PHYSIOLOGY 
water to the leaves, and death follows with surprising suddenness. In 
other cases, since in plants there are no means for quick distribution of 
poisons locally produced, nor any regulatory centers whose injury up- 
sets the whole system, death is likely to be merely local. In animals, 
on the contrary, a parasitic plant, restricted to a limited region, may 
produce poisons which are quickly spread through the body by the 
blood, attack the central nervous system or important viscera, and 
soon cause death. Thus, in diphtheria, the bacteria flourish chiefly 
a 
FIG. 655 . Galls : a, on leaf of rose ; b, on stem of grape. From PART III. 
in the throat, where they may produce no serious lesion, but the 
toxins produced reach the heart and kidneys and sometimes fatally 
injure them. 
Saprophytes. The association of a plant with a dead organism or 
organic debris is called saprophytism, and the live member is a sapro- 
phyte. Since a parasite may kill its host and then continue to live upon 
the body, the distinction between parasites and saprophytes is not always 
clear. Thus there are obligate parasites and obligate saprophytes; 
plants, namely, that are obliged to live in one relation or the other. Cor- 
respondingly there are facultative parasites and facultative saprophytes, 
which may pass part of their lives in one way and part in the other or 
wholly in either. Often the full cycle can be completed only if the given 
plant can establish the preferred relation. 
