3.1 The root #
3.1.1 Form, hairs, and endogenous branches #
The embryonic root is the radicle. Many eudicots and gymnosperms keep it as a taproot. Many monocots replace it with adventitious roots in a fibrous system. Behind the tip the order is cap, meristem, elongation, then differentiation, where short-lived root hairs and the first mature xylem appear. Older regions are often sealed with suberin and serve anchorage and conduction. Absorption belongs to young zones and to mycorrhizae.
Lateral roots arise in the pericycle and push out through the cortex. They are organs. A root hair is one epidermal cell. Shoot branches are exogenous; root branches are endogenous.
3.1.2 Primary anatomy and the Casparian strip #
A young root has a broad cortex and a narrow stele, the reverse of many stems. The innermost cortex is the endodermis. Its Casparian strip, lignin and suberin in the radial and transverse walls, is a gasket. Apoplastic water cannot cross it and must enter a protoplast, meeting membrane selectivity, before it reaches the xylem. The transporters are Cell Biology and Plant Physiology. Later suberin and thick walls seal many of these cells. Passage cells, often opposite protoxylem, may stay open. Many roots also have an exodermis, a second strip under the epidermis.
Orchid Reproductive Anatomy
The stele is a protostele: xylem in a star, phloem in the bays, maturation exarch. Eudicot roots are often diarch to pentarch and usually lack a pith. Monocot roots are often polyarch, with a pith. In a thickening eudicot root the cambium becomes a cylinder and cork cambium usually arises in the pericycle, so cortex and epidermis are shed. Exarch primary xylem in the centre still marks the organ as a root.
| Feature | Typical eudicot root | Typical monocot root |
|---|---|---|
| Xylem poles | Few, often 2–5 | Many |
| Pith | Usually absent | Usually present |
| Vascular cambium | Often present later | Usually absent |
A carrot is mostly root; a storage beet is root plus hypocotyl. Legume nodules are roots reorganised around nitrogen-fixing bacteria (Microbiology). The chemistry of fixation is not taught here. Pneumatophores and aerenchyma are the structural answer to hypoxic soil.
3.2 The stem #
A stem is nodes, bearing leaves and axillary buds, and internodes. That repetition is the external test. Bud scales, leaf scars, and bundle scars on a winter twig are the same arrangement out of season.
A herbaceous eudicot stem has epidermis, cortex, a ring of bundles (eustele), and pith. In each bundle phloem is outside and xylem inside. A leaf trace enters the leaf. Fern siphonosteles leave a parenchymatous leaf gap above a trace; a eustele is already broken into bundles, but the trace is real either way. A typical monocot stem scatters bundles in ground tissue (atactostele), each often fibre-sheathed, phloem towards the periphery and xylem towards the centre, with no fascicular cambium and no separate cortex and pith.
| Feature | Typical eudicot stem | Typical monocot stem |
|---|---|---|
| Bundles | Ring | Scattered |
| Ground tissue | Cortex and pith | Not divided that way |
| Woody cambium | Common in shrubs and trees | Absent in the typical case |
Lycophyte stems are often protosteles. Many fern stems are siphonosteles, a cylinder around a pith. A eustele is the seed-plant arrangement from the start, not a siphonostele that has split.
3.3 Wood and bark #
Early wood has wider conducting cells than late wood. The junction is an annual ring only where the climate makes one ring a year; the wet tropics may lack rings, and a pause can insert a false one. Sapwood still conducts. Heartwood has dead parenchyma, vessels often blocked by tyloses, and decay-resistant extractives. It is not the main water path. Rays are the radial path while alive. Conifers add compression wood below a lean; many angiosperms add tension wood above. Gravity is the stimulus. The hormone link is Plant Physiology.
As girth increases, epidermis and cortex split and cork replaces them. Tissue outside a cork layer dies. Later cork cambia form in the secondary phloem, and the dead accumulation is the rhytidome. Bark, in the working sense here, is everything outside the vascular cambium: living secondary phloem plus periderm. The cambium is only a few cells thick, immediately under the inner bark. Lenticels keep a gas path through it.
