|Will triploid marigolds set seeds if they are planted next to ordinary marigolds?|
|From what I understand of plant genetics, no. Or if they do set seeds, they will likely be sterile. According to the book Botany for Gardeners, by Brian Capon:
"Normally, sex cells contain the haploid [1n] number of chromosomes. But, sporadically, diploid gametes [2n] are formed when reduction fails to occur during meiosis. Thus, when a diploid gamete combines with a normal haploid sex cell, the resultant 3n zygote is the beginning of a triploid sporophyte generation. Vegetative propagation is possible in a 3n plant, since mitotic cell divisions simply replicate the triploid chromosomes in each newly-formed cell....but reduced fertility is virtually assured when what seems to be normal reproduction occurs between triploids. During meiosis, the irregular distribution of the three sets of chromosomes to the spores...eventually results in incomplete chromosome pairing when the sex cells unite."
I would guess this type of incomplete pairing would occur between triploid and "ordinary" marigolds--and thus seeds would not form, or if they did, they would not mature properly.