Dicot root in the zone of tissue maturation

Vascular cylinder

Developing primary vasculature of buttercup (Ranunculus sp. sp.) showing the vascular cylinder at higher magnification with light microscopy.

The primary xylem in buttercup is tetrarch. There are four strands of mature tracheary elements (stained red due to lignification of their secondary walls) of the protoxylem (Px) immediately contacting the pericycle (1) the outermost cell layer of the vascular cylinder. Large empty-looking cells are differentiating metaxylem vessels (Mx) prior to secondary wall formation.

Four primary xylem strands alternate with four strands of primary phloem providing radial symmetry of the root. Primary phloem matures ahead of primary xylem, providing for nutritional needs of the developing root. Functional metaphloem sieve tubes (2) are already seen at this level of the root length. Note the endodermis (3) -- the innermost layer of cortex, with Casparian strips on its radial cell walls.