Plant Structure and Function

This is Unit Eleven of Biology, Part One: tissues, growth, and organs. A vascular plant has two organ systems — a shoot above ground and a root system usually below. Note what shoot includes: stems and leaves, but also flowers and fruits. Reproductive structures are not a third category. Then the distinction that everything else here rests on. Meristematic tissue keeps dividing; permanent tissue has differentiated and stopped. Permanent does not mean long-lived or short-lived — it names lost division ability, and many permanent cells work for years afterward. The central fact about plant growth is where it happens. A stem lengthens only at its tip, from new cells added at the apical meristem — never by stretching along its existing length. Which produces a consequence people find genuinely surprising: a nail hammered into a trunk stays at exactly that height for the tree's whole life. Thickening is a separate process, from two lateral meristems, and it is why that same lower trunk gets wider every year while getting no taller at all. By the end you will be able to do five things. One: tell meristematic from permanent. Two: tell simple tissue from complex. Three: say why the nail never rises. Four: tell primary from secondary growth. Five: name a leaf's three elements.

Three entries. The body in two systems, the division between tissue that still divides and tissue that has finished, and the three places where dividing tissue sits — one of which exists in only half the flowering plants.

Above and below.

Bio 2501 Plant Organ Systems: Shoot and Root

A vascular plant organises into two organ systems: an aboveground shoot of stems, leaves, flowers and fruits, and a usually underground root system that anchors the plant and absorbs water and minerals. Shoot includes the reproductive parts — flowers and fruits belong there rather than in a category of their own.

Dividing, or done.

Bio 2503 Meristematic versus Permanent Tissue

Meristematic tissue is continuously dividing undifferentiated cells; permanent tissue is differentiated cells that no longer divide — a root tip's meristem giving rise to cells that specialise. Permanent describes lost division ability, not lifespan; many permanent cells go on functioning for years.

Three places.

Bio 2505 Three Meristem Locations: Apical, Lateral, Intercalary

Apical meristems at stem and root tips extend length, lateral meristems thicken, and intercalary meristems at leaf bases elongate — the last found only in monocots, which is why mowed grass regrows from below the cut. A mowed dicot leaf cannot do the same.

Three entries. What meristem cells differentiate into, the difference between a tissue of one cell type and a tissue of several, and the three ground-tissue cells that do the structural work.

Three destinations.

Bio 2507 Dermal, Vascular, and Ground Tissue

Meristematic cells differentiate into three permanent tissue types: dermal for covering, vascular for transport, and ground for photosynthesis and support — a stem's epidermis, its xylem and phloem, its pith and cortex. All three trace to one meristematic origin, so the distinction is function rather than lineage.

One type, or several.

Bio 2509 Simple versus Complex Tissue: Xylem and Phloem

A simple tissue is made of one cell type, a complex tissue of several — xylem combining vessel elements, tracheids and parenchyma; phloem combining sieve-tube cells, companion cells, parenchyma and fibres. Xylem's conducting cells are dead at maturity while phloem's stay alive, a distinction worth keeping straight.

Three ground cells.

Bio 2511 Stem Anatomy: Parenchyma, Collenchyma, Sclerenchyma

A stem's ground tissue is built from parenchyma for metabolism and storage, collenchyma for flexible support, and sclerenchyma for rigid support — celery's strings are collenchyma, a pear's grittiness comes from sclerenchyma. Most sclerenchyma is dead at maturity; the rigid cell wall outlives the cell itself.

Four entries, and the middle two are a twinned pair. The first states where lengthening happens, the pair corrects the picture people carry, and the fourth explains why a plant grows up rather than out.

At the tip.

Bio 2513 Primary Growth: Apical Meristem Elongation

Primary growth lengthens a stem or root through cell division and elongation confined to the apical meristem at each tip — a seedling lengthens entirely from new cells added at its growing tip, not from stretching along its existing length. Herbaceous plants rely almost entirely on this, thickening little over a lifetime.

The popular image.

Bio 2515 "A Tree Grows Taller by Pushing Up From the Base"

Picturing a tree's trunk lengthening from its base overstates a real process: growth in length happens only at the tips. A tree's lower trunk, already differentiated, adds girth over the years but never grows any longer once that section has matured.

The consequence.

Bio 2517 Why a Nail in a Trunk Never Rises

A nail hammered into a tree trunk stays at the same height above ground for the tree's entire life, because primary growth happens only at apical meristems and the trunk section holding the nail never elongates again. The from-the-base image mistakes lower-trunk girth increase for lengthwise growth that does not occur there.

Suppressed branches.

Bio 2519 Apical Dominance

Apical dominance is the apical bud suppressing axillary bud growth, keeping shoot growth concentrated at the main tip — a conifer's strong dominance producing its conical shape, with axillary buds branching out once that tip is removed. Pruning exploits this deliberately to encourage a bushier plant.

Two entries. The second growth process, from two lateral meristems, and what a ring actually records — which is less than counting them suggests.

