A COMPARATIVE-STUDY OF FLORAL DEVELOPMENT IN TRILLIUM-APETALON AND T-KAMTSCHATICUM (LILIACEAE)

Trillium apetalon Makino is unique among Trillium in having apetalous flowers. Using scanning electron microscope, the early floral development was observed in comparison with that of T. kamtschaticum Pallas ex Pursh having petalous flowers. Morphologically petal primordia closely resemble stamen primordia in their more or less narrow and radially symmetric shape and are clearly distinct from sepal primordia with broad bases. Early in floral development sepal primordia are first initiated and subsequently two whorls of three primordia each are formed in rapid sequence, the first three at the corners and the second three at the sides of the triangular floral apex. Based on comparison in position and early developmental processes of their primordia, petals and outer stamens of Trillium kamtschaticum are equivalent to outer stamens and inner stamens of T. apetalon. The replacement of petals by outer stamens apparently leads to the loss of petals in Trillium apetalon flowers. Such a replacement can be interpreted in terms of homeosis. The replacement of the petal whorl leads to the serial replacement of the subsequent whorls: outer stamens by inner stamens, and inner stamens by gynoecium in Trillium apetalon. The term 'serial homeosis' is introduced for this serial replacement.