- Anatomy is the study of the internal structure of plants and other organisms.
- A tissue is a group of cells having common origin and function.
- Based on the capability of cell division, plant tissues are divided into two groups: Meristematic and Permanent.
Meristematic Tissues (Meristems)
These are tissues where active cell division and growth occur. Based on position, meristems are classified into three types:
- Apical Meristems: Occur at the tips of roots and shoots, producing primary tissues. They are of two types:
- Root Apical Meristem: Occupies the tip of a root.
- Shoot Apical Meristem: Occupies the distant most region of the stem axis. Some cells left behind constitute the axillary bud, present in the axils of leaves, capable of forming a branch or flower.
- Intercalary Meristems: Occur between mature tissues, often in grasses, regenerating parts removed by grazing herbivores.
- Secondary (Lateral) Meristems: Occur in mature regions of roots and shoots, seen in gymnosperms and dicots. Examples include fascicular vascular cambium, interfascicular cambium, and cork cambium, responsible for producing secondary tissues.
Apical and intercalary meristems are primary meristems because they appear early in a plant’s life and contribute to the primary plant body, producing dermal tissues, ground tissues, and vascular tissues.
Permanent (Mature) Tissues
- Cells produced by primary and secondary meristems become structurally and functionally specialized, losing the ability to divide. These are called permanent (mature) cells and constitute permanent tissues.
- They are divided into two types: Simple and Complex.
1. Simple Permanent Tissues
Tissues with all cells similar in structure and function. There are three types: Parenchyma, Collenchyma, and Sclerenchyma.
a. Parenchyma
- Forms the major component within organs.
- Cells are generally isodiametric, spherical, oval, round, polygonal, or elongated.
- Walls are thin and made of cellulose.
- Cells are closely packed or have small intercellular spaces.
- Functions: Photosynthesis, storage, secretion, etc.
b. Collenchyma
- Occurs in layers below the epidermis in dicot plants.
- Found as a homogeneous layer or in patches.
- Cells have thickened corners due to deposition of cellulose, hemicellulose, and pectin.
- Intercellular spaces are absent.
- Cells are oval, spherical, or polygonal and often contain chloroplasts.
- Functions: Provide mechanical support to growing parts like young stems and leaf petioles. Cells with chloroplasts assimilate food.
c. Sclerenchyma
- Consists of long, narrow cells with thick, lignified cell walls having few or numerous pits.
- Cells are usually dead without protoplasts.
- Based on form, structure, origin, and development, sclerenchyma is divided into two types:
- Fibres: Thick-walled, elongated, pointed cells, generally occurring in groups.
- Sclereids: Spherical, oval, or cylindrical, highly thickened dead cells with narrow cavities (lumen). Found in fruit walls of nuts, pulp of fruits like guava, pear, and sapota, seed coats of legumes, and leaves of tea.
- Function: Provides mechanical support to organs.
2. Complex Permanent Tissues
These are made of more than one type of cell, working together as a unit. There are two types: Xylem and Phloem.
a. Xylem
- Functions as a conducting tissue for water and minerals from roots to stems and leaves.
- Provides mechanical strength to plant parts.
- Composed of four elements: tracheids, vessels, xylem fibres, and xylem parenchyma.
- Tracheids: Elongated, tube-like dead cells with thick, lignified walls and tapering ends. Protoplasm absent. Inner wall layers have varying thickenings. Main water-transporting elements in flowering plants, along with vessels.
- Vessels: Long, cylindrical, tube-like structures made of many vessel members with lignified walls and large central cavities. Protoplasm absent. Vessel members are interconnected through perforations in common walls. Characteristic of angiosperms; gymnosperms lack vessels.
- Xylem Fibres: Highly thickened walls with obliterated central lumens, septate or aseptate.
- Xylem Parenchyma: Living, thin-walled cells with cellulosic walls. Store food materials (starch or fat) and substances like tannins. Radial conduction of water occurs via ray parenchymatous cells.
- Primary xylem is of two types:
- Protoxylem: First-formed primary xylem.
- Metaxylem: Later-formed primary xylem.
- In stems, protoxylem lies toward the center (pith), and metaxylem toward the periphery, called endarch.
- In roots, protoxylem lies toward the periphery, and metaxylem toward the center, called exarch.
b. Phloem (Bast)
- Transports food materials from leaves to other parts.
- In angiosperms, composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres. Gymnosperms have albuminous cells and sieve cells, lacking sieve tubes and companion cells.
- Sieve Tube Elements: Long, tube-like structures arranged longitudinally, associated with companion cells. End walls are perforated, forming sieve plates. Mature sieve elements have peripheral cytoplasm and a large vacuole but lack a nucleus. Functions are controlled by companion cell nuclei.
- Companion Cells: Specialized parenchymatous cells closely associated with sieve tube elements, connected by pit fields in common longitudinal walls. Function: Maintain pressure gradient in sieve tubes.
- Phloem Parenchyma: Elongated, tapering cylindrical cells with dense cytoplasm and nucleus. Cell walls are cellulosic with pits for plasmodesmatal connections. Absent in most monocots. Function: Stores food materials and substances like resins, latex, and mucilage.
- Phloem Fibres (Bast Fibres): Sclerenchymatous cells, generally absent in primary phloem but found in secondary phloem. Elongated, unbranched, with pointed, needle-like apices and thick cell walls. Dead at maturity. Used commercially in jute, flax, and hemp. Function: Mechanical support and protection to soft tissues.
- Primary phloem consists of protophloem (narrow sieve tubes) and metaphloem (bigger sieve tubes).