Anatomy of roots and stems

Dicot root (sunflower), in a transverse section from outside in:

• Epiblema: the outer layer, with root hairs.
• Cortex: many layers of thin-walled parenchyma.
• Endodermis: the innermost layer of the cortex, a ring of barrel-shaped cells. Their walls carry a waxy band of suberin, the casparian strips, which control what water and minerals pass inwards.
• Pericycle: a few layers inside the endodermis. Lateral roots start here, and so does the cambium later.
• Two to four separate patches of xylem and phloem, with conjunctive tissue between them, and a small pith.
• Everything inside the endodermis (pericycle, bundles, pith) is the stele.

Monocot root is similar, but usually has more than six xylem bundles (polyarch), a large pith, and no secondary growth: that is why monocot roots stay thin.

Dicot stem:

• Epidermis with a cuticle, sometimes with hairs.
• Cortex in three parts: a hypodermis of collenchyma (gives strength with flexibility, so the stem bends in wind without breaking); parenchyma; and an innermost endodermis rich in starch grains, the starch sheath.
• Pericycle as half-moon (semi-lunar) patches of sclerenchyma above the bundles.
• Vascular bundles arranged in a ring: conjoint, open (with cambium), and endarch (the first-formed xylem points to the centre). Medullary rays run between them, and there is a large pith.

Monocot stem (maize): the hypodermis is sclerenchyma (hard). Many vascular bundles are scattered through the ground tissue, each wrapped in a sclerenchymatous bundle sheath; bundles near the edge are smaller than those in the centre. They are conjoint and closed (no cambium), have no phloem parenchyma, and have water-filled cavities. There is a large ground tissue and no clear pith.

The quickest check: bundles in a ring and open means a dicot stem; bundles scattered and closed means a monocot stem.