40o PLANT PHYSIOLOGY 



All this points to the idea that death is not inherent in the individual 

 cell but is only the fate of more complicated organisms in which different 

 types of cells or tissues are dependent upon each other. In this case it 

 seems to happen that one or certain types of cells produce a substance or 

 substances which gradually become harmful to a vital organ like the 

 respiratory center of the medulla, or that certain tissues consume or de- 

 stroy substances which are needed for the life of some vital organ. The 

 mischief of death of complex organisms may then be traced to the activity 

 of a black sheep in the society of tissues and organs which constitute a 

 complicated multicellular organism. 



Plants and animals which are less differentiated are thus better 

 life insurance risks than highly differentiated forms. 



The question, then, as to why organisms die and why they die 

 when they do, has been summed up by Pearl, who has shown that 

 life is a continuum in which death is not inherent. Somatic death 

 is the result of physicochemical changes in the cells of the organism. 

 These changes result in organic disharmony ending in death, 

 which is a new phenomenon appearing with differentiation and 

 which occurs only in such soma cells as have lost the power of 

 reproducing and rejuvenating themselves. The time of death 

 is determined by the combined action of the environment and 

 heredity. The length of time required for the injurious effects 

 accompanying senescence and death to be completed, depends 

 upon the external conditions and upon the inherited characters 

 of the individual and species. 



QUESTIONS 



1. " There is no inherent reason for death." Explain this statement. 



2. What are the chief factors that limit the age and size of a tree? 



3. The Washington elm is now dead but close by on the Cambridge Common 

 is a thrifty tree grown from a cutting. Has this tree any right to the veneration 

 accorded the original elm? 



4. What is an individual? 



5. When is an organism dead? 



6. What is natural death? Is this a good choice of terms? Explain. 



7. Why is sour milk thought by the followers of Metchnikoff to favor 

 longevity in man? 



8. Since 1840 the average expectation of life in northern Europe and America 

 has been prolonged from about 36 years to about 56 years. Is this an un- 

 mitigated blessing to society? 



9. What and where is the oldest tree in the world? ^^ the article by Cham- 

 berlain. 



