DEATH 401 



products of differentiation, the power of regulation is first lost, then the 

 power of division, and finally the power of assimilation; and this is nor- 

 mally followed by the senescence and death of the cells. 



Benedict (1915) and Finardi (1925) found that in the wild 

 grape (Vitis vulpina) and in many other woody perennials the 

 meshes between the veins of the leaves get progressively smaller 

 as the plants grow older. That this condition is propagated by 

 cuttings is important because it shows the failure of asexual re- 

 production to produce rejuvenation in these cases. Benedict 

 interpreted this decrease in the size of the vein-islets as due to a 

 progressive decrease in the permeability of the cells. This re- 

 quires that the cells be closer to the veins, with the result that 

 "as the activities of life proceed, the cell is being gradually en- 

 tombed by the inevitable decrease in the permeability of its proto- 

 plasm." This may explain also the increased dissection with age 

 of the leaves of the dandelion (Taraxacum), as reported by Sears. 

 Bergamaschi (1926) also found in grape, oleander, camellia, and 

 other plants that the epidermal and cortical cells of the root 

 decreased in size as the plants grew older. As plant cells die they 

 also undergo many of the changes mentioned above : the cell walls 

 become firmer and less permeable, the amount of protoplasm de- 

 creases in amount relative to the cell sap, and the nucleus gradually 

 ceases to function. A large part of the increased electrical re- 

 sistance that occurs with age is probably due, however, to the 

 increased air space (Dixon). Belehradek (1927) found that the 

 viscosity of plant protoplasm increases with age. It is interesting 

 to note in this connection that the viability of seeds from old 

 trees is frequently less than that from younger ones. Although 

 Fabricius (1928) reported a slight increase of seed viability with 

 age in conifers, his results are quite contrary to those of Hofmann 

 (1921) and of Green (1920), who found that trees of Pinus syl- 

 vestris aged 15, 30, 45, 60, and 110 years gave seeds with a 

 germination capacity of 87, 50, 30, 15, and 0% respectively. 



Although the protoplasm degenerates as old age approaches, 

 there are some who believe that this degeneration does not start 

 in from birth. According to these observers, the protoplasm im- 

 proves until the period preceding senescence, i. e., the curve which 

 expresses the change of protoplasm with time, has a maximum 

 point before senescence begins. This is in accord with the re- 

 sults of Boyagu (1919) who found that the respiration curve in 



