branching repeatedly on the same side. In Adiantum, applied to anadromous fronds in which basal basiscopic pinnae are well-developed (i.e. pinnate), but corresponding acroscopic pinnae are reduced to simple pinnules. The stipe thus appears to branch dichotomously into two helically curved rachises, with pinnate pinnae apparently arising from their upper sides.
A warped surface which may be generated by a straight line moving in such a manner that every point of the line shall have a uniform motion in the direction of another fixed straight line, and at the same time a uniform angular motion about it.
The helicoid is one of the first minimal surfaces discovered. Its name derives from its similarity to the helix: for every point on the helicoid there is a helix contained in the helicoid which passes through that point.