4M 



also found mannit as one constitutent of tlie honey dew on the linden. 

 Czapek^ collected the results of more recent observations. From this it 

 may be concluded that the composition of the honey dew varies in different 

 plants. 



A harmony of the theories as to the causes of the phenomenon has not 

 been obtained as yet. While Biisg^en- studied carefully the aphid stings on 

 plants, he proved that the animals secrete through the anus much larger 

 amounts of honey dew (the secretions of the abdominal tubes are waxy) 

 than is usually assumed, and, on this account, he concludes that real honey 

 dew depends only on aphids. Bonnier^ made some experiments which showed 

 an artificial production of honey dew without the intervention of the 

 animals. 



Biisgen says the peculiarities of the cuticle allow neither an osmosis 

 or distillation of sugar saps from the interior of the cell nor, as Wilson 

 assumed, an osmotic withdrawal of liquids through drops of sugar to be 

 found on the surface of the leaf, such as are formed by the excretion of 

 the aphids. This statement, however, does not consider the fact that the 

 smooth surface of the cuticle can become broken and that secretions in 

 individual cases can find their way through the stomata. Bonnier's results 

 prove the later case. Leaves which had been exposed to great differences 

 in temperature (conifers, oaks, maples, etc.), showed under the microscope, 

 when examined by direct illumination, the formation of nectar-like drops 

 from the stomata when the light was sufficiently strong. 



My own observations confirm the occurrence of honey dew without the 

 intervention of aphids. In one case I found an abundant formation of honey 

 dew on the older leaves of pear seedlings grown in water cultures and 

 exposed to the hot July sun. This observation showed that deficient soil 

 water was not necessarily a factor. I believe that honey dew is produced 

 if there is a sudden excessive increase of transpiration in strongly function- 

 ing active leaves, caused by a strong light stimulus, and brings about too 

 high a concentration of the cell sap. If the disturbance continues beyond 

 a certain point, the leaf suffers permanently and falls prematurely. In 

 another case the rain gradually washed the sugar coating away, which made 

 possible an attack of the black fungi (sooty dew). The production of 

 honey dew is not always dependent upon extreme and absolutely high 

 temperatures and strong light stimuli, but sudden great contrasts as, 

 for example, the sudden shock to an organism caused by an intense morning 

 sun following a very cool night, which had suppressed its activity. 



Shading would be the best preventative measure and repeated 

 sprinkling an effective remedy. 



1 Czapek, Fr., Biochemie der Pflanzen. Jena. Gustav Fischer. 1905, Vol. T, 

 p. 408. 



2 Biisgen, M., Der Honigtau. Biolog. Studien an Pflanzen u. Pflanzenlauscn. 

 Sond. Biologisches Centralbl. Vol. XI, Nos. 7 and 8, 1891. 



3 Bonnier, G., Sur la miell^e des feuilles. Compt. rend. 1S96, p. 335, cit. Zeit- 

 RChrift f. Pflanzenkrankh. 1896, p. 347. 



