As summarized by Barr (1979), Ligniera and Polymyxa cannot be distinguished by their sporosori, but can, however, by their sporangial states. Another problem with Ligniera is that both sporogenic and sporangial phases of one species (L. verrucosa) may exist in roots of Veronica along with sporangial stages of Sorosphaera veronicae (Miller 1959).
I was interested in karyotyping Ligniera because of its similarity to Polymyxa. We had easy access to L. verrucosa on the Ohio University campus, but I held off with my karyological study because I knew some of Charles Miller's students were working on the ultrastructure of general development of L. verrucosa. Once their paper was accepted for publication, however, I felt free to take a walk around campus and pull up some Veronica in an area where I knew it would be infected with L. verrucosa. Since stages of development of Sorosphaera veronicae also could be in roots of Veronica, I spent hours sifting through fresh material with compound light microscopy to select only sections of roots that had sporogenic stages of Ligniera.
I do not know how Charlie and his students missed transitional plasmodia in their developmental study of L. verrucosa. Transitional sporogenic plasmodia were easy to locate with techniques described in Meiosis & Karyotyping: I would cut a 1/2 µm "thick" section and examine it with light microscopy. When I found a transitional plasmodium, I would trim the block down to it and make serial sections for electron microscopy.
In hindsight, it is good that the karyotype for L. verrucosa differed from those of the Polymyxas. The conclusion that Ligniera should be kept as a separate genus from Polymyxa was easy to to make, but would not have been as easy to do if the karyotypes had been the same.
Images of Ligniera
Selected References for Ligniera