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Pin and thrum is how primulas of the same species avoid self pollination. I could bore you with the details....
I am trying to remember back in the mists of time to primula aureata in which there was only one form in cultivation (pin)and I think that with perseverance some seed was produced which resulted in both pin and thrum being in cultivation. At about the same time somebody put pollen of another species on a pin plant which them produced fertile seed which produced plants identical to the parent(not hybrids) in both pin and thrum and again further off spring plants were produced.
Richards says that the original P. aureata introduction by Bailey consisted solely of pins, with the thrums coming in as a subsequent, separate, introduction. All seed set from the first introduction gave rise, of course, only to pins.