Plane of Bilateral Symmetry in Plant Cilia

Abstract

THE fact of bilateral symmetry in cilia has been much more easily demonstrated than the plane of symmetry itself. The fact is implicit in the numerical relations of the component fibrils¹ (a ring of nine around a central pair); but unless there are external appendages or some other criteria with respect to which the orientation of the whole organ can be interpreted the median fibril cannot be recognized. This difficulty was encountered by Fawcett and Porter², who interpreted their very fine sections of animal cilia in terms of a plane of symmetry passing through and not between the two central strands, and a similar interpretation was given by Manton, Clarke and Greenwood³ for the seaweed Dictyota on the basis of external morphology. In neither case, however, was the possibility excluded that the plane of symmetry might in fact be at right angles to the line joining the two central strands, and Fawcett and Porter's outline diagrams do indeed somewhat suggest this.