108 CULTURE OF FARM CROPS. 



nitrogen was directly absorbed from the atmosphere and 

 assimilated, M. Boussingault observed the growth of min- 

 ute green cryptogamia on the outside of the flower pots 

 which had been exposed to the air, but he failed to detect 

 any vegetable growth on those from which fresh air had 

 been carefully excluded. The sudden growth of varieties 

 of fungi during misty weather has often been noticed, 

 and their appearance may be accelerated by the introduc- 

 tion of a small quantity of vapour of ammonia into any 

 confined space where they are observed. I am not aware 

 that any extensive experiments have been made upon the 

 growth of fungi in an atmosphere rich in ammonia, such as 

 certain fogs. I have, however, remarked with surprise 

 their absence in an atmosphere from which ammoniacal va- 

 pours were probably abstracted by powdered charcoal, with- 

 out, however, drawing any conclusion from the observation, 

 until attracted by the curious discovery of M. Boussingault, 

 that fogs are eminently rich in ammonia. The presence of 

 a large quantity of this plant food in certain fogs is not 

 difficult to account for. Not only does the gradually in- 

 creasing quantity of aqueous vapour in the atmosphere be- 

 fore the positive appearance of mist in any locality, collect 

 and condense rare and widely-diffused ammoniacal vapours, 

 but the exhalations from the soil, produced by decompos- 

 ing vegetable matter, are arrested and accumulate. The 

 period of the year when fogs rich in ammonia may be ex- 

 pected depends naturally upon the frequency of the fall of 

 rain upon the moisture of the atmosphere, and upon 

 the winds. In Canada it appears reasonable to sup- 

 pose that we may expect to find fogs rich in ammonia 

 during the hot months of July and August, when the 

 rain-fall is not so great as in September. During these 

 months mists frequently hang over the fields, particularly 

 in low situations. The exhalations of vapour of water 

 from the leaves of plants being then checked, and their 

 juices partially stagnating in an atmosphere often rich in 

 ammoniacal vapours, all the conditions for the appearance 

 of the fungus called "rust " on the stems and leaves of the 



