Plant Anatomy: Stems, Leaves, and Roots

Comprehensive vocabulary flashcards covering plant anatomy and morphology across stems, leaves, roots, vascular systems, ecological adaptations, and rhizobial symbioses.

Plant Morphology

The branch of botany that studies the external form, structure, organisation, and modifications of plant organs.

Shoot Apical Meristem (SAM)

The terminal growth region of the shoot consisting of the tunica and corpus, producing leaf primordia on its flanks across zones of cell division, elongation, and differentiation.

Collateral Vascular Bundle

A vascular bundle arrangement where xylem and phloem lie together side-by-side on the same radius, with xylem directed toward the centre (pith) and phloem toward the outside (epidermis).

Bicollateral Vascular Bundle

A vascular bundle arrangement characterized by phloem occurring on both the outer and inner sides of the xylem; characteristic of cucurbits and the potato family.

Amphicribral Vascular Bundle

A concentric vascular bundle in which the phloem completely surrounds the central xylem, found commonly in many ferns.

Amphivasal Vascular Bundle

A concentric vascular bundle in which the xylem completely surrounds the central phloem, found in Dracaena and select monocots.

Open Vascular Bundle

A vascular bundle containing intrafascicular cambium between the xylem and phloem, enabling secondary growth; typical of dicot stems.

Closed Vascular Bundle

A vascular bundle lacking cambium between the xylem and phloem, preventing secondary growth; characteristic of monocot stems.

Protoxylem

The first primary xylem to differentiate in actively elongating tissues, possessing narrow vessels with stretchable annular or spiral wall thickenings.

Metaxylem

Primary xylem that differentiates after protoxylem, featuring wider vessels with rigid pitted or reticulate cell wall thickenings.

Endarch Xylem Development

A pattern of xylem development where protoxylem is located towards the inner part of the xylem strands (closer to the pith) and maturation progresses centrifugally toward the periphery; typical of stems.

Exarch Xylem Development

A pattern of xylem development where protoxylem is located towards the outer periphery (closer to the phloem/pericycle) and maturation progresses centripetally toward the centre; characteristic of roots.

Sclerenchymatous Bundle Sheath

A ring of thick-walled, lignified, dead sclerenchyma fibres enclosing each vascular bundle in monocot stems that provides mechanical support, protects conduction tissues, and resists bending forces.

Vascular Cambium

A lateral secondary meristem consisting of fusiform initials and ray initials that divides inward to produce secondary xylem (wood) and outward to produce secondary phloem.

Cork Cambium (Phellogen)

A lateral secondary meristem that divides to produce cork (phellem) outward and phelloderm inward, together forming the protective periderm that replaces the epidermis.

Earlywood

Secondary xylem produced in spring during rapid cambial division stimulated by auxin, characterised by wide vessels or tracheids with thin walls specialized for large-volume water transport.

Latewood

Dense secondary xylem produced toward autumn as cambial division slows, characterised by narrow, thick-walled cells providing mechanical support.

Heartwood

The non-functional, dead, dark central secondary xylem filled with tannins and resins that provides structural support and decay resistance.

Sapwood

The outer, functional, lighter-coloured secondary xylem containing living parenchyma cells that actively conducts water and dissolved mineral nutrients.

Ring-Porous Wood

Hardwood anatomy in which large vessels are concentrated exclusively in earlywood, creating distinct boundaries between annual rings (e.g., oak, ash).

Diffuse-Porous Wood

Hardwood anatomy where vessels of similar diameter are evenly distributed throughout the entire annual growth ring (e.g., maple).

Rhizome

A horizontal underground modified stem with distinct nodes, internodes, scale leaves, axillary buds, and adventitious roots that stores reserves and functions in perennation (e.g., ginger, turmeric).

Stem Tuber

A swollen tip of an underground stem (stolon) adapted for starch storage, bearing spirally arranged nodes with axillary buds known as "eyes" (e.g., potato, Solanum tuberosum).

Corm

A short, vertically oriented, swollen underground stem covered by papery scale leaves with clear nodes and a terminal bud, replaced annually by a new corm (e.g., taro/songe).

Bulb

A modified underground shoot with a highly reduced, flattened stem (basal plate) bearing fleshy, concentric food-storing scale leaves and adventitious roots (e.g., onion, garlic).

Sub-aerial Stem Modifications

Specialised stems that grow along or near the soil surface for vegetative reproduction, including runners (prostrate surface stems), stolons (arching branches), offsets (short thick rosettes), and suckers (underground shoots emerging upward).

Stem Tendril

A slender, spirally coiling modified axillary or terminal stem bud that wraps around physical structures to support climbing plants (e.g., grape, passion fruit).

Thorn

A hard, woody, pointed modified axillary branch with internal vascular connections to the stem, functioning in plant defence (e.g., Citrus, Bougainvillea).

Cladode

A green, photosynthetic stem modification comprising a phylloclade limited to a single internode (e.g., Ruscus, asparagus).

Bulbil

A fleshy, modified vegetative axillary bud specialized for vegetative propagation that detaches from the plant and develops into a new individual (e.g., Agave, Dioscorea).

Bast Fibres

Long, thick-walled sclerenchyma fibres extracted from the secondary phloem region of dicot stems, used for textiles and ropes (e.g., jute, flax, hemp, kenaf).

Parts of a Typical Leaf

The principal structural components of a foliage leaf: lamina (photosynthetic blade), petiole (stalk attaching blade to stem), midrib (primary central vein), lateral veins, leaf apex, leaf base, and margin.

Simple Leaf

A leaf whose blade is a single undivided unit, where incisions or lobes never extend entirely to the midrib (e.g., mango, guava, banana).

Compound Leaf

A leaf whose blade is divided into multiple completely separate distinct units called leaflets, all attached to a shared rachis or petiole tip.

