Anatomy of Flowering Plants — NEET Notes

Meristematic tissues — where the plant keeps growing

A plant grows only at points where its cells retain the ability to divide. Such permanently embryonic tissue is called a meristem. The cells are small and roughly isodiametric, packed without intercellular spaces, thin-walled with cellulose, and crowded with cytoplasm and a prominent nucleus; their vacuoles are negligible. From these dividing cells, every other tissue in the plant body is ultimately derived.

NCERT classifies meristems on the basis of their position in the plant body into three types — apical, intercalary and lateral. The first two are primary meristems, responsible for the primary plant body and for growth in length. The third is a secondary meristem that produces the secondary plant body and is responsible for growth in girth.

Apical and intercalary meristems together compose the primary meristem, because they appear early in the life of a plant and contribute to the primary plant body. Lateral meristems are secondary — they appear later in mature regions, especially of dicots, and they are entirely responsible for the wood you see in a tree trunk.

Simple permanent tissues — parenchyma, collenchyma, sclerenchyma

Where meristems stop dividing, their daughter cells differentiate into permanent tissues. A simple permanent tissue is made of only one cell type doing one job. Three of them carry the bulk of the plant body, and NEET expects you to know each one's wall thickness, living status, and where it occurs.

Complex tissues — xylem and phloem

A complex permanent tissue is built from more than one cell type that work as a single functional unit. There are only two: xylem, which conducts water and minerals upward, and phloem, which translocates the food synthesised by the leaves in both directions. Both are the workhorses of the vascular tissue system; together they make up every vascular bundle.

Xylem is composed of four elements. Tracheids are elongated, tapering, dead cells with pitted lignified walls — present in all vascular plants and the only conducting element of gymnosperms. Vessels are shorter, broader, dead tube-cells whose end-walls have dissolved away to form an open pipe; they are characteristic of angiosperms. Xylem fibres are dead, thick-walled and provide mechanical strength. Xylem parenchyma is the only living component, storing food and helping with radial water transport.

Phloem is also four-celled. Sieve tube elements are living but enucleate — their end-walls are perforated into characteristic sieve plates through which sap moves. Companion cells sit beside each sieve element, connected by plasmodesmata, and provide all the nuclear and metabolic services the sieve cell has surrendered. Phloem parenchyma stores food and resins. Phloem fibres are dead, sclerenchymatous and provide support — they are absent from the phloem of the monocot stem and explain why option-(d) is correct in NEET 2020's monocot-stem identification question.

The three tissue systems

Tissues are grouped by location into three tissue systems — sets of tissues that occupy specific positions and share a common functional brief. NCERT names exactly three: the epidermal system that covers the plant, the ground (fundamental) system that fills it, and the vascular (conducting) system that transports through it.

Within the epidermal layer of the leaf, two specialised cells form the stomatal apparatus: a pair of bean-shaped (or in grasses, dumb-bell-shaped) guard cells, sometimes accompanied by modified neighbouring epidermal cells called subsidiary cells. The inner walls of the guard cells are thicker than the outer walls, so when the cells become turgid they bow apart and the stoma opens. NEET 2016 asked about subsidiary cells directly, and NEET 2018 about dumb-bell guard cells in grasses — both straight memory questions if you know the names.

Vascular bundle types — open vs closed, radial vs conjoint

Vascular bundles are classified on two independent axes. On the presence of cambium they are open (dicot stem) or closed (monocot stem). On the spatial relation between xylem and phloem they are radial (alternating on different radii — roots) or conjoint (xylem and phloem on the same radius — stems and leaves, usually with phloem on the outer side).

The position of the first-formed xylem (protoxylem) gives two more terms tested at NEET. Endarch means the protoxylem lies towards the centre, with metaxylem outside — the default in stems. Exarch means protoxylem lies towards the periphery, with metaxylem towards the centre — the default in roots. NEET 2023 Q.122 hinged on the fact that exarch and endarch describe primary xylem, not secondary, and that exarch is the rule in roots.

Anatomy of the root — dicot vs monocot

A transverse section of a young dicot root, taken from the maturation zone, shows a now-familiar concentric layering. The outermost layer is the epiblema (root epidermis), many of whose cells protrude as unicellular root hairs. Below it lies the cortex, several layers of thin-walled parenchyma with intercellular spaces, and the innermost cortical layer — the endodermis — is a single barrel-shaped layer that carries the Casparian strip: a deposition of waxy suberin on the radial and tangential walls that blocks the apoplast and forces water into the symplast. Inside the endodermis sits the pericycle, a thick-walled layer that gives rise to lateral roots and, during secondary growth, contributes to the vascular cambium. The vascular bundles are radial — xylem and phloem on alternate radii — and parenchyma between them is called conjunctive tissue. The whole stele is centred on a small inconspicuous pith.

The monocot root has every one of those layers in the same order, but the bundle count is dramatically higher and the pith is much larger.

Anatomy of the stem — dicot vs monocot

A young dicot stem in T.S. shows a tidy ring. The epidermis is a single cuticled layer that may bear trichomes and a few stomata. Below it the cortex is built in three sub-zones: an outer hypodermis of collenchyma cells (mechanical support), a middle parenchymatous cortex with intercellular spaces, and an inner endodermis rich in starch grains (the starch sheath). Outside the phloem lies the pericycle, here in the form of semi-lunar sclerenchyma patches. The vascular bundles are arranged in a ring; each one is conjoint, open, and endarch. Parenchyma between the bundles forms medullary rays. The centre is occupied by a large parenchymatous pith.

