Histological differentiation of inflorescences in two cultivars of geranium (Pelargonium x hortorum)

Differentiation of the first inflorescence was determined in two seed propagated cultivars of geranium, 'Multibloom Scarlet' and 'Red Elite'. Histological evidence was recorded in photomicrographs of paraffin sections of shoot apex tissues and scanning electron micrographs of shoot tip surfaces. Morphological evidence was recorded in photographs when the first flowers were showing red petals. Inflorescence primordia appeared at the 5th and 8th leaf stages in 'Multibloom Scarlet' and 'Red Elite' respectively. Earlier growth stages of seedling shoots bore pairs of opposite leaves. An inflorescence formed in lieu of a leaf, and inflorescence primordia and leaf primordia formed laterally from a central apical meristem. Whether or not this indeterminate pattern of seedling growth continued indefinitely was not determined. Photomicrographs of sectioned tissue detected inflorescence differentiation one leaf stage earlier than scanning electron micrographs, and thus was the more sensitive method for dating the event precisely. However, scanning electron micrographs revealed the structure of a whole shoot apex in a single micrograph and avoided reconstructive interpretation of three dimensional structure from serial sections.