Plant Anatomy (Botany): plant tissues, anatomy of dicot and monocot root, stem and leaf, secondary growth

(c) Mesophyll

Mesophyll is the ground tissue of the leaf, made of chlorenchyma cells (palisade and spongy parenchyma) that contain chloroplasts, so it is the main site of photosynthesis. Aerenchyma is air-filled tissue for buoyancy/ventilation, xylem conducts water, and sclerenchyma is dead supportive tissue without chloroplasts.

Internal structure of a monocot root (T.S. of maize, Zea mays)

A transverse section shows the following tissues from outside to inside.

1. Epiblema (rhizodermis): Outermost single layer of thin-walled cells without cuticle. Some cells form unicellular root hairs that absorb water and minerals.

2. Cortex: Broad zone of thin-walled parenchyma with intercellular spaces; it stores food and conducts water to the stele. The outer layer may form an exodermis.

3. Endodermis: Innermost layer of the cortex, of barrel-shaped cells with Casparian strips on their radial and transverse walls. Thin-walled passage cells lie opposite the protoxylem and let water enter the stele.

4. Pericycle: Single layer of parenchyma inside the endodermis. It gives rise to lateral roots only, because there is no secondary growth.

5. Vascular bundles: Many (polyarch, usually more than six, 8 to 30), radial, with xylem and phloem on separate alternating radii.

• Xylem: exarch (protoxylem towards the periphery, metaxylem towards the centre).
• Phloem: sieve tubes and companion cells, lying between the xylem arms.

6. Conjunctive tissue: Parenchyma between xylem and phloem.

7. Pith: Large, well-developed central mass of parenchyma that stores food.

Diagram (labelled, T.S. of monocot root):

 root hair  <-- Epiblema (outermost layer)
 +-------------------------------------+
 |  Cortex (parenchyma)                |
 |   +-----------------------------+   | <-- Endodermis (Casparian strips,
 |   |  Pericycle                  |   |     passage cells)
 |   |   +---------------------+   |   |
 |   |   |  Xylem (exarch)     |   |   |
 |   |   |  Phloem (alternate) |   |   |
 |   |   |  Conjunctive tissue |   |   |
 |   |   |   PITH (large)      |   |   |
 |   |   +---------------------+   |   |
 |   +-----------------------------+   |
 +-------------------------------------+

Key features: polyarch, exarch xylem, large pith, and no secondary growth.

(a) The plant is monocot.

Scattered vascular bundles distributed throughout the ground tissue are characteristic of monocot stems; dicot stems have vascular bundles arranged in a ring.

Simple permanent tissue vs complex permanent tissue

Key point: Simple tissues are homogeneous and mainly protective, storage or supporting in function, while complex tissues are heterogeneous and mainly conducting in function.

(b) Vascular cambium and cork cambium

The vascular cambium adds secondary xylem and phloem (increasing girth), while the cork cambium forms the periderm that replaces the ruptured epidermis — together they bring about secondary growth in thickness.

Differences between monocot and dicot stems (anatomical features)

Key point: Scattered closed bundles with no distinct ground-tissue layers mark a monocot stem. A ring of open bundles with a clear cortex and pith marks a dicot stem. Both stems are endarch.

Parenchyma tissue

Definition (1 mark) Parenchyma is a simple permanent living tissue made up of thin-walled, loosely packed, unspecialised cells that form the bulk of soft parts of a plant such as cortex, pith, mesophyll and pulp of fruits.

Structure (2 marks)

1. Cells are living, thin-walled and made of cellulose (primary wall only).
2. Cells are roughly isodiametric — oval, round or polygonal — with no thickening at the corners.
3. Cells are loosely packed, leaving prominent intercellular spaces between them.
4. Each cell has a large central vacuole, dense cytoplasm and a distinct nucleus.
5. The wall is thin and permeable, so materials can pass through easily.
6. In some parts the cells may develop special features, giving subtypes:
   • Chlorenchyma — contains chloroplasts (as in leaf mesophyll).
   • Aerenchyma — has large air cavities (as in aquatic plants).
   • Prosenchyma — cells become elongated and pointed (as in pericycle).

Functions (1 mark)

1. Fills up the soft parts of the plant and gives mechanical support by maintaining turgidity.
2. Stores food in the form of starch, sugar, fat and protein (e.g. potato tuber, fruit pulp).
3. Chlorenchyma carries out photosynthesis.
4. Aerenchyma provides buoyancy and helps in gaseous exchange in aquatic plants.
5. Provides a passage for the movement of water and dissolved substances from cell to cell.
6. Helps in wound healing and regeneration by becoming meristematic when required.

