302 



DISEASES OF ECONOMIC PLANTS 



more advanced stages, or in sections taken lower on the 

 stem, the wood is found to be penetrated longitudinally by 

 black streaks which are so abundant in stages immediately 



preceding death 

 that the whole or 

 nearly all of the 

 wood seems to be 

 so affected. Simi- 

 lar streaks pene- 

 trate the pith only 

 in extreme cases. 

 The blackening 

 often progresses 

 from the wood out- 

 ward through the 

 bark, produciiig 

 shrunken, black- 

 ened patches on 

 the surface of the 

 stem. 



In more ad- 

 vanced stages, 

 when all the leaves 

 are wilting, the 

 wood and bark at 

 the base of the 

 plant are blackened almost throughout, and the pith is 

 decayed, leaving the stem hollow or filled with a soft, 

 rotten residue. The bark, near the level of the ground, 

 turns black, and becomes dry and hard. The pith and 

 wood in the upper portions of the plant usually dry up, 



Fig. 132. — Tobacco plant in late stages of the Gran- 

 ville wilt. Original. 



