Dicot Stem Anatomy – A First Look
Imagine you cut a young sunflower or a bean stem straight across. What you see under a microscope is not a random jumble of cells. It is a highly organised city, built around a single principle: separate the transport highways from the storage zones, and keep the whole structure flexible enough to grow in girth.
That is the core idea. A dicot stem is designed to move water and food up and down, store some reserves, and — crucially — get thicker over time. The anatomy you see in a cross-section is the blueprint for that job.
The Big Picture: Three Concentric Regions
If you look at the cross-section from outside to inside, you see three clear zones:
- Epidermis – the single outer layer of protective cells.
- Cortex – a broad region of ground tissue just inside the epidermis.
- Stele – everything inside the cortex: the vascular bundles arranged in a ring, the pith in the centre, and the medullary rays between bundles.
The most striking feature is that ring of vascular bundles. Each bundle is open (has cambium between xylem and phloem) and conjoint (xylem and phloem lie on the same radius). This ring is what makes a dicot stem different from a monocot stem, where bundles are scattered.
Layer by Layer
Epidermis – A single layer of tightly packed cells with a waxy cuticle on the outside. Its job is protection and preventing water loss. Some cells have outgrowths called trichomes (hairs).
Cortex – This is not one tissue but several. Just under the epidermis you find collenchyma (living cells with thickened corners, giving flexible support). Deeper in the cortex is parenchyma (thin-walled storage cells). The innermost layer of the cortex is the endodermis — a single layer of barrel-shaped cells that often contain starch grains. In stems, the endodermis is not as sharply defined as in roots, but it marks the boundary between cortex and stele.
Stele – This is where the action is. The outermost layer of the stele is the pericycle — one or more layers of sclerenchyma (thick-walled, dead fibres) that give mechanical strength. Inside the pericycle lies the ring of vascular bundles.
The Vascular Bundles: Open and Conjoint
Each bundle is a discrete strand. From outside to inside of a bundle:
- Phloem – towards the outside. It has sieve tubes (for food transport) and companion cells. In older stems, you also see phloem fibres.
- Cambium – a thin layer of actively dividing cells between phloem and xylem. This is the "open" part — it remains alive and can produce new xylem and phloem, allowing the stem to grow thicker.
- Xylem – towards the inside. It has vessels and tracheids (for water transport) and xylem fibres. The xylem is endarch, meaning the protoxylem (first-formed xylem) lies towards the pith and the metaxylem (later-formed) lies towards the cambium.
Between the vascular bundles, you find medullary rays — strips of parenchyma that connect the cortex to the pith. They allow lateral transport of water and food.
Pith – The large central region of parenchyma. It stores food and sometimes contains tannins or crystals. …