(c) Cell elements are interconnected by pits in tracheids and pores in vessels

Tracheids are long, tapering cells whose end walls are not perforated, so adjacent cells communicate through bordered pits. Vessels are made of many vessel elements joined end to end with perforations (pores) in their end walls, forming a continuous tube. Vessels have wider, not narrower, cavities than tracheids. Tracheids, not vessels, occur in all vascular plants, since vessels are found mainly in angiosperms.

Vascular cambium vs cork cambium

In short: the vascular cambium is a lateral meristem of vascular origin that produces secondary vascular tissues (xylem and phloem), while the cork cambium is a lateral meristem of cortical origin that produces the protective periderm (cork and phelloderm).

(c) To produce new xylem and phloem

The vascular cambium is a lateral meristem. It cuts off secondary xylem towards the inside and secondary phloem towards the outside, so the stem and root of perennial dicots increase in girth every year.

Diagram: T.S. of a typical dicot stem (e.g. sunflower)

Draw a circle and, from outside to inside, show and label:

• Epidermis: single outer layer with cuticle and multicellular hairs.
• Cortex: hypodermis (collenchyma), middle layer of parenchyma (chlorenchyma), and endodermis (starch sheath) as the innermost layer.
• Pericycle: patches of sclerenchyma lying above each vascular bundle.
• Vascular bundles: arranged in a ring, each with xylem (inner), cambium (middle) and phloem (outer).
• Medullary rays: parenchyma between the bundles.
• Pith: large central parenchymatous region.

Two anatomical characteristics of the vascular bundles

1. They are conjoint, collateral and open: xylem and phloem lie on the same radius, and a strip of cambium is present between them, so secondary growth is possible.
2. They are arranged in a ring around the pith, with endarch xylem, meaning protoxylem lies towards the pith and metaxylem towards the periphery.

(b) Sieve tube

A mature sieve tube element of phloem loses its nucleus but keeps living cytoplasm, and its function is controlled by the neighbouring companion cell. Vessels and tracheids are dead at maturity with no cytoplasm, while companion cells have a nucleus.

Diagrams

• Collenchyma: draw cells in T.S. with unevenly thickened corners (angular type) and a few intercellular spaces, with labels for cell wall thickening, cytoplasm and nucleus. Add an L.S. showing elongated cells.
• Sclerenchyma: draw (a) fibres, long and spindle-shaped with pointed ends, and (b) sclereids, short and irregular, with a narrow lumen and pits in a very thick lignified wall.

Distinction

(a) Exarch

In an exarch bundle the protoxylem lies towards the periphery and the metaxylem towards the centre, so xylem differentiates centripetally. This is typical of roots. In endarch bundles (stems) protoxylem is toward the centre, and in mesarch it is in the middle.

(a) Characteristics of layer Y (1 mark)

Layer Y is the cambium: a thin layer of meristematic, thin-walled, living cells with dense cytoplasm and large nuclei, lying between the xylem and phloem, which divide repeatedly to add new tissue.

(b) Tissue responsible for secondary growth (3 marks)

Diagram: draw a transverse section (circular outline) showing from outside to inside epidermis, cortex, phloem, vascular cambium, xylem and pith, and label the cambium.

The vascular cambium (with the cork cambium later) is responsible for secondary growth. Formation of the cambial ring in a dicot stem:

1. The intrafascicular cambium is the strip of cambium lying within each open vascular bundle between xylem and phloem.
2. Cells of the medullary rays (parenchyma) lying between neighbouring bundles, in line with the fascicular cambium, become meristematic and form the interfascicular cambium.
3. The intra- and interfascicular strips join to form a continuous cambial ring.
4. The ring then cuts off secondary xylem inwards and secondary phloem outwards, thickening the stem.

(c) Activating cell division

Meristematic tissue is made of young cells that keep dividing actively, which gives growth to the plant. Permanent tissue is formed when these cells stop dividing and differentiate, so ability to divide is the basis of distinction. Supporting and providing food are functions of some permanent tissues only.

Vascular bundle is a strand-like conducting unit of a plant that contains xylem and phloem (with or without cambium) and carries water, minerals and food.

Type in monocot root: radial vascular bundle, in which xylem and phloem lie on separate radii, alternating with each other. It is polyarch (usually more than six xylem groups) and exarch.

Functions (any two)

• Xylem conducts water and dissolved minerals from roots upward to the leaves.
• Phloem translocates prepared food from leaves to the storage and growing parts.
• It gives mechanical support to the plant body.