388 ROBBINS 



trees, for example, have existed for more than four thousand years. Not only 

 does the apical meristem persist for long periods of time, it resists change 

 even though subjected to wide variation in environmental conditions; that 

 is, it does not age in the sense that its cells become adult and the meristem as 

 such disappears even when subjected to considerable ranges of temperature, 

 oxygen supply, and other factors. The only instance I can recall where an 

 apical meristem differentiates into mature cells and disappears as such is the 

 transformation of some stem meristems into flowers. 



To avoid confusion I should point out that although apical meristems per- 

 sist as such for the life of the plant and therefore have an inherent stability, 

 the juvenile meristem is functionally unstable since it is transformed to an 

 adult meristem as the plant ages. A distinction is made, therefore, between 

 the stability of meristems as regions of cell division and their functional in- 

 stability as they pass from a juvenile to an adult condition. 



Although division may be regarded as a distinguishing characteristic of 

 young cells, they may exist for long periods of time without division and still 

 retain the capacity for renewed division. I have referred to embryos of some 

 kinds of seeds which may be dormant for centuries. A similar situation exists 

 for dormant buds and also characterizes the periodicity of cell division in the 

 apical meristems of perennial plants. This is another evidence of the inherent 

 stability of young cells. 



While the young cells of the meristem are so stable, they are at the same 

 time extremely sensitive. The displacement of cells in the process of growth 

 for a fraction of a millimeter from the meristematic zone initiates the aging 

 changes which result in their becoming mature or adult cells. The stability of 

 young cells under some conditions and their instability under others is a 

 paradox which I present for your consideration. 



Some cells, once they have become adult, also have reached a relatively 

 steady state ; they may remain alive and unchanged for long periods of time. 

 Cells in the pith of birch trees have been reported to remain alive for 40 

 years, and parenchyma cells of the pith of some of the giant cacti for 100 

 years or more. Other mature cells are short-lived. Those of the petals of 

 Tradescantia virginica, for example, degenerate and die a few hours after 

 the flower has opened. It appears, therefore, that in their aging some cells 

 may pass from a relatively steady state characteristic of young cells to an- 

 other steady state in their maturity. 



This process, however, is reversible. Some mature cells under some condi- 

 tions rejuvenate. Kraus and his associates found indole acetic acid and other 

 auxins to induce mature cells of decapitated bean plants to return to a 

 meristematic condition. Mature and fully differentiated epidermal cells of 

 flax, begonia, and Crassula under some conditions become meristematic. 

 Severely defoliated apple trees develop meristematic structures, the sphaero- 

 blasts, by the dedifferentiation of cells of the cortex. 



