118 PRIZE essay: 



quence of which water and matters soluble in it are driven in- 

 wards, and raised from without with facility ; and this external 

 pressure, along with capillary attraction, is the chief cause of 

 the motion and distribution of plant juices." (^) 



211. When the plant has taken up a maximum of moisture, 

 and evaporation is suppressed by a low temperature, or by con- 

 tinued wet weather, the supply of food, the nutrition of the plant 

 ceases ; the juices stagnate, and are altered ; they now pass into 

 a state in which they become a fertile soil for microscopic 

 plants. ('^) When rain falls after hot weather, and is followed by 

 a great heat without wind, so that every part of the plant is sur- 

 rounded by an atmosphere saturated with moisture, the cooling 

 due, to further evaporation, ceases, and the plants are destroyed 

 by fire-blast or scorching.(3) 



212. I now proceed to consider the conditions favorable to the 

 growth of rust, whose spores and sporules are at all times floating 

 in the air. Having already discussed this subject at some length 

 before the Horticultural and Agricultural Central Club, at To- 

 ronto, in April, 1856, I venture to append the views of the rapid 

 appearance of rust then advanced, with some additional proofs 

 and remedial suggestions. 



212. Ammonia, we know, exists in the atmosphere, probably 

 to the extent of one part in ten million parts on the average. At 

 times the quantity of ammonia present is much greater than the 

 above ratio, at other periods less. Rain water contains on an 

 average nearly one part of ammonia to the million, and of nitric 

 acid about five parts to the million. ^^^ Dew always contains am- 

 monia, and mists have prevailed so rich in this substance that 

 the water had an alkaline reaction. Barral analyzed the water 



(1) Leibig on Hales' Experiments—" Motion of the juices in the animal body." 



(2) Leibig on the motion of the juices of the animal body. (3) Ibid. 

 (4) Experiments of Dr. Gilbert and Mr. Lawes. 



