(from the Children's Britanica)

The plant known as the scarlet pimpernel is often called the shepherd's barometer or the poor man's weather-glass.  This is because the delicate flowers, which blossom in July and August, close when the air becomes damp and the weather is getting dull or cold.  In the sunshine, however, they are like little scarlet stars on slim stalks, creeping among the ripening corn, or among the furze and broom on a sandy heath, or on roadside banks.
    Two other pimpernels grow in Great Britain, the wood or yellow pimpernel and the bog pimpernel.  All belong to the Primulaceae (primrose) family. but the yellow one belongs to a different group than the other two.  All are small, creeping plants with flowers on slender stems.
    The yellow pimpernel is found in woods and shady places, where it creeps over the ground among the grasses.  It likes sandy soil and damp places, but it is also fond of sunshine so it is usually seen in clearings and round the margins of woods.  It flowers all the summer.
    The bog pimpernel is found in the western countries of Great Britain and in Ireland.  Its flowers are pale pink and rather transparent.