ANATOMY OF FLOWERING PLANTS
- The branch of Biology that deals with the study of internal organization of plants.
- Higher Plants have complex body organization that is made up of many cells that form tissues, tissues give rise to organs and organs to organ systems.
- This whole process takes place by cell division and cell differentiation.
- In the beginning, all the newly formed cells arise from meristematic cells that are almost alike. During maturation, they undergo various structural changes by a process called differentiation. Differentiation leads to the formation of different kinds of cells and tissues from the parent cell where each cell performs a specific function. So, differentiation leads to physiological differentiation or division of labour i.e., performing special functions.
- The branch of Biology that deals with the study of internal structures of organisms is called Anatomy.
- Nehemiah Grew, is known as the “Father of Plant anatomy” and also coined the term ‘Tissue.
A tissue is a group of cells having a common origin and usually performing a common function. A plant is made up of different kinds of tissues. Tissues are classified into two main groups, namely, meristematic and permanent tissues based on whether the cells being formed are capable of dividing or not.
plant tissues is two types-
- Meristems or Meristematic Tissues – ever dividing tissue
- Permanent tissues non- dividing tissue formed after differentiation
- Meristematic Tissues –
Growth in plants is largely restricted to specialised regions of active cell division called meristems (Gk. meristos: divided).
- Plants have different kinds of meristems. The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems
- Root apical meristem occupies the tip of a root while the shoot apical meristem occupies the distant most region of the stem axis. During the formation of leaves and elongation of stem, some cells ‘left behind’ from shoot apical meristem, constitute the axillary bud.
- Such buds are present in the axils of leaves and are capable of forming a branch or a flower. The meristem which occurs between mature tissues is known as intercalary meristem. They occur in grasses and regenerate parts removed by the grazing herbivores.
- Both apical meristems and intercalary meristems are primary meristems because they appear early in life of a plant and contribute to the formation of the primary plant body.
- The meristem that occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis and appear later than primary meristem is called the secondary or lateral meristem. They are cylindrical meristems. Fascicular vascular cambium, interfascicular cambium and cork-cambium are examples of lateral meristems.
- These are responsible for producing the secondary tissues. Following divisions of cells in both primary and as well as secondary meristems, the newly formed cells become structurally and functionally specialised and lose the ability to divide. Such cells are termed permanent or mature cells and constitute the permanent tissues.
- During the formation of the primary plant body, specific regions of the apical meristem produce dermal tissues, ground tissues and vascular tissues.
Characteristics of Meristematic Tissue:
- The cells can divide throughout their life.
- These cells are thin-walled and are compactly arranged i.e., without intercellular spaces.
- These cells have only primary cell wall which is made up of cellulose.
- These cells may be isodiametric, rounded, oval or polygonal in shape.
- Cytoplasm is dense means vacuoles are either absent or are exceedingly small.
- The cells do not store reserve food as they are in highly active stage.
- Ergastic substances are usually absent in meristematic cells.
Classification of Meristems
Based on their origin, position, and functions:
- MERISTEMS ON THE BASIS OF ORIGIN ARE OF TWO TYPES:
⚪ Promeristem (Primordial Meristem or Embryonic Meristem)
- These are groups of meristematic cells in germinating embryos or young seedlings.
- Promeristems give rise to primary meristems viz. apical and intercalary meristems
⚪ Secondary Meristem-
Derived from permanent tissues (like parenchyma) by the process of differentiation.
- Interfascicular cambium (in dicot stem)
- Vascular cambium (in dicot roots)
- Cork cambium or phellogen
- Wound cambium
- Accessory cambium
- MERISTEMS ON THE BASIS OF POSITION
Meristems are classified into three types on the basis of position :
- Apical Meristems: These arise from Promeristem and form growing points at stem apex and roots apex.
- Being terminal in position, these meristems are called apical meristem.
- Intercalary Meristems
- These are the cells of the apical meristems, separated during the formation of permanent tissues.
- Present at the base of leaves (e.g., Pinus), above the nodes (e.g., Grasses) or just below the nodes (e.g., Mint).
- Intercalary meristems assist in lengthening of plant parts
- located along the lateral sides of stems, branches and roots.
- These meristems increase the girth or diameter of the plant organs.
- Lateral meristems are cylindrical, running throughout the plant body.
