Sentence:  Plant breeding is always a numbers game....The wild species we use are rich in genetic variation, .... In addition, we are looking for rare alleles, so the more plants we try, the better. These rarities may be new mutations, or they can be existing ones that are neutral—or are even selected against—in a wild population. A good example is mutations that disrupt seed dispersal, leaving the seeds on the <WSD>heads</WSD> long after they are ripe. 
Meaning of the word marked with <WSD> tags:  A clump of seeds, leaves or flowers; a capitulum.