Read a Root Cross-Section

A primary root has an outer epiblema, a multilayered cortex, a single-layered endodermis, pericycle, and a vascular cylinder with xylem, phloem, and conjunctive tissue; the pith lies centrally. Root hairs are extensions of epiblema. Endodermal Casparian strips are suberin deposits on radial and tangential walls. They block apoplastic flow; they do not make the entire living endodermis an impermeable wall, and passage cells allow selective movement.

A typical dicot root has two to four xylem/phloem patches and a small or inconspicuous pith. A monocot root is usually polyarch (more than six xylem bundles), has a large well-developed pith, and does not undergo secondary growth. In roots, xylem is exarch: protoxylem is toward the outside and metaxylem toward the centre.

Conjunctive tissue is parenchyma between xylem and phloem patches in a root. Medullary rays are parenchymatous radial strips between vascular bundles in a dicot stem. Both are parenchymatous and common in dicots, but neither is generally photosynthetic. For a tissue-count question, count epiblema, endodermis, pericycle, conjunctive tissue, and pith as simple parenchymatous components; xylem and phloem are complex tissues, and cork is not part of the primary root list.

Casparian strips and passage cells point to roots. Hypodermis, a starch-rich endodermis in the dicot stem, and a water cavity in a monocot stem point to stems. A parenchymatous region between vascular bundles can be conjunctive tissue in a root or medullary rays in a dicot stem, so identify the organ before counting the feature.

A lateral root begins inside the parent root, from the pericycle just inside the endodermis. In section it pushes through the cortex and emerges at the surface, unlike an adventitious root or bud that originates from stem tissue. The arrowed outgrowth is therefore a lateral root.