Malpighi’s seventeenth-century girdling is the clearest functional evidence. A ring that removes phloem and cambium, and leaves the xylem, does not immediately wilt the crown, because water still rises in the sapwood. Tissue above the cut swells with sugar. Roots below starve, and the tree dies when they fail. The downward food path is outside the wood. A three-year twig, from the outside inward, is periderm, secondary phloem, cambium, ringed secondary xylem, primary xylem, and pith.
3.4 The leaf #
3.4.1 Marks and anatomy #
A leaf typically has a lamina, a petiole, and in many eudicots stipules. Grasses sheath the stem and often have a ligule. Venation is usually reticulate in eudicots and parallel in monocots, with exceptions (Trillium is a net-veined monocot), so venation is evidence, not a clade diagnosis. Leaflets of a compound leaf do not have axillary buds; the bud sits in the axil of the whole leaf. Adaxial and abaxial are developmental faces, not “up” and “down”. Inside a vein, xylem is adaxial and phloem abaxial even if the leaf has twisted.
A dorsiventral leaf has an upper epidermis, palisade and spongy mesophyll, veins, and most stomata below. Carbon dioxide moves through the air space and dissolves in the water film of the wall, which is why gaining carbon costs water. The pathways of fixation are Plant Physiology. The leaf’s contribution is a short air path and veins close to the fixing cells. A bundle sheath is general. Kranz anatomy is the C4 form of it: a chloroplast-rich sheath against the mesophyll. A pine needle, with a thick cuticle and sunken stomata, is one xeromorphic tendency, not a universal design. Floating leaves put their stomata above.
Leaves are shed at an abscission zone, a separation layer and a suberised scar. Auxin and ethylene time the separation (Plant Physiology). Bundle scars in that scar are what a winter key uses.
3.5 Modifications #
Identify the organ first. A stem has nodes, leaves or scars, and buds. A root has a cap, endogenous branches, and no leaves. A leaf has an axillary bud at its base and a leaf trace. Function comes second, and explains why the form changed.
| Structure | Organ | Test and function | Example |
|---|---|---|---|
| Tuber | Stem | Eyes are buds; stores starch | Potato |
| Storage root | Root | No nodes; stores | Sweet potato; carrot |
| Rhizome | Stem | Scale leaves; spreads and stores | Iris |
| Stolon | Stem | Plantlets at nodes, fed by the parent | Strawberry |
| Bulb | Stem plus leaves | Fleshy leaf bases on a short stem | Onion |
| Corm | Stem | Swollen stem, papery leaves | Crocus |
| Tendril | Stem or leaf | Axillary shoot, or a leaflet; climbs | Grape; pea |
| Spine | Leaf | Leaf position; defends | Cactus |
| Thorn | Stem | Axillary branch; defends | Hawthorn |
| Prickle | Emergence | No vascular trace; defends | Rose |
| Cladode | Stem | Photosynthetic stem, leaves reduced | Butcher’s broom |
Potato and sweet potato store starch and are different organs. A cactus spine is a leaf on an areole, which is a short shoot, and the flat pad is a cladode. A rose prickle has no stele. The same job does not make the same organ.
3.6 Key terms, summary, and self-check #
Key terms. Pericycle; Casparian strip; eustele; atactostele; leaf trace; sapwood; heartwood; rhytidome; palisade; Kranz anatomy; abscission zone; tuber; rhizome; spine; thorn; prickle.
Summary. Roots branch from the pericycle, seal the stele with a Casparian strip, and hold an exarch xylem star in a broad cortex; monocot roots are often polyarch with a pith. Stems are nodes and internodes, bundles in a ring or scattered. Secondary growth adds rings of wood, a non-conducting heartwood, and bark outside a thin cambium; girdling shows that the food path is in that bark. Leaves put chlorenchyma next to an air path and a vein path; carbon biochemistry is not repeated here. Tubers, rhizomes, tendrils, spines, thorns, and prickles are named by nodes and traces, then understood by storage, climbing, or defence.
Self-check.
1. What does the Casparian strip force an apoplastic solute to do?
2. Give one root difference and one stem difference between a typical eudicot and a typical monocot.
3. Why can a girdled tree stay green for a time and still die?
4. In a dorsiventral leaf, where is xylem relative to phloem in a vein, and where are most stomata?
5. Which organ is a potato, which is a sweet potato, and how does a thorn differ from a prickle?
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