Anatomy of Flowering Plants

Morphology of Flowering Plants already covers the plant's external structure, roots, stems, leaves and flowers, and how each is recognised and named. This note goes inside those same organs, to the tissue-level structure that actually explains how a plant grows, thickens and functions.

Meristems: where new plant tissue actually comes from

A plant's growth is concentrated in specific, permanently dividing regions called meristems, classified by their location into three types. Apical meristems, at the very tips of roots and shoots, add length to the plant. Intercalary meristems, located at the base of leaves or at nodes rather than at a growing tip, also add length, and are the reason grass continues to grow back after its tip is cut or grazed, since the growing region sits below the cut rather than at it. Lateral meristems (the cambia), running along the sides of roots and stems rather than at their ends, add girth rather than length, and are specifically responsible for the secondary growth covered below.

Simple permanent tissues: parenchyma, collenchyma, sclerenchyma

Once a cell stops dividing and matures into its final form, it becomes part of a permanent tissue. A simple permanent tissue is made of one cell type throughout, and three named simple tissues recur across nearly every plant organ. Parenchyma cells are living, thin-walled and roughly isodiametric (equal in all dimensions), and serve general-purpose functions including photosynthesis, storage and secretion, essentially the plant's own "default" filler tissue. Collenchyma cells are also living, but with walls thickened unevenly at their corners by cellulose, hemicellulose and pectin, providing flexible mechanical support to growing parts of the plant (young stems, petioles) that still need to bend and elongate. Sclerenchyma cells, by contrast, are generally dead at maturity, with walls thickened uniformly and hardened by lignin, providing rigid, permanent mechanical support rather than flexible support, and occurring in two forms: elongated fibres and shorter, irregularly shaped sclereids.

Complex tissues: xylem and phloem, and how they're arranged

Complex tissues, unlike simple ones, are made of more than one cell type working together, and xylem (water and mineral conduction) and phloem (conduction of the products of photosynthesis) are the two named complex tissues that together constitute a vascular bundle. Whether a vascular bundle can keep growing thicker over time depends entirely on whether it retains a cambium between its xylem and phloem. An open vascular bundle, found in dicot stems and gymnosperms, retains a strip of cambium between the xylem and phloem, keeping the door open for further growth in girth later in the plant's life. A closed vascular bundle, found in monocot stems, has no cambium between the xylem and phloem at all, permanently closing off that option, which is exactly why monocot stems characteristically do not thicken with age the way dicot stems do.

Secondary growth: how a dicot stem gets visibly thicker

Secondary growth, the increase in girth seen in dicot stems and roots (but essentially absent in monocots, following directly from their closed vascular bundles), is driven by the continued activity of the lateral meristems, chiefly the vascular cambium. As the vascular cambium divides year after year, it adds new xylem toward the inside and new phloem toward the outside, and in plants growing in climates with a distinct seasonal cycle, this produces visible concentric growth rings (annual rings) in the wood, a direct physical record of the plant's own year-by-year growth history, and the basis of dendrochronology, dating a tree's age by counting its own rings.

Quick revision points

• Meristem types by location: apical (root/shoot tips, adds length), intercalary (leaf bases/nodes, adds length, lets grass regrow after cutting), lateral/cambium (sides of stem/root, adds girth, drives secondary growth).
• Simple tissues: parenchyma (living, thin-walled, general-purpose: photosynthesis, storage, secretion), collenchyma (living, unevenly thickened corners, flexible support in growing parts), sclerenchyma (generally dead, uniformly lignified, rigid support; fibres and sclereids).
• Complex tissues: xylem (water/mineral conduction) and phloem (conduction of photosynthesis products) together form a vascular bundle.
• Open vascular bundle (dicot stems, gymnosperms): cambium present between xylem and phloem, allows further growth in girth. Closed vascular bundle (monocot stems): no cambium, growth in girth is not possible, which is why monocot stems don't thicken with age.
• Secondary growth (dicots, essentially absent in monocots): driven by ongoing vascular cambium activity, adding xylem inward and phloem outward; produces visible annual growth rings in seasonal climates, the basis of dendrochronology.