Allelopathy – The release of chemicals from a plant into the environment which inhibits the development, growth, and seed production of neighboring plants. The word “allelopathy” comes from the Greek words allelon, which means "of each other," and pathos, which means "to suffer.” Ragweed, sunflowers, and diffuse knapweed are among the invasive plants with allelopathic properties in northern Arizona. These can all produce large monocultures.

Alternate – The arrangement of leaves or buds on a stem (or twig), one per node. They may alternate from one side of the stem to the other. Contrasted with opposite, see below.

Annual – A plant that lives one year, goes to seed, and dies. Summer annuals germinate in the summer, grow, produce seeds, and die in the same year. Winter annuals germinate in autumn, over-winter, produce seeds, and die within one year. Contrast with biennial and perennial, below.

Awn – A stiff bristle, especially one growing from the ear or flower of barley, rye, and many grasses.

Axil – The angle between a leafstalk and stem. Flowers sometimes appear in the axil.

Axis – The main stem of a structure, such as a plant or inflorescence (plural: axes).

Biennial – A plant that lives two years, goes to seed, and dies. In the first year, the plant lives in the form of a rosette. In the second year, the plant develops a stem, grows flowers, drops seed, and dies.

Bract – A small leaf-like structure below a flower. It is neither a leaf nor a petal, although it is often mistaken for a petal. Thistles and knapweeds have prominent bracts.

Compound leaf – A leaf that is divided into two or more distinct, smaller leaflets.

Cotyledons – The first two tiny leaves to develop from the seed, well before true leaves emerge. They become the food source in the early stages of plant growth.

Disk flowers  – Tiny, tubular flowers of an Aster.  They sometimes look like a “button” in the center of a ring of ray flowers. Compare with Ray flowers, see below.

Dissected – A leaf shape with deep divisions and many very small segments. Contrasted with lobed, see below.

Family – A level of plant classification that is higher and more inclusive than the genus. Among the plant families represented on this site are Asteraceae (aster or sunflower family), Fabaceae (pea family), and Poaceae (grass family).

Flower – The sexual reproductive structure of a plant. The sole function of the flower, which is often the showiest part of the plant, is sexual reproduction. Fragrance and color are devices to attract pollinators.

Genus (plural genera) – A collective name for a group of plants classed together on the basis of similarities in structure and descent. In the classification of plants, a plant family consists of multiple genera, and each genus contains multiple species. The scientific name for each plant is composed of the genus and the specific (species) name. The Asteraceae family, for example, includes the Bull thistle (Cirsium vulgare), Ragweed (Ambrosia artemisiifolia), and Horseweed (Conyza canadensis).

Germinate – Begin to grow. A seed germinates.

Herbaceous plants – Plants with non-woody stems. Their above-ground growth largely or totally dies back in winter, but they may have underground plant parts (roots, bulbs, etc.) that survive. All annuals are herbaceous, as are many perennials and biennials.

Leaflet – A single leaf-like part of a compound leaf. A leaflet may resemble an entire leaf, but it does not appear on the main plant stem or branch, as a leaf does, but rather on a branch of the leaf.

 Ligule - The membrane at the junction of the leaf blade and sheath in many grasses. It is important in identifying the species of grass.

Lobed – A leaf shape in which shallow indentations extend less than halfway to the middle of the leaf base. A lobe may be a curved or rounded. Contrasted with dissected, above.

Node – The areas along a stem where leaves are attached.

Opposite – The arrangement of leaves or buds on the same node (two per node) on the opposite side of the stem. Contrast with alternate, above.

Perennial – A plant that lives on year after year, given favorable conditions. In the first year, the plant may appear as a rosette, and grows only leaves. In subsequent years, it develops a more extensive root system, produces flowers, and generates seeds.

Petiole – The leaf stalk that connects the leaf to a stem.

Prostrate – A plant stem growing horizontally, along the ground.

Raceme – A flower cluster with the separate flowers attached by short equal stalks at equal distances along a central stem. The flowers at the base of the central stem develop first.

Ray flowers – Flat, strap-like flowers, often encircling the disk flowers of an Aster. Compare with disk flowers, above.

Rhizome – A perennial underground stem, growing horizontally, which sends out additional roots below ground. Contrast with tiller, below.

Rosette – A flat, circular cluster of leaves that grows from a very short (not noticeable) stem at the surface of the soil.

Species – A type of plant with certain characteristics that differentiate it from other members of the genus, and which retains these distinctions through successive generations. These include molecular and structural characteristics, ecological niches, and distribution patterns. The species name is the basic unit of classification. The genus Centaurea, for example, includes both species of knapweed, Diffuse knapweed (Centaurea diffusa) and Spotted knapweed (Centaurea stoebe).

Spike – A flower cluster with the separate flowers attached directly to the central stem (without the short stalks that attach flowers to the central stem in a raceme). The flowers at the base of the central stem develop first.

Spikelet – A small or secondary spike; flower cluster in grasses or sedges consisting of usually two bracts, and one or more florets.

Succulent – Fleshy, juicy; a plant with a fleshy habit.

Taproot – One main root, often very thick, from which smaller lateral roots can grow.

Tiller – A shoot that grows vertically from underground rootstock. Contrast with rhizome, above.

Toothed – A leaf shape defined by sawtooth-like projections.

Tufted – A plant shape defined by dense, matlike clumps or tufts with many short, crowded stems all arising from about the same point.

Veins – The ribs of a leaf.

Whorl – A ring of 3 or more buds, flowers, or leaves encircling a stem.