LUMINOSITY. 649 



by over-stimulating vegetative action, as is observed in the Vine in the 

 tropics, and commonly also in badly managed exotics in our hothouses. 



Sect. 2. LUMINOSITY. 



In most botanical works we find noticed the observation of the 

 daughter of Linnaeus, that she perceived a peculiar flashing lumi- 

 nosity of the flowers of Tropceolum majus on a hot summer's night 

 together with a statement that the same appearance has been ob- 

 served in the orange Lilies, Sunflower, Marigolds, &c. The fact 

 that most physiologists, from Saussure downward, who have sought 

 to repeat these observations have failed, and that the appearance 

 is always assigned to orange or red flowers, leads to the belief that 

 the statements are founded on error, arising from the peculiar 

 effect of these tints upon the eye. The influence upon the eye of 

 the brilliant orange and crimson flowers of some of the Rhododen- 

 drons and Azaleas now grown in our gardens is very similar to 

 that of looking upon a luminous body. The asserted luminosity 

 of flowers is certainly at present a very questionable matter. The 

 cases that have been recorded are cases of optical illusion, or in 

 some instances of exudation of inflammable vapour. 



Phosphorescence. On better authority, namely that of Humboldt, Nees 

 von Esenbeck, Unger, and others, rests the fact that the thallus of 

 some Fungi is luminous in the dark. The imperfect thalloid structure 

 described as Rhizomorpha subterranea, occasionally met with in mines, 

 exhibits upon its ramified structure points which possess an irregular 

 phosphorescence, sometimes rising to such a degree of luminosity as to 

 enable surrounding objects to be distinguished. According to Unger's ob- 

 servations, the light is not emitted from decaying matter, but from a 

 peculiar superficial layer of cellular tissue. Phosphorescence has been 

 observed in other mycelia, and it is also exhibited in the perfect Fungus 

 of Agaricus olearius and other species. 



Rotting wood is well known to be often phosphorescent ; and some 

 authors state that this does not depend upon the presence of Fungi ; but, 

 seeing the proved occurrence of phosphorescence in the mycelium of Fungi, 

 it is most probable that the luminosity is attributable to them, especially 

 as it is removed irrecoverably by drying up the damp rotten wood. 



The statement that the Moss Schistostega osmundacea, which grows on 

 the roofs of caves, is phosphorescent, has been declared by Milde to be 

 erroneous. We could detect no trace of it in observations made some 

 years ago, and we agree with Milde in attributing the appearance to the 

 glistening caused by the reflections and refractions of light on the wet 

 surface of the cellular leaves. 



According to Martius, the milky juice of Euphorbia phosphor ea becomes 

 luminous when removed from the plants and heated gently. 



The fullest details, with illustrations, on the subject of phospho- 

 rescence are to be found in the volumes of the ' Gardeners' Chronicle,' 

 1871, 1872, 1874. Dr. Cooke's paper on the same subject in ' Science 

 Gossip ' for 1871 should also be consulted on the subject. 



