Plant Anatomy: Tissues, Organs and Labeled Diagrams

Plant anatomy is the branch of botany that studies how a plant is built inside: the development, pattern and significance of its cells, tissues and organs, and the microscope methods used to see them (University of Guelph). A standard textbook works organ by organ, through the stem, root, leaf, flower, pollen, seed and fruit, with tools from the hand lens to the electron microscope, and calls the subject fundamental to classifying plants and essential to physiology, ecology and evolutionary biology (Rudall, Anatomy of Flowering Plants).

The split between anatomy (internal) and morphology (external form) follows university botany courses (University of Rwanda); the topics of physiology follow OpenStax (OpenStax ch. 30). In practice the three overlap: you need the anatomy of the xylem to explain how water moves.

A flowering plant has two organ systems. The shoot system, usually above ground, holds the vegetative organs (stems and leaves) and the reproductive ones (flowers and fruits). The root system, usually underground, anchors the plant and absorbs water and minerals (OpenStax 30.1). Every plant body is built of stems, roots and leaves (OpenStax ch. 30).

Plant cells have three things most animal cells lack: a rigid cell wall of cellulose, chloroplasts and other plastids, and a large central vacuole. Most plant cells, in turn, have no centrosome and no lysosomes (OpenStax 4.3). For a full 3D tour of the organelles, see cell anatomy.

Table sources: OpenStax on the wall, vacuole and plastids (OpenStax 4.3) and on plasmodesmata, which play the part that gap junctions play between animal cells (OpenStax 4.6); the Wood Handbook on the middle lamella (Wood Handbook, ch. 3). The vacuole is why a growing plant cell can become much larger without making much new cytoplasm.

A wall is built in layers. Each cell first lays down a thin primary wall, with cellulose microfibrils running in random directions. Inside it, many cells, including those of wood, add a secondary wall in three layers (S1, S2 and S3), each with its microfibrils at a different angle. Every layer combines cellulose microfibrils, hemicelluloses and a matrix: mostly pectin in primary walls and lignin in secondary walls (Wood Handbook, ch. 3). That chemical difference is what stains tell apart on a slide (see how plant anatomy is studied, below): the lignified walls of sclerenchyma and xylem take a different stain from the thin walls of parenchyma. For the inside of a chloroplast, see chloroplast structure.

Plants grow from meristems, regions of continuously dividing cells. Meristem cells are undifferentiated or only partly differentiated; the cells they leave behind become the permanent tissues, which no longer divide (OpenStax 30.1).

Table from OpenStax (OpenStax 30.1) (OpenStax 30.2). The vascular cambium is the cylinder of meristem that makes the secondary xylem and phloem of shrubs and trees (Bellairs 3.1). To see secondary growth in 3D, from the cambium to the growth rings, explore the tree anatomy model of an oak trunk.

All the tissues of a plant belong to three tissue systems: dermal, ground and vascular (Bellairs 2.1). Dermal tissue covers and protects the plant and controls gas exchange; vascular tissue transports water, minerals and sugars; and ground tissue is where photosynthesis happens, supports the vascular tissue and stores water and sugars (OpenStax 30.1).

Simple tissues: parenchyma, collenchyma and sclerenchyma

A simple tissue is made of similar cells; a complex tissue, such as xylem or phloem, is made of several cell types (OpenStax 30.1).

• Parenchyma: the most common plant cells, with thin cellulose walls. They carry out photosynthesis, store starch and help heal wounds (OpenStax 30.2). Parenchyma with large air spaces, called aerenchyma, keeps water plants afloat and aerated (Bellairs 2.2).
• Collenchyma: elongated, living cells with unevenly thickened walls that support the plant, found below the epidermis (OpenStax 30.2); celery is the classic example (Bellairs 2.2).
• Sclerenchyma: support cells with lignin-thickened walls, many of them dead at maturity (OpenStax 30.2). Long, thin fibres and variously shaped sclereids, such as the gritty stone cells of a pear (Bellairs 2.2).

Xylem and phloem

Xylem carries water and minerals from the roots; its water-conducting cells, tracheids and vessel elements, are dead at maturity, and vessel elements join end to end through perforation plates. Phloem carries sugars and amino acids in sieve-tube elements, which are alive and supported by companion cells (OpenStax 30.2). In stems the two form vascular bundles; in roots they form the vascular cylinder, or stele (OpenStax 30.1).

