Plant Anatomy: Vascular Tissue and Stele

Stele types and vascular arrangement

The stele is the central vascular cylinder of a root or stem. It consists of the primary xylem and phloem together with the pericycle and the associated fundamental tissue, usually pith. Steles are classified by the arrangement of xylem and phloem and by whether a pith is present. A protostele has a solid core of xylem surrounded by phloem and no pith, whereas a eustele has separate collateral bundles arranged in a ring around a central pith. An atactostele has scattered vascular bundles embedded in ground tissue without any ring arrangement, which is typical of monocot stems. A siphonostele has a pith enclosed by a xylem ring. It is called ectophloic when phloem lies only on the outer side of the xylem, and amphiphloic when phloem lies on both sides.

Vascular bundles are also classified by the relative position of xylem and phloem. A bundle is conjoint when xylem and phloem lie on the same radius, and collateral when the phloem lies on one side of the xylem. An open collateral bundle has a vascular cambium between the two tissues, which permits secondary growth, whereas a closed bundle lacks cambium, as in monocot stems. Endarch xylem, with protoxylem facing the centre, is the usual condition in such open collateral bundles of dicot stems. Roots instead have radial bundles in which xylem and phloem alternate on different radii.

The combination of conjoint, collateral, open bundles, endarch xylem, medullary rays between the bundles and a starch-rich endodermis (starch sheath) identifies the dicot stem. Dicot and monocot roots differ by having radial bundles with exarch xylem, and dicot leaves have closed bundles without medullary rays.

• Protostele — a stele with a solid xylem core and no pith. Example: a young axis with a central solid xylem strand surrounded by phloem.
• Eustele — separate collateral bundles in a ring around the pith, typical of dicot stems.
• Atactostele — scattered bundles without a ring, typical of monocot stems.
• Conjoint — xylem and phloem on the same radius; collateral — phloem on one side of the xylem.
• Medullary rays — parenchymatous tissue between vascular bundles in dicot stems.

Modified plant organs and prototypes

A modified organ is a structure that has changed in form and function from its basic prototype, which is a leaf, stem or root. The prototype is identified by the organ's origin rather than its present function. Tendrils may be modified stems, as in grape vine, or modified leaves or leaflets, as in garden pea. Spines and thorns are both protective but differ in origin. Spines are modified leaves, as in Opuntia, whereas thorns are modified axillary stems, as in Citrus and Bougainvillea.

Most insectivorous plants trap prey with modified leaves. The pitchers of Nepenthes, the bladder traps of Utricularia, the sticky tentacles of Drosera and the snapping lobes of Dionaea are all leaf-derived structures.

• Prototype — the basic, unmodified organ type from which a modified organ is derived.
• Tendril — a slender climbing organ that may be stem-derived or leaf-derived; check the origin before answering.
• Spine — a sharp leaf-derived structure, as in cacti.
• Thorn — a sharp stem-derived structure that arises in the axil of a leaf.
• Pitcher — a modified leaf that traps and digests prey.

Primary xylem maturation (exarch/endarch)

Exarch and endarch describe the position of protoxylem, the first xylem formed, relative to metaxylem, the later and wider xylem. In exarch xylem the protoxylem lies at the periphery and the metaxylem toward the centre, so maturation is centripetal. This is the normal condition in the roots of angiosperms.

In endarch xylem the protoxylem lies toward the centre, facing the pith, and the metaxylem toward the periphery, so maturation is centrifugal. This arrangement characterises stems, including both dicot and monocot stems.

• Exarch — protoxylem outside, metaxylem inside; centripetal maturation; roots.
• Endarch — protoxylem inside, facing the pith; centrifugal maturation; stems.
• Protoxylem — first-formed xylem, with narrow elements that may be stretched during organ elongation.
• Metaxylem — later-formed xylem, with wider vessels that mature after the protoxylem.