Tissues · Tissue systems · Root, stem & leaf anatomy · Secondary growth
Anatomy is the study of internal structure. Though flowering plants look very different outside, they are built from similar tissues inside. A tissue is a group of cells with a common origin, usually performing a common function.
On the basis of dividing ability, tissues are meristematic (actively dividing) or permanent (lost the ability to divide).
| Meristem | Location | Function |
|---|---|---|
| Apical | Tips of root & shoot | Primary growth — increase in length |
| Intercalary | At base of leaves / internodes (e.g. grasses) | Part of primary meristem left behind; helps regrowth after grazing/mowing |
| Lateral | Along the sides (vascular cambium, cork cambium) | Secondary growth — increase in girth/thickness |
Cells made by the apical meristem differentiate and mature into permanent tissues — this is differentiation. When a mature cell regains the ability to divide it is dedifferentiation (e.g. cork cambium, interfascicular cambium); when such tissue again loses dividing ability it is redifferentiation.
A simple tissue is made of only one type of cell. There are three: parenchyma, collenchyma and sclerenchyma.
| Feature | Parenchyma | Collenchyma | Sclerenchyma |
|---|---|---|---|
| Cell wall | Thin, cellulosic | Thickened at corners (cellulose, hemicellulose, pectin) | Thick, lignified with narrow lumen |
| Living? | Living | Living | Usually dead at maturity |
| Shape/arrangement | Isodiametric; little space | Oval/round; in layers below epidermis, often in patches | Long fibres or short sclereids (stone cells) |
| Main job | Storage, photosynthesis (chlorenchyma), secretion | Mechanical support to growing parts; flexibility | Mechanical strength & rigidity |
| Where | Everywhere — bulk of soft tissue | Petioles, leaf margins, dicot stem hypodermis | Fibres (jute, flax) & sclereids (nut shells, pear grit) |
Note: collenchyma is the only living mechanical tissue and the only one giving support plus flexibility to growing organs.
A complex tissue is made of more than one type of cell working as a unit. The two complex tissues — xylem and phloem — together make up the vascular bundle.
| Element | Living/Dead | Role |
|---|---|---|
| Tracheids | Dead, elongated, tapering, lignified | Chief water-conducting element; present in all vascular plants |
| Vessels (trachea) | Dead tube with wide lumen, perforated end walls | Efficient water conduction; characteristic of angiosperms |
| Xylem fibres | Dead, thick-walled | Mechanical support |
| Xylem parenchyma | Living | Stores food; radial conduction of water |
Primary xylem is of two kinds by the order in which it matures: protoxylem (first formed) and metaxylem (later formed).
| Arrangement | Protoxylem position | Found in |
|---|---|---|
| Endarch | Towards the centre (proto inside, meta outside) | Stems |
| Exarch | Towards the periphery (proto outside) | Roots |
Based on structure & location, all tissues of a plant fall into three tissue systems (a classification by J. von Sachs).
| Tissue system | Made of | Function |
|---|---|---|
| Epidermal | Epidermal cells, stomata, trichomes & root hairs | Outer protective covering; gas exchange; reduces water loss (cuticle) |
| Ground | Parenchyma, collenchyma, sclerenchyma (everything except epidermis & vascular) | Photosynthesis, storage, support; includes cortex, pericycle, pith, medullary rays |
| Vascular | Xylem + phloem (the vascular bundles) | Conduction of water, minerals & food |
| Type | Description |
|---|---|
| Radial | Xylem & phloem on different radii, alternating — typical of roots |
| Conjoint | Xylem & phloem on the same radius, phloem usually outside xylem — typical of stems & leaves |
| Open | Cambium present between xylem & phloem → can show secondary growth (dicot stem) |
| Closed | No cambium → no secondary growth (monocot stem) |
| Feature | Dicot root (e.g. gram) | Monocot root (e.g. maize) |
|---|---|---|
| Xylem arches | 2–4 (di/tetrarch); limited number | Many — polyarch |
| Pith | Small or absent | Large, well developed |
| Secondary growth | Present | Absent |
| Common | Both: epiblema (with root hairs) → cortex → endodermis with Casparian strips → pericycle → radial & exarch vascular bundles. Pericycle gives rise to lateral roots. | |
| Feature | Dicot stem (e.g. sunflower) | Monocot stem (e.g. maize) |
|---|---|---|
| Vascular bundles | In a ring; conjoint, open, endarch | Scattered in ground tissue; conjoint, closed |
| Bundle sheath | Absent | Sclerenchymatous sheath present |
| Ground tissue | Differentiated into hypodermis (collenchyma), cortex, endodermis, pericycle, pith | Not differentiated; ground parenchyma throughout |
| Special feature | Medullary rays between bundles | Water-containing cavity in each bundle (in monocots like maize) |
| Secondary growth | Present (open bundles) | Absent (closed bundles) |
The leaf blade in section shows three parts: epidermis, mesophyll (the photosynthetic ground tissue) and vascular system (veins).
| Feature | Dorsiventral (Dicot) leaf | Isobilateral (Monocot) leaf |
|---|---|---|
| Orientation | Held more or less horizontal; two distinct faces | Held vertical/erect; both faces alike |
| Mesophyll | Differentiated into palisade (upper, columnar) & spongy (lower, loose) | Not differentiated — uniform spongy-type cells |
| Stomata | More on the lower surface | Equal on both surfaces |
| Special cells | — | Bulliform (motor) cells in upper epidermis — lose turgor to roll/fold the leaf, reducing water loss |
| Vein endings | Reticulate venation | Parallel venation; similar-sized bundles in a row |
In C₄ plants (maize, sorghum, sugarcane) the vascular bundles are surrounded by a ring of large, thick-walled bundle-sheath cells rich in chloroplasts and without intercellular spaces — the Kranz ("wreath") arrangement. This separates the two stages of C₄ photosynthesis spatially and makes these plants very efficient.
Secondary growth increases the girth/diameter of stems & roots. It is brought about by two lateral meristems: the vascular cambium and the cork cambium.
| Spring / Early wood | Autumn / Late wood | |
|---|---|---|
| Cambium activity | High | Low |
| Vessels | Wider, more numerous | Narrower, fewer |
| Appearance | Lighter, less dense | Darker, denser |
One ring of spring + autumn wood = one annual ring. Counting rings estimates a tree's age (dendrochronology).
| Heartwood (duramen) | Sapwood (alburnum) |
|---|---|
| Inner, dark, dead; tylose-filled; no conduction; gives strength & durability | Outer, lighter, living; conducts water |