THE ROTATED-LAMINA SYNDROME .4. RELATIONSHIPS BETWEEN ROTATION AND SYMMETRY IN MAGNOLIA AND OTHER CASES

Further variations of the rotated-lamina syndrome are described in Mag nolia spp. and Rhamnus imeretinus, as well as an abnormal adult shoot of Ulmus glabra without lamina rotation. All magnolias investigated sh ow lamina rotation, but there are four possible forms of shoot symmetr y: (i) dorsiventral distichous shoots with the form of rotated-lamina syndrome previously described, i.e., laminae of young leaves all face towards the same (upper) side of the bud or towards the parental axis in axillary buds; (ii) another form of dorsiventral symmetry in which lamina rotation occurs in the reverse direction; (iii) spiral phyllota xis with laminae rotated to face up the genetic spiral; and (iv) spira l phyllotaxis with laminae rotated to face down the genetic spiral. Sh oot symmetry and development of lamina rotation in leaf primordia corr elate with the taxonomic subdivision of the genus. Shoots of R. imeret inus are dorsiventral, with leaves arranged in four ranks, and lamina rotation occurs towards the upper side of the shoot. The sense of rota tion of leaf primordia reverses with a periodicity of two plastochrons . In the abnormal shoot of Ulmus without lamina rotation, phyllotaxis was distichous and leaf primordia were symmetrical. The various cases are discussed in relation to the previously erected hypothesis that co ntrol of development in dorsiventral shoots with the rotated-lamina sy ndrome resides in alternating states of asymmetry in the shoot apex, a nd the corollary that a shoot with spiral phyllotaxis and one sense of lamina rotation should result if the state of asymmetry is maintained and does not alternate.