Vascular bundle

Longitudinal strand of vascular tissue in the roots, stems and leaves of higher plants

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**Types of Vascular bundles**
(black: Xylem, green: Phloem, white: Cambium)
A concentric, periphloematic
B concentric, perixylematic
C radial with inner xylem, here with four xylem-poles, left closed, right open
D collateral closed
E collateral open
F bicollateral open

A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will include supporting and protective tissues. In addition, there is also a tissue between xylem and phloem which is the cambium.

The xylem typically lies towards the axis (adaxial) with phloem positioned away from the axis (abaxial). In a stem or root this means that the xylem is closer to the centre of the stem or root while the phloem is closer to the exterior. In a leaf, the adaxial surface of the leaf will usually be the upper side, with the abaxial surface the lower side.

The sugars synthesized by the plant with sun light are transported by the phloem, which is closer to the lower surface. Aphids and leaf hoppers feed off of these sugars by tapping into the phloem. This is why aphids and leaf hoppers are typically found on the underside of a leaf rather than on the top. The position of vascular bundles relative to each other may vary considerably: see stele.

Cross section of a leaf showing parts of a vascular bundle

Bundle-sheath cells

The bundle-sheath cells are the photosynthetic cells arranged into a tightly packed sheath around the vein of a leaf. It forms a protective covering on leaf vein, and consist of one or more cell layers, usually parenchyma. Loosely arranged mesophyll cells lie between the bundle sheath and the leaf surface. The Calvin cycle is confined to the chloroplasts of these bundle sheath cells in C₄ plants. C₂ plants also use a variation of this structure.^[1]

References

^ Sage, Rowan F.; Khoshravesh, Roxana; Sage, Tammy L. (1 July 2014). "From proto-Kranz to C4 Kranz: building the bridge to C4 photosynthesis". Journal of Experimental Botany. 65 (13): 3341–3356. doi:10.1093/jxb/eru180. PMID 24803502.

External links

Curtis, Lersten, and Nowak cross section of a vascular bundle
Mauseth another cross section of a vascular bundle

Botany

History
Outline

Subdisciplines

Plant groups

Plant anatomy

Plant morphology
(glossary)

Plant cells	Cell wall Phragmoplast Plastid Plasmodesma Vacuole
Tissues	Cork Ground tissue Mesophyll Meristem Storage organs Vascular tissue Vascular bundle Wood
Vegetative	Bulb Root Rhizoid Rhizome Shoot Bud Leaf Cataphyll Petiole Sessility Stem
Reproductive (incl. Flower)	Archegonium Antheridium Androecium Pollen Stamen Staminode Tapetum Flower Aestivation Flower development Floral diagram Floral formula Floral symmetry Whorl Fruit Anatomy Berry Capsule Nut Pyrena Seed Dispersal Endosperm Gametophyte Gynandrium Gynoecium Ovary Locule Ovule Stigma Hypanthium (Floral cup) Inflorescence Bract Pedicellate Raceme Umbel Perianth Tepal Petal Sepal Plant embryo Receptacle Sporophyll Sporophyte
Surface structures	Cuticle Epicuticular wax Epidermis Nectar Stoma Thorns, spines, and prickles Trichome