opuntia ficus-indica and engelmannii
hello. does anyone know of a definitive guide to opuntia, particularly ficus-indica and engelmannii varieties? I am a grad student working on a project with SW cacti and I am finding it very difficult to figure out the exact subspecies (varieties) of many of my plants. They were purchased a few years ago from a nursery in AZ, but I don't trust them b/c many were mislabeled. All the books/field guides/websites I have checked seem to contradict each other; for example, the images I have found of O. engelamnnii lindheimeri often look quite different from each other. Any help with this daunting task would be greatly appreciated!!