Two cambiums.

Bio 2521 Secondary Growth: Vascular Cambium and Cork Cambium

Secondary growth thickens a stem through division in two lateral meristems: the vascular cambium adding xylem inward and phloem outward each season, and the cork cambium adding bark. It occurs in some dicots and rarely in monocots, which mostly lack the lateral meristems it depends on.

One year, one ring.

Bio 2523 Annual Rings

An annual ring forms from the contrast between wide thin-walled spring wood and denser thick-walled autumn wood produced later the same year, so counting rings estimates a tree's age. A ring records one season's growing conditions rather than a fixed clock — width varies with rainfall and temperature.

Five entries. Stems modified until they look like other organs, two root architectures, the three zones packed into a root tip, and a waxy seal that decides what reaches the vascular tissue.

Stems in disguise.

Bio 2525 Stem Modifications

Modified stems include the horizontal rhizome, fleshy corm, ground-level stolon, above-ground runner, starch-storing tuber and layered bulb — a strawberry spreading by runners, a potato a tuber at a stolon's swollen end. Tendrils and thorns are aerial stem modifications too, despite looking like roots or leaves.

Deep, or wide.

Bio 2527 Taproot versus Fibrous Root Systems

A taproot system has one deep main root with few laterals, typical of dicots; a fibrous system spreads a dense shallow network typical of monocots — a dandelion regrowing from a taproot fragment, lawn grass resisting erosion near the surface. Root type correlates with that split without defining it; some plants combine both.

Three zones.

Bio 2529 Root Growth Zones

A root tip divides into three zones: cell division nearest the tip, elongation directly behind it, and maturation where cells differentiate. The elongation zone lengthens the root by stretching newly formed cells rather than dividing them further. All three sit within roughly the first centimetre.

A waxy seal.

Bio 2531 Root Anatomy: Endodermis and Casparian Strip

A root's endodermis, sealed by a waxy Casparian strip, forces water and solutes through cell membranes rather than between cells, screening what enters the vascular tissue — blocking toxins and pathogens from slipping past while still admitting water. The root cap protecting the growing tip is a separate structure.

Roots in other roles.

Bio 2533 Root Modifications

Modified roots include storage taproots such as carrots, aboveground prop and aerial roots for support, and epiphytic roots absorbing moisture from rain and air while anchored on another plant — a banyan's aerial roots descending to the soil. An epiphytic root anchors on its perch and draws nothing from the host.

Three entries closing the part. What a leaf is made of, how a blade divides, and how leaves sit on a stem — the last being a species-level constant rather than something a plant varies.

Three elements.

Bio 2535 Leaf Structure: Mesophyll, Stomata, Veins

A leaf's interior mesophyll houses the chloroplasts driving photosynthesis, its epidermis carries stomata flanked by guard cells for gas exchange, and its veins carry vascular tissue — palisade mesophyll packed above looser spongy mesophyll whose air spaces reach the stomata. Guard cells are the only epidermal cells with chloroplasts.

One blade, or many.

Bio 2537 Simple versus Compound Leaves

A simple leaf has one undivided or lobed blade; a compound leaf divides fully into leaflets attached to a shared rachis — a banana leaf against a locust leaf split into many leaflets. A compound leaf's leaflets are not separate leaves; the whole cluster counts as one leaf at its base.

How they sit.

Bio 2539 Leaf Arrangement: Phyllotaxy

Phyllotaxy names a plant's characteristic leaf arrangement: alternate or spiral placing one leaf per node, opposite placing two directly across from each other, and whorled placing three or more. It is species-characteristic — each species keeps one consistent pattern rather than mixing arrangements.

Five things to carry out of Part One. Meristematic tissue divides continuously and stays undifferentiated; permanent tissue has differentiated and stopped dividing. Permanent names the lost ability, not a short life — plenty of permanent cells function for years. Meristem sits at tips, along the sides, and in monocots at leaf bases. Dermal tissue is simple, made of one cell type. Vascular tissue is complex: xylem combines vessel elements, tracheids and parenchyma, and phloem combines sieve tubes, companion cells, parenchyma and fibres. Xylem's conducting cells are dead at maturity; phloem's are alive. A nail never rises because primary growth happens only at apical meristems. A stem lengthens by adding cells at its tip, never by stretching along its length, so once a section of trunk has matured it never gets any longer — it only gets wider. Primary growth lengthens, secondary growth thickens. Primary comes from apical meristems at the tips; secondary from two lateral meristems, the vascular cambium laying down xylem and phloem and the cork cambium laying down bark. Herbaceous plants do almost no secondary growth at all. And a leaf has three structural elements. Mesophyll holds the chloroplasts and does the photosynthesis, in a dense palisade layer above a looser spongy one. Stomata in the epidermis handle gas exchange, opened and closed by guard cells — the only epidermal cells with chloroplasts of their own. Veins carry the vascular tissue. Part Two turns to transport and the plant hormones.