398 PLANT PHYSIOLOGY 



death. To prove the former is not within its realm, and to prove 

 the latter is to prove a negative, which is not within its power. 



Although man is interested primarily in the death of the human 

 species, because of its direct bearing upon his own life, any facts 

 about death either in the animal or plant worlds may be of im- 

 portance in helping him to solve the problem; and, for this reason, 

 it is valuable to review briefly some of the facts known concern- 

 ing death. 



Known Facts Concerning Death.— 1. There is an enormous 

 variation in the length of life of different species and groups of 

 organisms. The following table expresses the range found within 

 the organic world : 



Group Age limit 



Protozoa and simpler plants including bacteria 24 hrs.-no limit 



Insects 24 hrs.- 17 yrs. 



Fish ? " 267 y rs - 



Amphibians ? ~ . 2? yrs ' 



Reptiles ? -J75 yrs. 



Birds 9 yrs.- 118 yrs. 



Mammals 1.5 yrs - 100 yrs. 



Trees {Sequoia) ? - 3 > 000 y rs - 



In trees, which live so much longer than animals, there is present 

 much dead tissue. At the growing regions, plants have an indefinite 

 form of growth, although the bulk of the tissue is dead. Owing 

 to the lack of specialization of plant tissues as compared with 

 those of animals, it is possible for only a small region to be alive 

 and the entire organism, nevertheless, to be spoken of as "living." 

 Unlike old Rover of the nursery rhyme, biennials and perennials 

 do not " die all over." In deciduous species, new leaves are formed 

 each year, and in those which are " evergreen," the leaves last, at 

 the longest, only a few seasons before they are replaced. The wood 

 dies in time and the old phloem is pushed off with the bark. Mac- 

 Dougal (1926) reported cactus cells which, he thinks, are over 100 

 years old, and later (1928) he and Brown reported living ray 

 cells more than two centuries old in the heart wood of Parkinsonia. 

 If the ray cells are that old, they are probably extremely ex- 

 ceptional in the plant kingdom; plant cells, in general, live for 

 only a relatively few years. It is thus seen that the life of a sper- 

 matophyte is not exactly comparable with that of a mammal; 

 the simple animals and plants are much more alike in their struc- 

 ture and in their longevity. 



It will be noted from the table that whales and elephants (mam- 



