Check out Karl Blossfeldt`s work. Fantastic botanical macro. Andreas Feininger also produced some very interesting images. Spanish photographer Emili Godez was doing stunning macro work in the 1930`s
Today I haven`t found many impressive B&W macro photographers, but there`s quite an array of people doing color work. Rosamond Wolff Purcell is a great example.
Tough challenge.

Peter