Anatomy of Flowering Plants

A tissue is a group of cells with a common origin and usually a common function. Plant tissues are first divided by whether their cells can still divide. Meristematic tissues (meristems) are regions of actively dividing cells responsible for growth, while permanent tissues are made of cells that have stopped dividing and have taken on specific roles. Knowing which meristem produces which kind of growth is a recurring NEET point.

There are three meristems by position. The apical meristem at the tips of roots and shoots drives primary growth (increase in length). The intercalary meristem, found at the base of internodes or leaves (as in grasses), also adds to length and lets a grazed grass regrow; apical and intercalary meristems together are the primary meristems. The lateral meristem — the vascular cambium and cork cambium — lies along the sides of the axis and brings about secondary growth (increase in girth/diameter).

Permanent tissues are simple or complex. A simple tissue is made of one cell type. Parenchyma has thin-walled living cells and carries out storage, photosynthesis and secretion. Collenchyma has living cells with cellulose thickenings at the corners and provides flexible mechanical support to young stems and petioles. Sclerenchyma consists of dead cells with thick, lignified walls and gives rigid support; it occurs as long fibres and short sclereids (the grit of a pear).

A complex tissue is made of more than one cell type working as a unit, and the two conducting tissues are complex. Xylem conducts water and minerals upward and gives mechanical support; it comprises tracheids, vessels, xylem parenchyma and xylem fibres — and the tracheids and vessels are dead at maturity. Phloem conducts food (mainly sucrose), usually downward; it comprises sieve tube elements, companion cells, phloem parenchyma and phloem fibres. A NEET favourite is that sieve tube elements are living but lack a nucleus at maturity, so they are regulated by their adjacent nucleated companion cells.

On the basis of their structure and location, all the tissues of a plant are organised into three tissue systems (a scheme due to Sachs): the epidermal, the ground and the vascular tissue system. Recognising what belongs to each — and how their components differ in roots, stems and leaves — is the backbone of plant anatomy questions.

The epidermal tissue system forms the outermost covering. It includes the epidermis (a single layer of cells, often with a waxy cuticle on aerial parts), the stomata that regulate gas exchange and transpiration (each bordered by two bean-shaped guard cells, and sometimes subsidiary cells), as well as trichomes (epidermal hairs on the shoot that reduce water loss) and root hairs (unicellular extensions that absorb water and minerals).

The ground tissue system is everything between the epidermis and the vascular tissue, made mostly of simple tissues. In stems and roots it includes the cortex, the inner endodermis, the pericycle and the central pith; in leaves it is the photosynthetic mesophyll. It carries out storage, photosynthesis and packing/support.

The vascular tissue system consists of the conducting tissues, xylem and phloem, grouped into vascular bundles. Bundles are described by the relative position of xylem and phloem and by the presence of cambium. In radial bundles (roots) xylem and phloem lie on different radii, alternating; in conjoint bundles (stems and leaves) they lie together in the same bundle, usually with phloem outer. A bundle with a strip of cambium that allows secondary growth is open (dicot stem), while one lacking cambium is closed (monocot stem). These distinctions — radial vs conjoint, open vs closed — are precisely what NEET tests.

The internal structure of an organ differs systematically between dicots and monocots, and a side-by-side comparison is the most efficient way to learn it. In the dicot root the outermost layer is the epiblema (bearing root hairs), within which lie the cortex, a single-layered endodermis with Casparian strips, and the pericycle from which lateral roots and the vascular cambium arise. The vascular bundle is radial with usually two to four xylem strands (di- to tetrarch) and little or no pith. The monocot root is similar but has many xylem strands (polyarch, more than six) and a large, well-developed pith.

In the dicot stem, beneath the cuticle-covered epidermis (with trichomes) lies a hypodermis of collenchyma, then the cortex, an endodermis often rich in starch (the starch sheath), and the pericycle. The conjoint, open vascular bundles are arranged in a ring around a large central pith. The monocot stem has a sclerenchymatous hypodermis, undifferentiated ground tissue (no cortex/pith distinction), and conjoint, closed vascular bundles scattered throughout, each wrapped in a sclerenchymatous bundle sheath; the bundles have a characteristic 'Y'-shaped xylem and a water-containing cavity (the protoxylem lacuna).

Leaves are classified as dorsiventral (dicot) or isobilateral (monocot). The dorsiventral (dicot) leaf has its mesophyll differentiated into an upper palisade parenchyma (columnar, packed with chloroplasts) and a lower spongy parenchyma with air spaces; the stomata are more numerous on the lower (abaxial) surface. The isobilateral (monocot) leaf has undifferentiated mesophyll, stomata on both surfaces in equal numbers, and large, empty bulliform cells in the upper epidermis that roll the leaf inward to cut water loss in dry conditions.

These contrasts are extremely high-yield, so it pays to fix the cleanest discriminators: in the root, xylem number (dicot di-tetrarch vs monocot polyarch); in the stem, bundle arrangement (ring vs scattered) and type (open vs closed); and in the leaf, mesophyll (differentiated vs not), stomatal distribution (lower surface vs both) and the presence of bulliform cells (monocot only). NEET diagrams of these sections are common, so being able to label them from a description is valuable.

Secondary growth is the increase in the girth (thickness) of stems and roots, brought about by the lateral meristems. It is prominent in dicots and gymnosperms and is what produces wood. Two lateral meristems are involved: the vascular cambium, which forms secondary vascular tissue, and the cork cambium, which forms the protective bark.

In the dicot stem the vascular cambium becomes a complete ring by the joining of the cambium within the bundles (intrafascicular) with newly formed cambium between them (interfascicular). This cambial ring then cuts off new cells on both faces: secondary xylem (wood) toward the inside and secondary phloem toward the outside. Because far more secondary xylem is produced than phloem, the woody core enlarges greatly while the older phloem is pushed outward and crushed.

The activity of the cambium varies with the seasons, and this produces annual rings. In spring, when growth is active, the cambium makes spring (early) wood with wide vessels and a lighter colour; in autumn it makes autumn (late) wood with narrow vessels and a darker colour. One ring of spring + autumn wood represents one year, so counting the rings estimates a tree's age — the basis of dendrochronology. In old stems the central, non-conducting, darker wood is the heartwood (which gives mechanical strength), surrounded by the lighter, water-conducting sapwood.

As the stem thickens, the epidermis is replaced by the work of the cork cambium (phellogen), which arises in the cortex. It cuts off cork (phellem) on the outside — dead, suberised, waterproof cells that protect the stem — and secondary cortex (phelloderm) on the inside. Together the phellogen, phellem and phelloderm constitute the periderm. To allow the living inner tissues to breathe through this impervious cork, small lens-shaped openings called lenticels develop for gaseous exchange. The full sequence — cambium, secondary tissues, annual rings, heart/sapwood, periderm and lenticels — is a frequent and scoring NEET storyline.