Paripinnate Leaf

A pinnately compound leaf ending in an even pair of terminal leaflets with no single apical leaflet (e.g., tamarind).

Imparipinnate Leaf

A pinnately compound leaf terminating in an odd single central leaflet (e.g., rose, neem).

Palmately Compound Leaf

A compound leaf in which all leaflets radiate outward from a single common point at the tip of the petiole (e.g., Bombax, cannabis).

Phyllotaxy

The pattern of leaf arrangement along a plant stem, strategically organized to optimize light interception and reduce self-shading, often separated at angles close to $137.5^\circ$.

Decussate Phyllotaxy

An opposite leaf arrangement where each successive pair of leaves is oriented at right angles ($90^\circ$) relative to the pair directly below it (e.g., mint).

Leaf Margin

The boundary edge of a leaf blade, classified into shapes including entire (smooth), serrate (saw-toothed forward), dentate (teeth outward), crenate (rounded teeth), lobed (deep indentations), and undulate (wavy).

Leaf Apex

The structural tip of the leaf blade, classified as acute (pointed), acuminate (long tapering tip), obtuse (blunt and rounded), retuse/emarginate (notched tip), or mucronate (short sharp point).

Leaf Shapes

The overall geometric outline of the lamina, including linear, lanceolate, ovate, cordate (heart-shaped), reniform (kidney-shaped), palmate, and peltate (stalk attached to blade centre).

Guttation

The exudation of drops of xylem sap from specialized pores called hydathodes at the margins or tips of leaves, driven by positive root pressure during periods of high humidity and closed stomata.

Water Potential Gradient in Plants

The decreasing gradient of water potential from the soil through root hairs, stem xylem, and leaf mesophyll to the atmosphere (e.g., from $\sim -0.2\,\text{MPa}$ in roots to $\sim -100\,\text{MPa}$ in air) that drives the transpiration stream.

Kranz Anatomy

A specialized leaf internal arrangement characteristic of $C_4$ plants (e.g., sugarcane, maize) consisting of chloroplast-rich bundle sheath cells surrounded by mesophyll cells, spatially separating initial $CO_2$ fixation from the Calvin cycle to suppress photorespiration.

Abscission Zone

A specialized structural region at the petiole base consisting of a separation layer where cell wall enzymes degrade the middle lamella, and a suberized protective layer that seals the attachment point before leaf detachment.

Taproot System

A root system originating directly from the persistent embryonic radicle, producing a dominant, deep vertical central taproot and secondary lateral branches; common in eudicots (e.g., carrot, radish).

Fibrous Root System

A root system composed of a dense mass of thin, adventitious roots of uniform size originating from the base of the stem to replace the short-lived radicle; common in monocots (e.g., grasses, maize).

Adventitious Root

Any root that arises from non-root tissue, such as stems, nodes, or leaves, functioning in support, vegetative propagation, or specialized aeration.

Pneumatophores

Negatively geotropic, aerial respiratory roots that grow vertically upward above waterlogged mud or soil, containing lenticels and aerenchyma to deliver oxygen to submerged roots (e.g., Avicennia).

Haustoria

Specialised invasive root organs produced by parasitic plants (e.g., Cuscuta) that penetrate the host plant's vascular tissues to extract water, carbohydrates, and minerals.

Velamen

A multi-layered, sponge-like hygroscopic epidermis covering the aerial roots of epiphytic orchids that absorbs atmospheric moisture, reduces water loss, and provides mechanical protection.

Rhizosphere

The narrow zone of soil directly influenced by living root growth, respiration, and exudates, characterised by elevated microbial activity relative to bulk soil.

Casparian Strip

A hydrophobic band of suberin and lignin deposited within the radial and transverse walls of root endodermal cells that blocks apoplastic movement, forcing water and solutes to cross selectively permeable plasma membranes into the vascular cylinder.

Apoplastic Pathway

The route of water and dissolved mineral transport that moves exclusively through the continuous matrix of non-living cell walls and intercellular spaces without crossing plasma membranes until reaching the Casparian strip.

Symplastic Pathway

The pathway of water and solute movement across plant tissues through the continuous interconnected cytoplasm of living cells linked by plasmodesmata.

Arbuscular Mycorrhizae (AM)

Endomycorrhizal mutualisms formed between Glomeromycota fungi and plant roots, developing intracellular branched exchange structures called arbuscules and lipid-storing vesicles inside cortical cells.

Ectomycorrhizae (ECM)

Mutualistic associations primarily involving Basidiomycota and Ascomycota on woody trees, characterised by a fungal mantle encasing fine roots and an intercellular Hartig net surrounding cortical cells without penetrating inside the cells.

Nod Factors

Chitin-like lipochitooligosaccharide signalling molecules produced by rhizobia in response to legume root flavonoids, recognized by plant LysM receptor kinases to trigger infection and nodule organogenesis.

Infection Thread

An invaginated tubular structure composed of plant cell wall and plasma membrane through which rhizobia travel from a curled root hair pocket into the root cortex.

Bacteroid

The enlarged, differentiated, non-dividing symbiotic form of rhizobial bacteria residing within host plant cells enclosed by a peribacteroid symbiosome membrane to fix dinitrogen.

Nitrogenase

A bacterial two-component metalloenzyme complex consisting of an iron protein and a molybdenum-iron protein that catalyzes the biological reduction of atmospheric nitrogen: $N_2 + 8 H^+ + 8 e^- + 16 ATP \rightarrow 2 NH_3 + H_2 + 16 ADP + 16 P_i$.

Leghemoglobin

A plant-synthesized, heme-containing protein with high oxygen affinity found inside active legume root nodules that buffers free oxygen to protect nitrogenase while delivering adequate oxygen to bacteroid respiratory chains.