Image courtesy of John Crellin
Take a look at this excellent Blogpost by BSBI member Alan, and then see what you think of the hairs on Sisymbrium officinale.
And if you think Buddleja davidii Butterfly Bush could hold no surprises for you, try turning the leaf over and look closely with a handlens. You should see the textbook stellate hairs that John Poland talks about, like tiny parasols. A great way to conserve water, if you are a chunky ruderal plant intending to grow out of a wall and not die of thirst :-)