erect and almost thornless American hawthorn with somewhat pear-shaped berries
<noun.plant>
Blackthorn \Black"thorn`\, n. (Bot.) (a) A spreading thorny shrub or small tree ({Prunus spinosa}), with blackish bark, and bearing little black plums, which are called sloes; the sloe. (b) A species of {Crat[ae]gus} or hawthorn ({Crat[ae]gus tomentosa}). Both are used for hedges.
I began to understand why blackthorn sticks were so scarce, compared with hazel, which can be cut with relative ease. More often than not a good deal of preliminary clearing had to be done even to reach a likely-looking piece.
The story, from an eyewitness, is that O'Hara claimed blackthorn was the only wood that couldn't be broken over an Irishman's head. Hemingway bet he could break that very stick over the head of that very Irishman.