See the three tissue systems in 3D

A leaf shows all three systems at once. In this 3D block of a lilac leaf, the epidermis and cuticle are dermal tissue, the palisade and spongy mesophyll are ground tissue, and the veins are vascular tissue. Hide a system with the buttons at the bottom of the viewer, or open the full leaf cross section guide.

An organ is a group of tissues working together. These are the six organs of a flowering plant, with a short answer for each and a link to the full guide where one exists.

Secondary growth is the thickening of stems and roots by the two lateral meristems. The vascular cambium, a thin cylinder of dividing cells between the bark and the wood, makes secondary xylem (wood) to the inside and secondary phloem to the outside; the cork cambium makes cork, whose walls contain waxy, water-repelling suberin. Herbaceous plants mostly show primary growth only (OpenStax 30.2), and monocot stems rarely form wood (EBSCO).

As the cambium adds wood each year, the young stem's tissues are rearranged. The secondary xylem gives shrubs and trees extra support and extra water-conducting tissue, new secondary phloem replaces the primary phloem (Bellairs 2.5), and the expanding wood squeezes the pith into the centre (Bellairs 3.1). Wood made early in the season has large cells; later wood is denser, and the contrast between them shows as annual rings (OpenStax 30.2). The cambium also grows in circumference by dividing sideways, and its two kinds of starting cells make the vertical cells of the wood and the horizontal rays (Wood Handbook, ch. 3).

Wood does three jobs for the plant: it conducts water from the roots to the leaves, holds the plant body up, and stores food and other chemicals (Wood Handbook, ch. 3). Everything outside the vascular cambium is bark: the inner bark (phloem) and the outer bark (cork). To follow all of these layers through a real trunk, from bark to heartwood, explore the tree anatomy model of an oak.

Flowering plants are split mainly into monocots, with one seed leaf, and eudicots (true dicots), with two. The difference runs through every organ, so it is the first thing to check on a slide (OpenStax 26.3).

Sources: OpenStax for cotyledons, veins, stems, roots, flowers, pollen and examples (OpenStax 26.3) (OpenStax 30.3); EBSCO for wood and species numbers (EBSCO). OpenStax's statement that eudicots make up two-thirds of flowering plants matches EBSCO's counts. A few early-branching groups, such as water lilies and the magnoliids, fit neither column neatly (OpenStax 26.3).

Plant anatomists cut thin slices (sections) of an organ, stain them so that different walls show in different colours, and examine them under a microscope. Because a single slice is flat, wood scientists cut in three planes: a transverse section across the stem, a radial section from the centre outward along its length, and a tangential section at right angles to the radius; only together do they show the structure in three dimensions (Wood Handbook, ch. 3).

You can make sections by hand. Hold the leaf or stem between two halves of a carrot, potato or elder pith, keep the tissue, blade and fingers wet, and cut with one smooth stroke of a razor blade, never sawing. Cut many slices, float them in water, and mount the thinnest, almost transparent ones in a drop of water (White, Prometheus protocols). Laboratory slides are often cut from tissue embedded in paraffin, or with a vibratome (University of Vigo atlas).

Safranin and fast green compete, so the more green a wall takes up, the less red it shows (University of Vigo atlas). Stains are needed because most cell parts are colourless, but staining kills the cells. A light microscope can show living cells; an electron microscope magnifies far more but only works on specially prepared, dead specimens. The scanning electron microscope shows the surface of a specimen, and the transmission electron microscope shows the structures inside its cells (OpenStax 4.1). Plant anatomy uses the whole range, from the hand lens to the electron microscope (Rudall, Anatomy of Flowering Plants). To see what a stained leaf section looks like, compare the leaf cross section.

• Flower anatomy: petals, sepals, stamens, pistil, ovary and ovules of a lily.
• Leaf cross section: every tissue layer of a lilac leaf, plus monocot, C4 and pine leaves.
• Stomata diagram: guard cells and how the pore opens and closes.
• Chloroplast structure: envelope, stroma, thylakoids and grana.
• Seed anatomy: bean and corn seeds, seed coat, cotyledons and embryo.
• Tree anatomy: bark, cambium, sapwood, heartwood and growth rings of an oak.
• Cell anatomy: the organelles of a cell in 3D.