bundles are numerous, scattered, conjoint, collateral and closed (no cambium). - The bundles are surrounded by a sclerenchymatous sheath. - No distinction of cortex, pericycle and pith, and there is no medullary ray. - The vascular bundles are arranged in two rings in maize: the peripheral smaller bundles and the central larger bundles.
The dicot leaf shows two types of mesophyll: the upper palisade parenchyma (elongated cells with chloroplasts) and the lower spongy parenchyma (irregular cells with intercellular spaces). The epidermal layers on both surfaces have stomata, though they are more numerous on the lower surface. The vascular bundles in the veins are surrounded by bundle sheath cells.
The monocot leaf has isobilateral symmetry, with stomata present on both surfaces. The mesophyll is not differentiated into palisade and spongy parenchyma. The vascular bundles in the veins are surrounded by a bundle sheath.
Secondary growth is the increase in the girth of stems and roots, which occurs by the activity of the lateral meristems, the vascular cambium and the cork cambium. Secondary growth occurs in most dicotyledonous plants, and it is usually absent in monocots.
In the dicot stem, the vascular cambium is present between the xylem and phloem in the vascular bundles (fascicular cambium). The cambium also arises from the medullary rays, forming the interfascicular cambium. The fascicular and interfascicular cambia join to form a complete cambium ring. The cambium ring divides periclinally, cutting off xylem towards the inside and phloem towards the outside. The cells of the cambium that give rise to xylem and phloem are of two types: fusiform initials (give rise to axial system) and ray initials (give rise to radial system). The xylem formed earlier in the season is called early or spring wood (with larger, thinner-walled vessels), while the xylem formed later is late or autumn wood (with smaller, thicker-walled vessels). The annual rings are formed by the alternating bands of spring wood and autumn wood. The number of annual rings indicates the age of the tree. The activity of the cambium is under hormonal and environmental control.
In the older dicot stems, the central region of the wood becomes dark, hard and resistant to the attack of microorganisms and insects because of the deposition of tannins, resins and other substances. This region is called heartwood, which provides mechanical support. The outer, lighter region of the wood is called sapwood, which is involved in the conduction of water and minerals.
As the stem continues to increase in girth, the epidermis ruptures. The cork cambium (phellogen) arises in the cortex (or pericycle) and cuts off cork (phellem) towards the outside and secondary cortex (phelloderm) towards the inside. The cork is impervious to water and gases due to the deposition of suberin. The cork cells are dead and compactly arranged. Lenticels, which are minute openings on the cork, help in gaseous exchange. The cork, cork cambium and secondary cortex together constitute the periderm. The outer bark is the tissue lying outside the cork cambium, including the cork and secondary phloem.
In roots, the vascular cambium arises from the conjunctive tissue between the xylem and phloem, forming a wavy cambium ring. This cambium produces secondary xylem and secondary phloem, and the root becomes circular in outline. The pericycle produces the cork cambium in roots.
| Tissue | Living/Dead | Wall | Function | Example |
|---|---|---|---|---|
| Parenchyma | Living | Thin cellulosic | Storage, photosynthesis | Cortex, mesophyll |
| Collenchyma | Living | Thick at corners | Mechanical support, flexibility | Hypodermis of dicot stem |
| Sclerenchyma | Dead | Thick, lignified | Strength | Fibres, sclereids |
| Feature | Dicot Root | Monocot Root | Dicot Stem | Monocot Stem |
|---|---|---|---|---|
| Vascular bundles | Radial, few | Radial, polyarch | Conjoint, open, in ring | Conjoint, closed, scattered |
| Pith | Small/absent | Large | Large | Present |
| Hypodermis | - | - | Collenchymatous | Sclerenchymatous |
| Cambium | Absent | Absent | Present | Absent |
| Term | Meaning |
|---|---|
| Fascicular cambium | Cambium within vascular bundle |
| Interfascicular cambium | Cambium from medullary rays |
| Periderm | Cork + cork cambium + secondary cortex |
| Heartwood | Dead central wood, dark, non-conducting |
| Sapwood | Outer wood, conducting |
| Lenticel | Opening in bark for gaseous exchange |
Anatomy of flowering plants explains the internal architecture that supports every physiological function of the plant. The organisation of meristematic and permanent tissues into epidermal, ground and vascular systems gives each organ its characteristic structure, visible in the transverse sections of roots, stems and leaves. The comparative anatomy of dicots and monocots reveals the adaptive differences between the two groups, while secondary growth explains how woody plants increase in girth and form bark and annual rings. Understanding the arrangement of xylem and phloem, the role of cambium, and the formation of periderm provides the structural foundation for the study of transport in plants, photosynthesis and other physiological processes discussed in subsequent chapters.