In this chapter: meristematic and permanent tissues, xylem and phloem, the three tissue systems, types of vascular bundles, and the anatomy of dicot and monocot roots, stems and leaves.Meristematic tissues
Growth in plants is restricted to regions of active cell division called meristems.
- Apical meristems: at the tips of roots and shoots; produce the primary plant body. Some cells "left behind" by the shoot apical meristem form axillary buds.
- Intercalary meristems: between mature tissues, as in grasses; they regenerate parts removed by grazing herbivores.
- Lateral meristems: in mature regions, cylindrical; produce secondary tissues. Examples: fascicular vascular cambium, interfascicular cambium and cork cambium. These are secondary meristems, since they appear later in life.
Apical and intercalary meristems are primary meristems because they appear early in life and contribute to the primary plant body.
Permanent tissues
Simple tissues (one type of cell)
| Tissue | Cells | Function / where |
|---|---|---|
| Parenchyma | Living, thin cellulose walls; isodiametric (spherical, oval, polygonal, elongated); small intercellular spaces or closely packed | Major component within organs; photosynthesis, storage, secretion |
| Collenchyma | Living; walls thickened at the corners by cellulose, hemicellulose and pectin; no intercellular spaces; may contain chloroplasts | Mechanical support to growing parts; in layers below the epidermis of dicots, in leaf petioles; absent in monocots |
| Sclerenchyma | Dead, without protoplasm; thick, lignified walls with pits. Fibres (long, pointed) and sclereids (spherical, oval, with a narrow lumen) | Mechanical support. Sclereids in fruit walls of nuts, pulp of guava, pear and sapota, seed coats of legumes, leaves of tea |
Complex tissues (more than one type of cell)
Xylem conducts water and minerals from roots and gives mechanical strength.
- Tracheids: elongated, dead, thick lignified walls, tapering ends.
- Vessels: long, tube-like, made of many cells (vessel members) joined end to end with perforated walls; dead, lignified. Vessels are characteristic of angiosperms; gymnosperms lack them.
- Xylem fibres: highly thickened walls, obliterated central lumen.
- Xylem parenchyma: living, stores food (starch, fat) and tannins. It is the only living element of xylem.
The first-formed xylem is protoxylem; the later-formed is metaxylem. In stems protoxylem lies towards the centre (endarch); in roots it lies towards the periphery (exarch).
Phloem transports food, usually from leaves to other parts.
- Sieve tube elements: long, tube-like, end walls perforated in a sieve-like pattern (sieve plates). A mature sieve element has a peripheral cytoplasm and large vacuole but no nucleus; its functions are controlled by the companion cell.
- Companion cells: specialised parenchyma, connected to sieve tubes by pit fields; help maintain the pressure gradient.
- Phloem parenchyma: stores food, resins, latex and mucilage. Absent in most monocots.
- Phloem fibres (bast fibres): sclerenchymatous, generally absent in primary phloem. The only dead element of phloem. Jute, flax and hemp fibres are used commercially.
Gymnosperms have albuminous cells and sieve cells; they lack sieve tubes and companion cells.
The three tissue systems
Epidermal tissue system
- Epidermis: usually a single layer of parenchyma with a waxy cuticle that prevents water loss. The cuticle is absent in roots.
- Stomata: regulate transpiration and gaseous exchange. Two guard cells enclose the pore; guard cells are bean-shaped in most plants and dumb-bell shaped in grasses. Their outer walls are thin and inner walls (towards the pore) highly thickened. Guard cells have chloroplasts. Specialised epidermal cells around them are subsidiary cells. Pore + guard cells + subsidiary cells = stomatal apparatus.
- Root hairs are unicellular elongations of epidermal cells that absorb water and minerals. Trichomes on stems are usually multicellular, branched or unbranched, soft or stiff, sometimes secretory; they help prevent water loss.
Ground tissue system
This system includes all tissues except the epidermis and vascular bundles: parenchyma, collenchyma and sclerenchyma of the cortex, pericycle, pith and medullary rays. In leaves, the ground tissue is the thin-walled, chloroplast-containing mesophyll.