- Both primary and secondary in origin
- Meristem types (Based on Function):
Haberlandt (1914) classified primary meristem into three types: Protoderm
- it is the outermost layer of promeristem or apical meristem.
- It leads to epidermal tissue system.
- Epidermal tissue system includes epidermis, root hair, stem hair etc.
- SIMPLE PERMANENT TISSUES:
- A simple permanent tissue is homogeneous in nature viz. made up of similar types of permanent cells. These carry out the same or similar set of functions.
- Types of Simple Permanent Tissues:
- It is made up of thin-walled, isodiametric (equal diameter) living cells.
- The cells may be oval, rounded or polygonal in shape.
- Cell wall is cellulosic, thin and elastic.
- Small intercellular spaces are generally present between the cells for exchange of gases.
- Plasmodesmata are commonly present.
- It is made up of living cells which possess unevenly thickened cell walls.
- The thickening is made up of cellulose, pectin and hemicellulose.
- The cells are either isodiametric or somewhat elongated.
- They appear circular, oval or angular in transverse section. Each cell has a large central vacuole and peripheral cytoplasm
- It is a simple permanent tissue of dead and highly thick-walled cells.
- The thickening of walls may be made up of cellulose or lignin or both.
- Several unlignified or unthickened area called pits are often present on the walls.
- At maturity, these cells lose protoplasmic contents and become dead.
- Often the thickening in the cells is more so, central cavity (lumen) is obliterated.
- COMPLEX PERMANENT TISSUES
- The complex tissues are made of more than one type of cells and these work together as a unit.
- Xylem and phloem constitute the complex tissues in plants (Figure 6.3).
- Xylem functions as a conducting tissue for water and minerals from roots to the stem and leaves.
- It also provides mechanical strength to the plant parts. It is composed of four different kinds of elements, namely, tracheids, vessels, xylem fibres and xylem parenchyma.
- Gymnosperms lack vessels in their xylem. Tracheids are elongated or tube-like cells with thick and lignified walls and tapering ends.
- These are dead and are without protoplasm. The inner layers of the cell walls have thickenings which vary in form. In flowering plants, tracheids and vessels are the main water transporting elements.
- Vessel is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity. The vessel cells are also devoid of protoplasm.
- Vessel members are interconnected through perforations in their common walls. The presence of vessels is a characteristic feature of angiosperms. Xylem fibres have highly thickened walls and obliterated central lumens.
- These may either be septate or aseptate. Xylem parenchyma cells are living and thin-walled, and their cell walls are made up of cellulose. They store food materials in the form of starch or fat, and other substances like tannins. The radial conduction of water takes place by the ray parenchymatous cells
- Primary xylem is of two types – protoxylem and metaxylem.
- The first formed primary xylem elements are called protoxylem and the later formed primary xylem is called metaxylem.
- In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ. This type of primary xylem is called endarch.
- In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre. Such arrangement of primary xylem is called exarch.
- Phloem transports food materials, usually from leaves to other parts of the plant. Phloem in angiosperms is composed of sieve tube elements, companion cells, phloem parenchyma and phloem fibres. Gymnosperms have albuminous cells and sieve cells. They lack sieve tubes and companion cells. Sieve tube elements are also long, tube-like structures, arranged longitudinally and are associated with the companion cells. Their end walls are perforated in a sieve-like manner to form the sieve plates.
- A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus.
- The functions of sieve tubes are controlled by the nucleus of companion cells. The companion cells are specialised parenchymatous cells, which are closely associated with sieve tube elements.
- The sieve tube elements and companion cells are connected by pit fields present between their common longitudinal walls.
- The companion cells help in maintaining the pressure gradient in the sieve tubes. Phloem parenchyma is made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus.
- The cell wall is composed of cellulose and has pits through which plasmodesmatal connections exist between the cells. The phloem parenchyma stores food material and other substances like resins, latex and mucilage.
- Phloem parenchyma is absent in most of the monocotyledons. Phloem fibres (bast fibres) are made up of sclerenchymatous cells.
- These are generally absent in the primary phloem but are found in the secondary phloem. These are much elongated, unbranched and have pointed, needle like apices. The cell wall of phloem fibres is quite thick. At maturity, these fibres lose their protoplasm and become dead.
- Phloem fibres of jute, flax and hemp are used commercially. The first formed primary phloem consists of narrow sieve tubes and is referred to as protophloem and the later formed phloem has bigger sieve tubes and is referred to as metaphloem