120 THE POTATO CROP. 



nal are produced by outward agents, and no diseases can 

 be more strictly internal than some of those produced by 

 parasites, such as "smut," '-'bunt," and "vibrio." 



A mere glance at the vegetable kingdom, as it is distri- 

 buted over the face of the globe, is sufficient to show that 

 plants are greatly affected by inorganic agents. Not 

 merely is there a visibly marked difference between the 

 vegetable productions of the tropical and temperate zones, 

 but in proportion as there are differences of soil and alti- 

 tude; we see this plainly enough at home. The driest and 

 the wettest places have their peculiar denizens ; the mea- 

 dows and the woods have again theirs ; and, according to 

 the nature of the soil, these are clothed with different 

 vegetables, which are wholly absent in some spots, and in 

 others barely maintain an existence; while very slight 

 alterations of drainage or tillage completely drive away 

 some, and others take their place. Some species, again, 

 which abound for miles in certain districts as the fox- 

 glove and the red campion in other tracts, where they 

 flourish perfectly when introduced into the garden, are 

 wholly wanting. If resident in a mountainous district, 

 especially one that ascends up as high as perpetual 

 snow, we see distinct zones of vegetation; and if the 

 plants of the upper zones are brought into the garden, 

 with every care and skill, and a perfect knowledge of their 

 exigencies of a long rest, and of abundant light and warmth 

 at the period of active growth, it is frequently found im- 

 practicable to preserve them. 



It is clear, then, that certain conditions of soil, atmo- 

 sphere, and other inorganic agents, are not merely neces- 

 sary to the healthy development of particular vegetables, 

 but that any great derangement of these conditions, or of 

 any one of them, is sufficient to prevent their growth ; for 

 if this were not the case there could not exist, as there 

 does at present, a distinctly marked distribution of vege- 



