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It is by thorns, spines, and prickles that plants often protect themselves against the attacks of grazing animals. But it must be remembered that these are by no means the only safeguard. Plants produce poisonous, bitter, or strong-smelling substances which keep off their enemies, and these indeed often afford a more efficient protection (see Chap. III.). These thorns, etc., can be produced in the most unexpected places. There is one rule, however, namely that they are invariably found in the exact spot where they can be most useful.