Vascular tissue system
| Type of vascular bundle | Meaning | Where |
|---|---|---|
| Radial | Xylem and phloem on different radii, alternating | Roots |
| Conjoint | Xylem and phloem on the same radius, phloem usually on the outer side | Stems and leaves |
| Open | Cambium present between xylem and phloem; can form secondary tissues | Dicot stems |
| Closed | No cambium | Monocots |
Anatomy of root
| Feature | Dicot root (sunflower) | Monocot root (maize) |
|---|---|---|
| Outer layer | Epiblema (epidermis) with unicellular root hairs | |
| Cortex | Several layers of thin-walled parenchyma with intercellular spaces | |
| Endodermis | Innermost cortex layer; barrel-shaped cells without intercellular spaces; tangential and radial walls have a band of water-impermeable, waxy suberin: the Casparian strips | |
| Pericycle | Thick-walled parenchyma just inside the endodermis; gives rise to lateral roots (and to vascular cambium in dicot secondary growth) | |
| Xylem bundles | 2 to 4 (diarch to tetrarch) | Usually more than 6 (polyarch) |
| Pith | Small or inconspicuous | Large, well developed |
| Secondary growth | Occurs | Does not occur |
The parenchyma between xylem and phloem is the conjunctive tissue. All tissues inside the endodermis (pericycle, vascular bundles, pith) form the stele.
Anatomy of stem
| Feature | Dicot stem (sunflower) | Monocot stem (maize) |
|---|---|---|
| Epidermis | Cuticle, trichomes, few stomata | Cuticle |
| Hypodermis | Collenchymatous (mechanical strength to young stem) | Sclerenchymatous |
| Endodermis | Innermost cortex layer; cells rich in starch, so called the starch sheath | Not distinct |
| Pericycle | Semi-lunar patches of sclerenchyma above the phloem | Not distinct |
| Vascular bundles | Arranged in a ring; conjoint, open, endarch protoxylem; separated by medullary rays | Scattered; each with a sclerenchymatous bundle sheath; conjoint, closed; peripheral bundles smaller than central ones; phloem parenchyma absent; water-containing cavities present |
| Pith | Large, with parenchyma and intercellular spaces | Ground tissue not differentiated into cortex and pith |
Anatomy of leaf
| Feature | Dorsiventral (dicot) leaf | Isobilateral (monocot) leaf |
|---|---|---|
| Stomata | More on the lower (abaxial) epidermis; may be absent on the upper | On both surfaces |
| Mesophyll | Differentiated into palisade (adaxial, elongated, parallel cells) and spongy parenchyma (oval or round, loosely arranged, with large air spaces) | Not differentiated |
| Venation | Reticulate: vascular bundles of varying size | Parallel: bundles of nearly similar size (except in main veins) |
| Special cells | – | In grasses, some adaxial epidermal cells along the veins become large, empty, colourless bulliform cells |
Vascular bundles in leaves are surrounded by a layer of thick-walled bundle sheath cells. Bulliform cells, when turgid, keep the leaf surface exposed; when they lose water (flaccid), the leaf curls inwards to reduce water loss.
Common traps: (1) Endarch = stem; exarch = root. (2) Collenchymatous hypodermis = dicot stem; sclerenchymatous = monocot stem. (3) Xylem parenchyma is the only living part of xylem; phloem fibres the only dead part of phloem. (4) Casparian strips are made of suberin, not lignin.NEET focus
- Types of meristems and examples; intercalary meristem in grasses.
- Elements of xylem and phloem: which are living, which dead; gymnosperm exceptions.
- Guard cell shape (bean vs dumb-bell), bulliform cells.
- Dicot vs monocot comparison tables for root, stem and leaf.
- Casparian strips and the role of pericycle.
Practice questions
Which is the only living element of xylem?
- Tracheids
- Vessels
- Xylem fibres
- Xylem parenchyma
Show answer
D.
Casparian strips are present in the:
- Pericycle
- Endodermis
- Epiblema
- Cortex outer layer
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B. Suberin bands on the tangential and radial walls of endodermal cells.
Scattered, closed vascular bundles with a sclerenchymatous bundle sheath are seen in:
- Dicot root
- Dicot stem
- Monocot stem
- Monocot root
Show answer
C.
Bulliform cells help in:
- Gas exchange
- Rolling of grass leaves during water stress
- Photosynthesis only
- Storage of starch
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B. When flaccid, they make the leaf curl inwards.
Gymnosperms lack:
- Tracheids
- Vessels and companion cells
- Sieve cells
- Xylem parenchyma
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B. They have tracheids, sieve cells and albuminous cells instead.
Lateral roots arise from the:
- Endodermis
- Pericycle
- Cortex
- Epidermis
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B. Lateral roots are endogenous, from the pericycle.
In a dicot root, the number of xylem bundles is usually:
- 2 to 4
- More than 6
- One
- Scattered, many
Show answer
A. Monocot roots are polyarch (more than 6).