400 PLANT PHYSIOLOGY 



difference between plants and animals. Animals are alive "all 

 over " while plants are not. 



3. Natural death (as distinguished from accidental death) is 

 preceded by structural and functional changes in the body known 

 as senescence. In man these visible changes are the bent posture, 

 which is the result of the bending and fusing of the vertebrae ; the 

 wrinkled visage, due to the loss of plasticity in the tissues; and 

 the shuffling gait, which is caused by a failing motor coordination. 

 These visible external signs of senescence are accompanied by 

 internal changes within the cells, especially in the nervous tissue. 

 As the cells age, the nucleus becomes smaller in proportion to the 

 rest of the cell, fewer nucleoli appear in the nuclei, and the pig- 

 mentation of the cells increases. The following data from the 

 work of Hodge (1894) show the change from a child at birth to 

 an old man of ninety-two years: 



In the antennary lobe of the nervous system of the honeybee, 

 changes have been found similar to those in man. The young 

 cells contain large, round nuclei and small amounts of cytoplasm. 

 In the older cells, the nuclei become stellate in shape, and only a 

 small proportion of the cell is rich in protoplasm. While Minot 

 (1908) considered these changes as the cause of senescence, it is 

 more probable that they are accompanying results. However 

 that may be, the correlation of the two phenomena, — senescence 

 and cellular changes, — are very striking as expressed by Conklin 

 (1913), who writes: 



By all odds the most important structural peculiarity of senescence 

 is the increase of metaplasm or differentiation products at the expense of 

 the general protoplasm. This change of general protoplasm into products 

 of differentiation and metabolism is an essential feature of embryonic 

 differentiation, and it continues in man}^ types of cells until the entire 

 cell is almost filled with such products. Since nuclei depend upon the 

 general protoplasm for their growth, they also become small in such cells. 

 If this process of the transformation of protoplasm into differentiation 

 products continues long enough, it necessarily leads to the death of the 

 cell, since the continued life of the cell depends upon the interaction 

 between the general protoplasm and the nucleus. In cells laden with 



