136 PLANT PHYSIOLOGY 



198. Death of the diseased phiiit or tissues may be very 

 early or may actually be postponed beyond the normal 

 time, the fungus continuin<^ to live in the living infected 

 tissues after the surrounding tissues are dead. In most 

 cases, however, the presence of the parasite so weakens 

 the host that part of it or even the whole plant dies. The 

 death may result from various causes. Thus a disease 

 involving the tissues of the roots may so interfere with the 

 al)sorption of water that the top of the pLant dies under 

 symptoms of wilting. It is sometimes hard to tell, 

 however, whether the wilting is really due to reduced 

 water supply from the roots or to poisons secreted by or 

 whose secretion is induced by the fungus so that the cells 

 of the top are poisoned and lose their turgor, i.e. wilt. 

 Or again, the leaf tissues may be so destroyed by the in- 

 vasion of a fungus that photosynthesis is not sufficient 

 and the plant is weakened and dies. In some cases the 

 mechanical rupture of the host tissues by the reproduc- 

 tive bodies of the parasites leads to the destructive loss of 

 water through the wounds thus formed. This is probably 

 why the black stem rust of grains {Pucdnia graminis) is 

 so destructive. 



199. External meteorological conditions often result in 

 harmful conditions in the plant. Thus low temperature, 

 even when the freezing point is not approached, may so 

 check certain functions that a plant remains dwarfed or 

 pale (as in Indian corn in a cold spring). Excessive heat 

 and atmospheric dr3'ness ma}" cause so much water loss 

 that the plant actually' dries out and dies. But aside 

 from these cases must ])e noted the diseased conditions 

 resulting from harmful substances in the air. Thus in 

 the vicinity of manufacturing cities some trees cannot 

 exist, owing to the sulphur dioxide given out in the smoke 

 and which gradually poisons some of the nutritive cells 



