384 



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 



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. 



