70 



Journal of Agricultural Research 



Vol. XVI, No. 2 



Efjpect oif Humidity 



Plants for this experiment on humidity (Table XIV) were placed 

 under two glass bell jars, under one of which there were exposed three 

 beakers filled with water to secure a humid atmosphere ; under the other 

 bell jar three plates with calcium chlorid were placed to absorb the 

 moisture in the air and that of transpiration by the plants. To prevent 

 the evaporation of moisture from the soil in one case and the absorption 

 of moisture by the soil in the other case, the surface of the pots was 

 covered with a paraffin layer before they were placed under the jar. A 

 third set of plants was kept as a control under normal greenhouse con- 

 ditions. For the purpose of determining the relative humidity, hy- 

 grometers were employed for each set, and readings were taken daily; 

 at the same time barometric readings in another end of the same building 

 were being taken. 



Table XIV. — Results showing the effect of humidity on the physiology and morphology 

 of urediniospores of Puccinia graminis tritici on wheat 



Ex- 

 peri- 

 ment 

 No. 



Humidity. 



Daily limits. 



Maxi- 

 mtmi. 



Per ct. 

 92- S 



67-5 

 54- o 



97- o 

 83- S 



66.5 



95- o 

 80.0 

 76.0 



Mini- 

 mum. 



Per ct. 



76-5 

 47-0 

 50- S 



91.0 



73- o 

 59-5 



90.5 

 68.5 

 67.5 



Total 

 aver- 

 age. 



Per ct. 

 85-3 



60. 4 

 52-3 



94-6 

 78.5 

 62. 2 



93-4 



74-3 

 71.0 



Moderate . 

 Heavy. . . . 



Weak. 



.do. 



.do. 



Heavy. 

 do. 



Degree of infection. Result 



Spore dimensions. 



Size limits. 



23- 36-40- 

 25.60-40. 

 24. 96-40- 



24. 96-40. 

 22. 40^40. 

 22. 72-39. 



24- 00-40- 

 25. 28-40. 

 23. 68-39. 



32X16. 

 00X16. 

 32X15- 



32X15- 

 32X16. 

 36X15. 



00X16. 

 32X16. 

 04X15. 



64-23.04. 

 00-23-04. 

 36-24.32. 



68-24. 96 • 

 32-23.68. 

 68-24. 64 . 



64-24.00. 

 96-23.04. 

 36-24.00. 



Modes. 



32.96X19.84 

 32.28X19-84 

 32.00X20. 16 



32.32X20.80 

 32.00X19.84 

 30.40X19-84 



32.00X20. 16 

 32.64X19.84 

 31.36X19-52 



From the results shown in Table XIV, it would seem as though either 

 excessively high or excessively low humidity causes a decrease in size 

 of spores, but the difference, however, is neither very pronounced nor 

 consistent. This can be explained by the fact that the inoculated plants 

 were placed under the restricted conditions only subsequent to the usual 

 confinement of 48 hours in the moist chambers. The germination period 

 would thus appear to constitute the critical period, the difiference in hu- 

 midity thereafter being apparently of lesser importance. 



