413 



on leaves, blossoms and young twigs of woody and herbaceous plants usually 

 covering the outer surface of the organs, sometimes as a shining uniform 

 varnish, sometimes in the form of yellowish tough drops. Meyen^ relates 

 that for some time the theory expressed by Pliny was accepted, namely, 

 that the honey dew was an actual falling from the air, occurring in the dog 

 days especially and coating not only the plants, but even the clothing of 

 men. J- Bauhin contradicts this theory and calls attention to the fact 

 that only isolated plants or species in any region become diseased. After 

 the excretion of a sweet sap from the anus or the abdominal tubes of the 

 aphids had been observed, they were considered to be the cause of the dis- 

 ease and at the time it w'as observed that aphids and honey dew were fre- 

 quently found together. To this, however, was opposed first of all, the fact 

 that the aphids usually occur on the under side of the leaf, the honey dew, 

 chiefly on the upper side. However, this fact is no very certain proof 

 since the aphids of the under side of the leaf can sprinkle the upper side of 

 the leaf lying next below. But gradually the observations on honey dew 

 were increased on isolated outdoor and indoor plants on which no aphids 

 could be found or upon which they did not appear until sometime later. 

 Hartig's observation, made in 1834, is interesting in this connection. A 

 rose plant, which had not been taken from the house, secreted small drops 

 on the under epidermis of the leaves from which the sugar was separated 

 in rhomboidal or cubical crystals. In this the green color of the leaves 

 changed to a grayish one, due to the disappearance of the chlorophyll in 

 the mesophyll at the secreting places and to the appearance of clear drops 

 in the cells. Treviranus", in the same way, frequently found such sugary 

 secretions in the warm, continuously dry air out of doors as well as in 

 greenhouses, on white poplars, lindens, orange trees, distils (Carduus 

 arctioides) and cited still older observations by Lobel, Pena, Tournefort et 

 al., according to which honey dew occurs on olive trees, varieties of maple, 

 walnuts, willows, elms and spruces. He, and later Meyen, were convinced 

 that the drops containing sugar w^ere secreted directly from the epidermal 

 cells, to which the former observers also added that the stomata did not 

 take part in this secretion. Further observations on honey dew occurring 

 in very different plants, especially oaks, were furnished later by Gasparrini''. 

 The honey dew on the linden has been chemically investigated by 

 Boussingault and that on the grape cherry (Prunus Padus) by Zoller**. 

 Boussingault found that the honey dew, collected at two different times, 

 differed quantitatively in regard to the different substances ; from which 

 fact it is evident, that the secretion does not always have the same per- 

 centage composition. But the nature of the substance seems to change also, 

 for although Boussnigault found only cane sugar (48 to 55 per cent.), in- 

 vert sugar (28 to 24 per cent.), and dextrin (22 to 19 per cent.), Langlois 



1 Pflanzenpathologie, 1841, p. 217. 



2 Physiologie der Gewachse, 18.38, Vol. II, Part I, p. 35-37. 



3 Sopra la melata o trasudamento di aspetto goommoso etc. Bot. Zeit. 1864, 

 p. 324. 



i Okonom. Fort.schr. 1S72, No. 2, p. 39. 



