Comprehensive Notes on Plant Anatomy and Tissues

Definition of Tissue: A tissue is defined as a group of cells that are similar in origin, form, and function.
Father of Plant Anatomy: Nehemiah Grew (N. Grew). He is credited with coining the term "tissue."
Living vs. Dead Tissue:
- Collenchyma: Classified as a living mechanical tissue.
- Sclerenchyma: Classified as a dead mechanical tissue, characterized by lignified cell walls.
- Parenchyma: A living tissue where plasmodesmata are quite common, maintaining cell-to-cell cytoplasmic connections.

Characteristics of Meristematic Cells:
- Cells are isodiametric in shape.
- They possess thin cellulosic cell walls.
- There is an absence of intercellular spaces and vacuoles.
- There is an absence of reserve food materials and plastids.
- These cells possess an active cell division capacity.
Classification of Meristems:
- Intercalary Meristem: These are portions of the apical meristem that have become separated by permanent tissues. They are responsible for the primary growth in grasses (e.g., regrowth after grazing).
- Secondary Meristem: These are derived from primary permanent tissues through the process of dedifferentiation.
- Examples of Meristems: Fascicular vascular cambium, interfascicular cambium, and cork cambium are all classified as meristems.

Parenchyma:
- Functions include storage, photosynthesis, and secretion.
- Note: Parenchyma does not provide support to the leaf petiole (that is typically the function of collenchyma).
Collenchyma:
- Consists of living cells with thin cell walls that are highly thickened at the corners.
- Absent in roots and monocot stems.
Sclerenchyma and Sclereids:
- Sclereids are dead cells with highly thickened walls and a narrow lumen.
- Commonly found in the fruit walls of nuts and the pulp of fruits like guava.
Tissue Groups (Matching):
- Meristematic: Cells with active cell division capacity.
- Simple Tissue: Tissue having cells similar in structure and function.
- Vascular Tissue: Tissue having different types of cells (Complex tissue).
- Sclereids: Dead cells with highly thickened walls and narrow lumen.

Main Conducting Elements:
- In soft wood containing plants (Gymnosperms), tracheids are the main water-conducting elements.
- Gymnospermic wood is characterized by the absence of vessels but the presence of tracheids.
Centripetal and Centrifugal Xylem:
- Exarch Xylem: The protoxylem is directed toward the periphery, while the metaxylem is toward the center (centripetal differentiation). This is a feature of roots.
- Endarch Xylem: The protoxylem is directed toward the center, while the metaxylem is toward the periphery (centrifugal differentiation). This is a feature of stems.
Xylem Components: Xylem vessels have thin cell walls that are highly thickened in corners while xylem parenchyma is a living component.

Sieve Tube Elements:
- Matured sieve tube elements possess a peripheral layer of cytoplasm and a large vacuole but lack a nucleus.
- Contrary to some statements, mature sieve tube elements do not possess a conspicuous nucleus.
Companion Cells: These are specialized parenchymatous cells closely associated with sieve tube elements.
Phloem Composition:
- Phloem parenchyma is absent in most monocots.
- In leaf anatomy, the phloem is typically directed towards the lower (abaxial) epidermis.

Root Characteristics:
- Vascular bundles are radial and exarch.
- Vascular bundles in monocot roots are characterized as polyarch (many xylem bundles).
- Roots do not show the presence of a collenchymatous hypodermis (hypodermis in roots is absent or different from stems).
- Pericycle: The initiation of lateral roots and the vascular cambium during secondary growth in dicot roots takes place in the cells of the pericycle.
Stem Characteristics:
- Monocot Stem: Characterized by a large number of scattered vascular bundles, each surrounded by a bundle sheath. Phloem parenchyma is absent, and vascular bundles are conjoint and closed (no cambium). Ground tissue is conspicuous and consists of parenchyma.
- Dicot Stem: Features vascular bundles arranged in a ring.
Stele: Includes all tissues internal to the endodermis, such as the pericycle, vascular bundles, and pith. The endodermis itself is not part of the stele.

Bulliform Cells:
- Large, empty, and colorless cells located on the adaxial (upper) epidermis of monocot leaf veins (e.g., grass leaves).
- They are responsible for the inward curling of leaves to minimize water loss.
Guard Cells and Stomata:
- Stomata in grass leaves are dumbbell-shaped.
- Specialized epidermal cells surrounding the guard cells are called subsidiary cells.
- Green cells in the epidermis that contain chloroplasts are the guard cells.
Vascular Bundle Types:
- Bicollateral: A vascular bundle with a $2:1$ ratio (implied arrangement) of phloem and xylem, typically with phloem on both the outer and inner sides of the xylem.
- Closed Bundles: These lack cambium and therefore do not undergo secondary growth.
- Water Cavities: Water-containing cavities within vascular bundles are found in plants like Maize (monocots).
Cortex: The region found between the epidermis and the stele (or vascular cylinder).
Medullary Rays: Cells of medullary rays that form part of the cambial ring are called interfascicular cambium.