THE HOP PLANT. 47 



capacity to furnish the necessary supply of water to the plant 

 above ground. When the disease is the result of drought 

 the evil may be counteracted by watering the plants ; this 

 treatment is, however, of no avail if the cause is due to 

 decreased activity of the roots through rotting. Besides, 

 in many cases, artificial watering is too expensive an opera- 

 tion to be practicable. 



3. Cones Dropping Off. 



This is due to the same causes as sun brand ; and is, in 

 fact, nothing more or less than sun brand itself, occurring 

 at a time when the hop has already begun to put forth 

 cones. 



4. Honey Dew. 



Two kinds of honey dew are spoken of — vegetable and 

 animal ^ — the former being nothing more than sap excreted 

 by the leaves and stems, on the surface of which it collects 

 in the form of a layer of sweet, sticky matter. These ex- 

 cretions of sap are of most frequent occurrence under rapid 

 and extensive fluctuations of temperature, as often happen 

 in summer time, when very cold nights are followed by hot 

 days. The formation of this honey dew may be regarded as 

 taking place in the following manner : In the daytime the 

 tissues of the leaf are surcharged with sap in consequence 

 of the energetic flow of this material ; so that when the tissue 

 contracts in consequence of a considerable fall in the tem- 

 perature at evening and in the night, the sap, which con- 

 tracts less readily than the cell walls on cooling, is exposed 

 to pressure on the part of the latter, and is thus forced out 

 on to the surface of the organs in question. 



Similar to vegetable honey dew is the excretory pro-, 

 duct of the aphis, to which Fruwirth has applied the 

 distinguishing term of " animal " honey dew. 



1 C. Fruwirth, Hopfenbau und Hopfenbehandlung. Berlin, 1888. 



