Nick recently watched an episode of QI where the difference between thorns and prickles was explained. The Egg is surrounded by bushes protected in one of these ways or the other. Prickles grow from the skin of the plant and are found on the tangle of impenetrable blackberries. Thorns derived from shoots, arise from a bud to guard the blackthorn. Interestingly the rose bush growing in the former Bofors Gun battery is protected by prickles and not as commonly supposed by thorns.