136 PLANT PHYSIOLOGY 
198. Death of the diseased plant or tissues may be very 
early or may actually be postponed beyond the normal | 
time, the fungus continuing 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 
absorption 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 indueed 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 (Puccinia 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 dryness may cause so much water loss 
that the plant actually dries out and dies. But aside 
from these cases must be 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 
