Other than sound like a character from Bucky O'Hare, Toadflax (Linaria vulgaris) are fascinating plants for several reasons. Taking centre stage though is how this plant potentially offers another method of inheritance beyond genes. We're now entering the slightly confusing world of epigenetics.
See, a particular species of these toadflax plants come in two distinct flavours: one consists of white, symmetrically arranged petals, whilst the other form is yellow and has five-pointed stars. Interestingly, this variation is not directly due to their DNA. Rather, these observed differences, which by the way are inherited by the offspring, are due to molecular caps attached to their DNA. As Carl Zimmer explains in his brilliant essay for the New York times:
These caps, made of carbon and hydrogen, are known as methyl groups. The star-shaped toadflax have a distinct pattern of caps on one gene involved in the development of flowers.
The importance of toadflax, and other instances of epigenetic phenomena, is not to be understated: it provides evidence for heredity through an additional channel. That said, we must be careful not to overstate the role of epigenetics by proclaiming mendelian inheritance is duly dead and, um, incorrect. It's not -- and I don't think many serious geneticists argue this, but you never know where some unscrupulous creationist is lurking.
One more point: citing Toadflax (even if accidentally referred to as Toadflux) in an exam will get you a first class mark. Fact.