THE ROTATED-LAMINA SYNDROME .2. THE SEEDLING OF ULMUS-GLABRA

Seedlings of Ulmus glabra show heteroblastic development of the plumul ar axis, involving a change from an initial radial symmetry with leave s in decussate pairs to dorsiventral symmetry with distichously arrang ed foliage leaves displaying the rotated-lamina syndrome. The change i s very variable in detail. The change to distichy usually precedes the appearance of the rotated-lamina syndrome and is usually accompanied by the formation of one or more bud scales. Contrary to an earlier sug gestion, there is no anisophyllous phase of development. Axillary buds in the radially symmetrical region of the seedling also show heterobl astic development but have distichous phyllotaxis from the beginning. During the first growing season they initiate a small number of bud sc ales and then a series of foliage leaves showing progressively accentu ated development of the rotated-lamina syndrome. Some early foliage le aves show evidence of rotation of the lamina region by torsion. This i s not detectable in later leaves in the seedling or in adult shoots, a nd it is suggested that the rotated-lamina syndrome may have arisen by heterochronic shift of the rotation process into progressively earlie r stages of leaf development.