WITH KEFEKENCE TO THE POTATO DISEASE. 67 



may be inferred from its prevailing in widely distant countries 

 at the same time. Great Britain, Canada, St. Helena may be 

 instanced. It must be concluded that a disease so universal must 

 proceed from some universal cause. Then it may be asked what 

 the causes are by wiiich vegetation is most imiversally affected. 



Vegetation is universally affected by Temperature, But to 

 this the disease cannot be attributed. We have had hot and 

 cold and temperate seasons since the disease visited us ; and it 

 has continued through them all. A peculiar sudden cliill may 

 have spread over great part of Europe and Nortli America, from 

 their proximity to tlie frozen regions ; but such could never 

 reach across the Line and affect the tropical island climate of St. 

 Helena, and the vegetation of the potato there. 



Vegetation is universally dependent on 3Ioisture : some plants 

 indeed require but little, yet all must have some. Moisture, as 

 we have seen, develops the latent disease or accelerates its pro- 

 gress, but cannot be said to cause it, else why did it not occur in 

 wetter and in drier seasons tlian any we have had since 1845? 

 The potato has been diseased where less than twenty inches depth 

 of rain have fallen in a year. Formerly it was not diseased in 

 seasons quite as dry ; nor was it formerly diseased, on the con- 

 trary, in localities wliere twice that quantity was not unusual. 

 Therefore the excess or deficiency of moisture has not certainly 

 been the cause of disease. 



Vegetation requires the presence of Air. This of course it 

 has liad. And with regard to tiie amount of atmospheric pressure, 

 this has not varied throughout these years in any considerable 

 degree. Besides, the potato grows at the level of the sea where 

 the atmospheric piessure averages about 2126 lbs. on every 

 square foot ; and it thrives likewise on the table-lands of South 

 America at an elevation of 10,000 feet, where the pressure is 

 743 lbs. less on every square foot than it is at the level of the 

 sea. Therefore it cannot be affected to any considerable extent 

 by minute differences in the density of the air. 



There is yet another important and universal agent necessary 

 for healthy vegetation, and that is Light. We have had some 

 experience in the use of instruments for estimating the intensity 

 of light ; and, in the general sense of the word, it does not 

 appear to have been so defective as to occasion anything like 

 absolute disease. The potato, it is well known, has frequently 

 elongated its shoots several feet or yards in a dark recess, succu- 

 lent and blanched, it is true, till they have reached the light ; 

 but on doing so, green foliage was produced, and no constitu- 

 tional disease was engendered. In very shaded situations, 

 potatoes produced weaker, paler, and altogether more imperfect 

 plants and tubers than in situations where they were well ex- 



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