4 
ROYAL HORTICULTURAL SOCIETY. 
The amount of manna, therefore, which exudes from the affected 
leaves is considerable, especially when one takes into consideration 
the amount of dextrine, a substance which does not exist in the 
healthy leaves at all.* Prom calculations made upon a tree of the 
same age and size, the leaves of the affected Lime-tree would have 
a surface of 240 square metres, or rather of 120 square metres 
(equal to 145 square yards), since the manna only covers one side 
of their surface. It would result from this, that on July 22, 1869, 
the Lime bore 2 to 3 kilogrammes (equal to about 4 to 7 lbs.) of 
honey-dew, reckoned in a dry state. 
In the normal conditions of vegetation the saccharine matters 
elaborated by the leaves, under the influence of light and heat, are 
distributed through the organism of the plant with the descending 
sap. In the abnormal state, which determines the production of 
the honey-dew, the saccharine matters are accumulated at the upper 
surface of the leaves, either because the movement of the sap is 
interrupted, or because it is retarded by the viscosity resulting from 
the formation of dextrine. 
The production of honey-dew cannot be due merely to meteoro- 
logical influences — to the effect of warm and dry summers. ISo 
doubt the Lime of Liebfrauenberg secreted it during a summer 
when there were periods of high temperature, accompanied by great 
dryness ; we must not, however, lose sight of the fact that it was a 
single tree that was attacked by the malady, and that at a little 
distance there were Limes which were perfectly healthy. 
It has been supposed that aphides, after having drawn the 
honey-dew from the parenchyma, discharge it again scarcely 
altered ; but it is contrary to the results of analysis to assign it a 
composition similar to that of leaf-sap. It is, however, admitted 
that certain insects possess the faculty of determining the production 
of manna. Thus it is to the punctures of a coccus that Ehrenberg 
and Heimprich attribute the formation of the manna which is still 
found on the mountains of Sinai. 
The manna falls to the ground from the air (that is to say, from 
the summit of a tree and not from the sky). The Arabs call it m«», 
and they, as well as the Greek monks, collect it to eat upon bread 
in the same way as honey. I have myself seen it fall, collected it, 
and brought it to Berlin with the plant and the remains of the 
* [Tbe occurrence of dextrine in the living tissues of plants is at the most 
hypothetical. Sachs, Fh^s. Veg. (Fr. ed.), 377.] 
