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

Seedlings of Ulmus glabra show heteroblastic development of the plumular axis, involving a change from an initial radial symmetry with leaves in decussate pairs to dorsiventral symmetry with distichously arranged foliage leaves displaying the rotated-lamina syndrome. The change is 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 suggestion, there is no anisophyllous phase of development. Axillary buds in the radially symmetrical region of the seedling also show heteroblastic development but have distichous phyllotaxis from the beginning. During the first growing season they initiate a small number of bud scales and then a series of foliage leaves showing progressively accentuated development of the rotated-lamina syndrome. Some early foliage leaves show evidence of rotation of the lamina region by torsion. This is not detectable in later leaves in the seedling or in adult shoots, and it is suggested that the rotated-lamina syndrome may have arisen by heterochronic shift of the rotation process into progressively earlier stages of leaf development.