Joint Convention. 201 



develop in plants. From experiments that have been tried, it 

 has been found that the force with which the sap is driven up in 

 a vine in the spring is in one instance equal to the weight of 

 fourteen and one-half inches of mercury or to a column of water 

 one hundred and ninety-five feet high; in another experiment 

 the sap pressure was found to be equal to thirty-eight inches of 

 mercury. It has also been ascertained that this force varies 

 with atmosphere pressure. Now rnst, blight and mildew all 

 develop most rapidly in extremely hot and damp weather. 

 This is the time of low atmospheric pr£ssure and also 

 is a time in which there is a greater flow of sap and con- 

 sequently of rapid growth. The atmospheric pressure is lessened 

 and presents little resisting force to the enormous pressure 

 within the plants, and more sap is forced into the leaves than 

 they can assimilate, and is even forced through the pores of the 

 leaves and bark of the tender twigs, giving the spores of fungi 

 just the opportunity they need to germinate and to send their 

 roots into the cells within. The moisture of the atmosphere at 

 such times also favors the development of the spores by pre- 

 venting the drying up of the sap and thus checking germJnation. 

 Some varieties are more susceptible to the influence of heat than 

 others, and hence are more apt to attacks of blight or mildew. 

 Their sensitiveness to the effect of heat causes them to make a 

 more rapid growth, thus increasing the pressure of the sap cir- 

 culation and the tendency to blight. Anytiiing in local condi- 

 tions that increases the power of this pressure, increases the 

 tendency to mildew and blight, and anything that diminishes it 

 will act as a preventive, as root pruning, underdraining, deep 

 plowing, loweriiig the temperature or fertility of the soil. 



There are two classes of fungi. One that has fibrous roots, or 

 mycelium, that spread out over the leaves and bark of the plants 

 and force their way into the cellular structure. Another that 

 has no roots, but consists only of cells. Microscopists say that 

 it is a variety of this last class that produces hog cholera; an- 

 other, diphtheria; and others cause Asiatic cholera, typhoid 

 fever, and the whole list of contagious diseases. There is an- 

 ther variety of this last class, called bacteria, that are the sole 



