The Calypso Borealis is considered as the rarest of the flowering plants which John Muir had discovered. He used the connotation 'the Hider of the North' to indicate the rare feature of it. Connotation refers to the words or phrases which is understood by the people belonging to a particular culture of belief. In the above case, the connotation 'Hider of the North' defines the rare flowering plant.
