Morphology of Flowering Plants

Roots and stems: recognising a modification by its underlying organ

Both roots and stems are frequently modified away from their ordinary function, and correctly identifying which organ a given modification actually started from, root or stem, is the recurring exam trap in this section. Root modifications include storage roots swollen with food reserves (carrot, turnip), prop roots that grow downward from branches to provide extra mechanical support (the banyan tree's aerial prop roots), and pneumatophores, specialised roots that grow upward out of waterlogged, oxygen-poor soil specifically to enable gas exchange, characteristic of mangrove plants adapted to swampy coastal habitats. Stem modifications include underground stems storing food (the potato tuber and ginger rhizome, both stems despite growing underground, not roots), runners and stolons that creep along or near the ground for vegetative propagation, thorns for defence, and flattened, photosynthetic stems that take over the leaf's usual function in plants like Opuntia, where the true leaves are reduced or absent.

Leaves: venation and phyllotaxy

Venation, the arrangement of veins within a leaf blade, comes in two main patterns: reticulate venation, a branching, net-like pattern typical of dicotyledonous plants, and parallel venation, where veins run alongside each other without a netted cross-pattern, typical of monocotyledonous plants, making venation pattern itself a quick, visible way to tell the two major flowering-plant groups apart. Phyllotaxy, the arrangement of leaves on a stem, comes in three named types: alternate (a single leaf at each node, alternating sides progressively up the stem), opposite (two leaves at each node, positioned directly across from each other), and whorled (more than two leaves arising together at a single node, forming a ring). Leaves are also frequently modified beyond simple photosynthesis: into tendrils for climbing (as in the pea plant), into spines for defence (as in cacti), or retained as fleshy, food-storing structures (the swollen leaf bases of onion and garlic).

Inflorescence: racemose versus cymose

An inflorescence is the arrangement of flowers on a plant's floral axis, and it splits into two fundamentally different growth patterns. A racemose inflorescence has a main axis that keeps growing indefinitely (indeterminate growth), continuously producing new flowers along its length in acropetal order, meaning the youngest flowers sit nearest the growing tip and the oldest sit further down toward the base. A cymose inflorescence instead has a main axis that terminates in a flower, consuming its own apical growing point in doing so, which necessarily halts any further elongation of that axis (determinate growth), and its flowers are consequently arranged in basipetal order, the oldest flower sitting at the very tip (where the axis terminated) and younger flowers arising progressively further down, the exact reverse sequence from a racemose inflorescence. The single clearest diagnostic test between the two is simply whether the main axis ends in a flower at all: a cymose axis does, a racemose one never does.

The flower: four whorls, arranged around a central axis

A complete flower is built from four concentric whorls attached to the floral axis (the receptacle), moving from outermost to innermost: the calyx (the outer whorl, made of sepals, typically green and protective in the flower bud stage), the corolla (made of petals, typically the most colourful, brightly displayed whorl, often serving to attract pollinators), the androecium (the flower's male reproductive whorl, made of stamens, each generally differentiated into a filament and a pollen-bearing anther), and the gynoecium (the flower's female reproductive whorl, at the very centre, made of one or more carpels, each differentiated into an ovary, style and stigma). How the sepals or petals overlap one another in the unopened bud is called aestivation, and several named patterns (valvate, where the margins just touch without overlapping; twisted; imbricate, where margins overlap irregularly; and vexillary, a specific overlapping pattern characteristic of pea-family flowers) are themselves used as a diagnostic, family-identifying feature.

Quick revision points

• Root modifications: storage (carrot, turnip), prop roots (banyan), pneumatophores (mangroves, for gas exchange in waterlogged soil).
• Stem modifications: underground food storage (potato tuber, ginger rhizome, both stems not roots), runners/stolons (vegetative propagation), thorns (defence), flattened photosynthetic stems (Opuntia).
• Venation: reticulate (net-like, dicots) versus parallel (monocots). Phyllotaxy: alternate (one leaf/node, alternating), opposite (two leaves/node, across from each other), whorled (more than two leaves/node).
• Racemose inflorescence: indeterminate growth (main axis never ends in a flower, keeps elongating), acropetal flower order (youngest nearest the tip). Cymose inflorescence: determinate growth (main axis ends in a terminal flower, growth stops), basipetal flower order (oldest at the tip).
• Flower's four whorls (outer to inner): calyx (sepals), corolla (petals), androecium (stamens, male), gynoecium (carpels, female, at the centre). Aestivation (how sepals/petals overlap in bud): valvate, twisted, imbricate, vexillary (pea-family characteristic).