Thistle Is The Common Name Of A Polyphyletic Group Of Flowering Plants Characterised By Leaves With Sharp Spines Or Prickles On The Margins, Mostly In The Plant Family Asteraceae. Their Prickles Often Occur All Over The Plant, Including On The Stem And Flat Parts Of The Leaf. These Are An Adaptation To Protect The Plant Against Herbivorous Animals, Discouraging Them From Feeding On The Plant. Photo