Root Vocabulary

Root:an absorbing and anchoring organ developing from the radicle and usually positively gravitropic

Root Apical Meristem Vocabulary
[See Cytology and Meristems Vocabulary]

Embryonic Root Vocabulary
[See Seeds and Seedlings Vocabulary]

Mature Root/Stem Parts
Aerenchyma: a parenchyma tissue with very large gas spaces forming passage canals for gas exchange, common in roots of plants living in swampy soils
vascular cambium:
a residual (lateral) meristem arising from an apical meristem and giving rise to secondary xylem and secondary phloem
Casparian Strip: a waxy, waterproofing deposit in the radial and transverse walls of endodermal cells
Companion Cell: small-diameter parenchyma cell in the phloem associated with sieve tube members and providing for their needs through plasmodesmata
Cortex: a ground tissue layer between the epidermis and the vascular tissues
Endodermis: a parenchyma layer outside (and sometimes in stems also just inside) the vascular tissue, having Casparian strips, and responsible for selective mineral uptake
Epidermis: the exterior parenchyma layer of roots, stems, and leaves
Hypodermis: a suberized layer of cells just under the epidermis, common in mature roots growing in arid soils
Metaxylem: primary xylem consisting of vessels; maturing after protoxylem
Pericycle: a parenchymatous layer just inside the endodermis, producing branch (secondary) roots in all species, and periderm in woody species
Pith: a tissue region inside the vascular tissues usually consisting of parenchyma, but perhaps also including hollow air-space
Primary Phloem: phloem tissue originating from a provascular tissue
Primary Xylem: xylem tissue originating from a provascular tissue
Protoxylem: primary xylem consisting of tracheids; the first to form from provascular tissue
Root Hair: an extension of an epidermal cell, growing out into soil particles and responsible for water and mineral intake
Secondary Phloem: phloem tissue originating from the vascular cambium and maturing to its outside
Secondary Xylem: xylem tissue originating from the vascular cambium and maturing to its inside; wood
Sieve Tube Member/Element: elongate thin-walled, but large-diameter cells in the phloem transporting organic solutes usually from leaves to the rest of the plant; alive but lacking nucleus, plastids, mitochondria, etc.
Stele: the combined tissues inside the cortex, in roots the pericycle, xylem, phloem, and any pith present.
Tracheid: a spindle-shaped cell transporting water and minerals in the xylem, with end walls; dead at maturity
Velamen: a multiple epidermal system responsible for water intake from mist, yet reducing water loss in aerial roots
Vessel: a tube-like cell transporting water and minerals in the xylem, lacking end walls or having perforated end walls; dead at maturity

Xylem/Phloem Arrangements
Bicollateral: a vascular bundle in which xylem and phloem share two surfaces
Collateral: a vascular bundle in which xylem and phloem share only one surface
Radial: a vascular cylinder with xylem and phloem in alternate ridges (in alternating radii of cross-section)

Xylem Maturation Patterns
Endarch: xylem matures centrifugally; metaxylem is closer to the epidermis than is the protoxylem
Exarch: xylem matures centripetally; protoxylem is closer to the epidermis than is the metaxylem
Mesarch: xylem matures both centripetally and centrifugally; metaxylem appears both interior to and exterior to the protoxylem

Stele types
Actinostele: a protostele with lobed or stellate strand of xylem surrounded by phloem
Amphiphloic: a siphonostele with phloem both on the inside and the outside of the xylem cylinder
Diarch: a stele with two ridges of protoxylem
Ectophloic: a siphonostele with phloem surrounding the outside of the xylem cylinder
Eustele: a dissected siphonostele consisting of collateral or bicollateral bundles
Haplostele: a protostele with xylem completely surrounded by phloem
Pentarch: a stele with five ridges of protoxylem
Plectostele: a protostele with bands of xylem surrounded by phloem
Polyarch: a stele with many ridges of protoxylem
Protostele: consisting of a solid column of vascular tissue
Siphonostele: consisting of a hollow cylinder of vascular tissue; pith occupies hollow portion
Tetrarch: a stele with four ridges of protoxylem
Triarch: a stele with three ridges of protoxylem

Appendage Origins
Endogenous: an appendage arising from internal layers
Exogenous: an appendage arising from dermal and perhaps a few external cortical layers

Root Interactions
Ectotrophic: a symbiosis in which the non-plant haplobiont lives outside the plant cells (but also between plant cells)
Endotrophic: a symbiosis in which the non-plant haplobiont lives inside the plant cells
leg-Hemoglobin: a type of hemoglobin produced by legumes in the external layers of root nodules typically containing Rhizobium bacteria
Mutualism: a symbiosis in which both haplobionts benefit from the association
Mycorrhiza: a fungus-root symbiosis
Nodule: a tumor-like formation in response to the presence of a microorganism, often housing an endotrophic haplobiont
Symbiosis: a close relationship between two species (haplobionts) living in continuous intimate contact with each other

Root Origins
Adventitious: developing from dedifferentiated parenchyma from a structure other than a root
Primary: developing from the apical meristem of the radicle
developing from the pericycle of another root

Specialized Root Types
Aerial: above-ground roots, usually extending into the air
Buttress: roots developing board-like growth on the upper side and assisting with support of the trunk against lateral forces
Contractile: roots capable of shrinking usually drawing the plant deeper in the soil, but in some cases drawing the plant horizontally through the soil.
Fibrous: consisting of fine or threadlike slender roots
Fleshy: succulent roots
Haustorial: roots of parasitic species penetrating into host tissues for nutrient absorption
vertical or horizontal above-ground roots, usually involved with plants living in swampy soils
Pneumatophorous: containing spongy aerenchyma tissues of plants living in swampy soils
adventitious roots arising at a steep angle from the lower portions of stems and rooting in soil to support the trunk against lateral forces
Tap: a persistent and well-developed primary root.
Tuberous: extremely fleshy roots resembling stem tubers

This page © Ross E. Koning 1994.

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