Thonner's analytical key to the families of flowering plants

For the identification of a flowering plant the first step usually is to discover to which family it belongs. With some experience, the families commonly encountered in one's area of interest are soon known, but when dealing with specimens from other places, notably those from the vast and rich subtropics and tropics, there is much less certainty. The pertinent literature is often not readily available as it is often found only in expensive, rare or obscure books, or journals, present only ...