The monocot stem breaks this pattern. The hypodermis is sclerenchymatous, not collenchymatous. The vascular bundles are scattered throughout the ground tissue rather than arranged in a ring; each one is conjoint and closed (no cambium); each is surrounded by a sclerenchymatous bundle sheath; phloem parenchyma is absent; and there are water-containing cavities within the bundle. NEET 2020 Q.42 listed these four features verbatim and asked the student to name the section — the only correct answer being a monocot stem.

Anatomy of the leaf — dorsiventral vs isobilateral

Cut a dicot leaf vertically through the lamina and three regions appear: epidermis on both surfaces, mesophyll sandwiched between them, and a vascular system running through the veins and midrib. The adaxial (upper) and abaxial (lower) epidermis both carry a cuticle, but the abaxial surface generally bears most of the stomata; the adaxial may lack them altogether. The mesophyll is differentiated into two zones — an upper palisade parenchyma of elongated, vertically arranged photosynthetic cells, and a lower spongy parenchyma of round, loosely arranged cells with conspicuous air-cavities for gas exchange. Vascular bundles are surrounded by thick-walled bundle sheath cells; in dicot leaves these are large and parenchymatous. Veins differ in size (reticulate venation), so vascular bundles also differ in size on the section.

The monocot leaf is similar in skeleton but markedly different in detail. The mesophyll is not differentiated into palisade and spongy zones; stomata occur on both surfaces; venation is parallel, so vascular bundles in T.S. are of near-identical size (except in the main veins); and along the adaxial epidermis of grasses, certain cells specialise into large, empty, colourless bulliform cells. When the bulliform cells are turgid the leaf is flat; when they lose water and become flaccid, the leaf curls inwards to reduce transpiration — the textbook explanation for why grass leaves curl in dry weather (NEET 2019 Q.3).

Secondary growth — how a stem becomes a tree

Primary growth puts up the scaffold; secondary growth turns it into timber. The process operates in two regions of the plant body — the vascular cambium produces secondary vascular tissue inside the stem, and the cork cambium (phellogen) produces the protective bark outside. Together they account for every gram of mass added to a tree-trunk after its first year.

The story begins inside an open vascular bundle. The strip of cambium already present there (called fascicular cambium) joins up with cells from the medullary rays that become meristematic (interfascicular cambium) to form a complete cambium ring. This ring then cuts off new cells in two directions: cells added to the inside differentiate into secondary xylem; cells added to the outside differentiate into secondary phloem. Because the cambium produces much more xylem than phloem each year, the inner mass grows much faster than the outer — pushing the primary phloem outward, where it is eventually crushed.

Cambium → Secondary xylem (inside) + Secondary phloem (outside) → over years, annual rings → cork cambium & periderm form the bark.

The whole arc of secondary growth in a single sentence

Spring wood, autumn wood and the annual ring

The cambium is not equally active throughout the year. In spring it works hard, producing wide-lumened xylem vessels efficient at conducting water for the new leaf-flush; this is the spring wood or early wood — light in colour, lower in density. In autumn and winter the cambium slows down and produces narrow-lumened xylem with fewer vessels — the autumn wood or late wood — darker and denser. One alternation of spring wood and autumn wood forms an annual ring. Count the rings in a transverse section of a tree trunk, and you have the tree's age.

Heartwood and sapwood

In an old tree, the secondary xylem itself differentiates into two zones. The inner, older wood becomes heartwood (duramen) — a dark, durable, non-conducting core whose vessels are plugged with tyloses and whose cell walls are saturated with tannins, resins, oils and aromatic substances. Heartwood provides mechanical support and resists insect attack (NEET 2022 Q.104). The outer, younger wood remains functional in conduction and is called sapwood (alburnum) — lighter in colour, still active in the transport of water and minerals from root to leaves (NEET 2020 Q.12; NEET 2017 Q.88).

Periderm, lenticels and bark

As the stem thickens, the original epidermis stretches and tears. To replace it, a second lateral meristem — the cork cambium, also called phellogen — differentiates from cortical cells. The phellogen is couple of layers thick. It cuts off two kinds of cells: phellem (cork) outwards, which becomes suberised, dead and waterproof; and phelloderm (secondary cortex) inwards, which remains parenchymatous and alive. Phellogen, phellem and phelloderm together make the periderm.

Cork is impermeable to water and gas, but the inner tissues still need to respire. At certain places, the phellogen produces loose, thin-walled parenchyma cells called complementary cells that rupture the epidermis and the cork to form lens-shaped openings called lenticels. Lenticels permit gas exchange between the atmosphere and the internal stem tissues (NEET 2021 Q.111; NEET 2023 Q.137).

The non-technical term bark refers to all tissues exterior to the vascular cambium — that is, the secondary phloem plus the periderm. Bark produced early in the season is called soft or early bark; bark formed towards the end of the season is hard or late bark.

Secondary growth in dicot roots

Dicot roots also undergo secondary growth, but the cambium has to be built before it can be used. Conjunctive parenchyma between xylem and phloem and a few pericycle cells just outside the protoxylem turn meristematic and join up into a wavy strip; this strip soon straightens into a cambium ring. From that point onwards the process is essentially identical to the stem — secondary xylem inwards, secondary phloem outwards, and a cork cambium (here arising from the pericycle, not the cortex) producing the periderm. Monocot roots have no such re-organisation and remain primary all their lives.