Thorny hawthorns

The robin sang to me from the hawthorn tree again this morning, though sadly there was no rising sun to be seen from behind the thick masses of grey cloud. It appeared later on however, and is now sending streaky tree shadows across the lawn.

After mentioning my previous robin serenade, a transatlantic friend, currently living on English shores, asked me if I would teach him how to identify the British Hawthorn. I replied that certainly I would, but I would endeavour to refrain from using the same lesson by which I myself learnt to identify this thorny tree. It happened in this way...
Some years ago, my father and I were walking in some wild parkland, high above Oxford, and my father was telling me about the different trees. Finally I felt the time had come for me to "repeat my lesson", so I walked boldly up to a hawthorn bush, stuck out my finger, and said "So this is a hawthor- OW!" Painful, but effective...