A Theory of the Forms of Floating Leaves in certain 
Plants. By W. P. Hiern, M.A. 
[Read March 13, 1871.] 
WHEN a flat leaf with flexible margins grows steadily under 
favourable conditions and floats in running water, let it be 
supposed that during growth the action between two con- 
tiguous portions of an undivided part of the margin is entirely 
tangential and of the nature of tension, and not submitted either 
to normal strains or wrenching couples. Let it also be sup- 
posed that the vital power of growth as exhibited at the margin 
at any instant may be expressed mechanically at each point 
