THE FLOWER 247 



corolla, its fantastic shapes and honey glands, very 

 perfect adaptations of the flower to the purposes already 

 indicated. Perfume probably has a like purpose. 

 Like colour it serves to attract insects. It has even been 

 noticed that flowers, which exhale in the night a very 

 strong perfume, are visited by night insects. Yet the 

 exhalation of volatile, aromatic substances can serve 

 another purpose in the plant. To begin with, plants 

 need a more or less high temperature in order to flower, 

 which is partly supplied to them by the above-mentioned 

 rise in temperature as the result of respiration ; but, on 

 the other hand, during clear, calm nights plants are 

 known to be exposed to considerable cooling owing to 

 strong irradiation. To prevent this cooling we have 

 only to cover the plant with a glass bell which arrests 

 the heat rays emitted by the plant, and thereby prevents 

 unnecessary cooling. But this very property of the 

 glass belongs also to all volatile substances such as are 

 exhaled by flowers ; they strongly arrest radiant heat. 

 In order to ascertain how considerable is the atmosphere 

 of the volatile, so-called ethereal oils, surrounding scented 

 plants, we have only to apply a lighted match to the 

 strongly scented flower of Dictamnus. We see in amaze- 

 ment one flower and then the whole bush of flowers 

 enveloped in a bluish flame. This means that the 

 vapours of ethereal oils excreted by glands with which 

 the flower organs of this plant are supplied have become 

 ignited. Therefore, during calm, clear, summer nights, 

 i.e. exactly when there is a danger of cooling owing to 

 irradiation, the flowers are surrounded with a trans- 

 parent cloud, an atmosphere of these emanations, 

 which, by arresting like the glass bell the heat radiating 

 from the flowers, preserve them from excessive cooling.^ 

 Thus one more disappointment has been added to the 



1 Plants are known to be preserved from cooling in the night by the 

 lighting of a fire in their neighbourhood, which will make a cloud round 

 them and prevent them from losing too much heat by irradiation. 



