The rotated-lamina syndrome. VII. Direct formation of lamina in the rotated position in Pterospermum (Sterculiaceae) and the appearance of hyper-rotation

Abstract

The genus Pterospermum presents an extreme case of lamina rotation in their dorsiventral distichous shoots. In Pterospermum suberifolium and an unidentified species, the young laminae emerge from the bud with their upper surfaces facing towards the upper side of the shoot, and, in this, they resemble most other cases of lamina rotation. In Pterospermum heterophyllum and Pterospermum acerifolium, the lamina emerge in hyper-rotated orientation, with the upper surface of the lamina facing outwards away from the parent axis in P. heterophyllum or outwards and towards the lower side of the shoot in P. acerifolium. The early development of the lamina is unusual in all cases, since it appears to arise directly in the rotated position. This unusual mode of lamina development appears to be a case of saltational evolution. In P. acerifolium and P. heterophyllum, the leaf buttress also develops asymmetrically in the manner of other lamina rotators previously described, so that the lamina is rotated further with the formation of a triangular axillary site. This may be a relic of a more "normal" form of lamina rotation. Shoots with spiral phyllotaxis also occur in P. acerifolium, and in P. heterophyllum to a lesser extent, and these shoots have palmately lobed peltate leaves in contrast with the simple leaves of the dorsiventral shoots. The laminae in these shoots arise in a peculiar orientation, so that the upper surface of the lamina faces in the direction of growth of the shoot and never faces towards the shoot apex on which it formed. The mode of early lamina development in the spiral shoots is very similar to that of the dorsiventral shoots, but in the spiral shoots, the lamina is symmetrically placed instead of facing to one side of the shoot. It is suggested that the peculiar orientation of the lamina in the dorsiventral shoots arose first, and then a symmetrical form of this mode of development became incorporated into the leaves of the spiral shoots. Key words: Sterculiaceae, leaf, development, dorsiventrality, lamina rotation, hyper-rotation.