Question

Which plant is described by these clues? Its genus name is a compound of the Greek words for water and for a small vessel, coined not for the shrub's thirst but for the little cup-shaped seed {capsule} that follows each fertile flower. Each seed-setting flower is small and open, with four or five quite separate {petals}, twice as many free {stamens} or more, and an {ovary} sunk below them that ripens into a dry many-seeded pod crowned by its persistent {styles}. Its leaves are set in opposite pairs at every {node}, each pair turned across the pair below, and there are no {stipules} where the stalks meet the stem. The big showy blooms round the rim of the cluster set no seed at all: their colour is carried by about four enlarged {sepals}, and the true fertile flowers are the beaded specks crowded behind them. Its epithet is nothing more than a promise of a big leaf, and the blades keep it — thick, firm, glossy and essentially hairless, 4 to 6 inches (10 to 15 cm) long with coarse triangular teeth. In the wild it is a coastal plant of the Pacific-facing peninsulas of central Honshu and the Izu and Ogasawara islands, growing in forest margins and grassland within sight of the sea. Blue flowers on it are not a direct reading of soil pH: acid soil keeps soil aluminium in solution where roots can take it up, and in the {sepals} the aluminium ion complexes with the {anthocyanin} delphinidin-3-glucoside to give blue, while limy soil locks that aluminium away as insoluble hydroxide and the identical pigment reads pink. Its white-flowered cultivars carry none of that pigment at all, so no amount of soil doctoring will ever push them to blue or pink. Its leaves and flower buds hold cyanogenic {glycosides}, which is why it is listed as a poisoning risk for dogs, horses and children who chew the foliage. The flower buds of its traditional forms are set at the shoot tips the summer before they open, so a hard winter or a tidy autumn prune leaves a bush of lush leaves and not one head of bloom. It is the shrub gardeners deliberately recolour, aluminium sulfate for blue and garden lime for pink, the florist's potted gift plant that wilts flat on a hot afternoon and stands up again by dusk, and the one every grower eventually blames themselves for cutting back at the wrong moment.

Accepted Answer

Hydrangea macrophylla

Name That Plant #062