Botany Terms Starting With B

Bark is the outermost protective covering of woody stems and roots, comprising all tissues outside the vascular cambium, including the phloem, cork cambium, and cork layers that replace the epidermis as the plant ages.

Bark develops as the vascular cambium produces secondary xylem inward and secondary phloem outward; as the stem expands, the original epidermis is replaced by the cork cambium, which generates impermeable suberized cork cells. Multiple functions are packed into this layered structure: it prevents desiccation, blocks pathogen and herbivore entry, insulates against fire and temperature extremes, and seals wounds. Lenticels, small pores scattered across the bark surface, allow gas exchange between the living tissues beneath and the atmosphere.

In giant sequoia (Sequoiadendron giganteum), the fibrous bark can reach 60 centimeters thick and provides substantial insulation against the low-intensity ground fires that periodically sweep through its Sierra Nevada habitat.

Benzaldehyde is an aromatic aldehyde produced by certain plants, characterized by an almond-like odor, and derived either through the phenylpropanoid pathway or by enzymatic breakdown of cyanogenic glucosides stored in seeds and woody tissues.

In plants, benzaldehyde is produced via the phenylpropanoid pathway from cinnamic acid or released enzymatically from the cyanogenic glucoside amygdalin stored in the seeds and woody tissues of plants in the family Rosaceae. When tissues of bitter almond (Prunus dulcis var. amara) are crushed, the enzyme beta-glucosidase cleaves amygdalin, releasing benzaldehyde and hydrogen cyanide simultaneously.

Benzaldehyde concentrations released from damaged tissue are high enough to deter many insect herbivores, and the compound also contributes to the characteristic scent of cherry, apricot, and peach kernels. Beyond defense, benzaldehyde volatiles attract certain pollinators and seed dispersers that associate the scent with food rewards.

Bipinnate describes a compound leaf that is divided twice, so that each primary division, called a pinna, is itself divided into smaller leaflets arranged along a secondary axis.

A bipinnate leaf results from two successive divisions of the leaf blade into primary and secondary pinnae; each primary pinna bears multiple leaflets arranged along a secondary rachis. This hierarchical branching reduces a large surface area into many small leaflets, increasing mechanical strength while reducing wind-induced stress and transpiration in exposed or seasonally dry environments. Bipinnate leaves are common in the families Fabaceae and Mimosaceae and can display movement patterns where individual leaflets fold in response to touch or darkness, a behavior well documented in sensitive plant (Mimosa pudica).

Each leaflet of a bipinnate leaf can close independently within seconds of mechanical stimulation, a response driven by rapid changes in turgor pressure in specialized cells called pulvini.

Blade is the flat, expanded portion of a leaf that captures light for photosynthesis and exchanges gases with the atmosphere through pores called stomata.

The leaf blade is the flat, light-capturing surface composed of upper and lower epidermis, palisade mesophyll for photosynthesis, and spongy mesophyll for gas exchange and internal transport. Blade morphology varies widely; broad, thin blades maximize light interception in shaded understory environments, while small, thick, or lobed blades reduce transpiration and mechanical stress in exposed or arid habitats. Veins embedded in the blade deliver water and minerals while removing photosynthetic products, and their spatial arrangement affects blade strength and flexibility.

In the giant water lily (Victoria amazonica), blades can exceed 3 meters in diameter and support the weight of a small child, with a ribbed underside architecture that distributes mechanical load across the entire surface.

Bract is a modified leaf associated with a flower or inflorescence, positioned below or around the floral structure, and ranging from small and scale-like to large and brightly colored depending on the species.

Bracts range from small, scale-like structures barely distinguishable from foliage leaves to large, brightly colored structures far more conspicuous than the actual flowers they subtend. In flowering dogwood (Cornus florida), four white or pink bracts surround a tight cluster of tiny true flowers at their center, and each bract can reach 5 centimeters in length. The bracts of poinsettia (Euphorbia pulcherrima) turn red in response to shortened day length, a photoperiodic response that signals the onset of the flowering season.

Bract size, shape, color, and arrangement are taxonomically informative characters used to distinguish genera and species within families such as Asteraceae and Euphorbiaceae.

Bracteate describes a flower or inflorescence that bears one or more bracts, which are modified leaves positioned at the base of a flower stalk or along the inflorescence axis.

A bracteate inflorescence bears bracts at the base of individual flower pedicels or along the main inflorescence axis, contrasting with ebracteate inflorescences where such bracts are absent or highly reduced. Bracts may be inconspicuously small and leaf-like, or enlarged and brightly colored, as in poinsettia (Euphorbia pulcherrima) and bougainvillea (Bougainvillea spectabilis), where colorful bracts attract pollinators while actual flowers remain small and inconspicuous. The presence, size, shape, and color of bracts are important diagnostic features in plant taxonomy and aid in identifying inflorescence type and evolutionary relationships.

In the family Asteraceae, each floret in the flower head is subtended by a small bract called a palea, and the entire head is enclosed by an involucre of bracts called phyllaries, making bracteate structure central to the family’s floral architecture.

Bulb is an underground storage organ consisting of a short, compressed stem called a basal plate surrounded by fleshy, nutrient-storing leaf bases that protect and supply the apical meristem during dormancy and early regrowth.

A true bulb consists of a highly compressed basal plate representing a modified stem, from which arise concentric fleshy leaf scales packed with carbohydrates, proteins, and minerals stored for future growth. Each scale is a modified leaf base that surrounds an apical meristem enclosed in the bulb’s center; when conditions favor growth, this meristem resumes activity and generates a new shoot that emerges from the bulb’s apex. True bulbs such as onions (Allium cepa), daffodils (Narcissus pseudonarcissus), and tulips (Tulipa gesneriana) differ from corms, tubers, and rhizomes, which are distinct underground storage organs lacking the leaf-scale structure characteristic of bulbs.

An onion bulb can store enough energy to support shoot emergence through several centimeters of soil before the new leaves begin photosynthesizing independently.

Bundle sheath is a cylinder of cells enclosing the vascular bundle in a leaf, forming a tight sleeve that regulates solute movement and, in C4 plants, concentrates carbon dioxide for the Calvin cycle.

In C3 plants, bundle sheath cells are thin-walled parenchyma or sclerenchyma that restrict apoplastic movement of solutes and provide structural support around the vascular tissue. C4 plants such as maize (Zea mays) show a dramatically different arrangement: their bundle sheath cells are large, contain abundant chloroplasts, and form an airtight ring that traps carbon dioxide released from four-carbon organic acids. This spatial separation of initial carbon fixation in mesophyll cells from the Calvin cycle in bundle sheath cells suppresses photorespiration and raises photosynthetic efficiency under high temperatures.

The anatomical pattern of tightly packed mesophyll cells surrounding a prominent bundle sheath, called Kranz anatomy, is visible in cross-sections of maize leaves and is a diagnostic feature of C4 grasses.