PLANT CELL GROWTH AND NUTRITION 475 



Stimuli in the form of coconut milk are added to tissue previously kept 

 in distilled water (or Ca CI2), its total respiration is increased as the 

 cells begin to grow and divide; at the same time, the amount of Cs'^'' 

 absorbed per day is such as to reach a lower concentration than in cells 

 which, lacking the coconut milk factors, are not dividing but only ex- 

 tending. These data then show again the sharp contrast between the 

 ways in which dividing cells and enlarging cells absorb ions from a 

 given solution, and the concentration levels attained. In the case of 

 dividing cells, limited "concentrations" are built up by ionic binding on 

 sites which multiply, and the internal concentration is proportional to 

 the first power of the external concentration. In the other case, the 

 growth of the vacuole invokes a mechanism of secretion from the cyto- 

 plasm into the vacuole, and in this situation a logarithmic relationship 

 obtains between the internal and external concentrations, so that the 

 degree of accumulation ( accumulation ratio ) is greater from the more 

 dilute solution. 



All these facts have been interpreted to mean that a given cell, in 

 its ontogeny, passes through a first phase in which the ions are bound 

 predominantly on given sites (or surfaces) which are themselves re- 

 newed or reduplicated as growth and synthesis proceeds. But as the 

 growth shifts to a condition in which the dividing cells mainly enlarge, 

 and vacuoles form and extend, the ions vacate their previous sites and 

 are secreted into the vacuoles, and so the process may be repeated. 

 ( For a general scheme which incorporates these ideas, see Figure 39 of 

 Steward and SutclifiFe, 1959. ) 



Growing and non-growing cells: Their physiology and metabolism 



Sites of action of the growth factors, etc. Some evidence does exist 

 concerning the site of action of the factors that induce or control 

 growth by cell division and by cell enlargement. This evidence flows 

 first from some data upon the effects of radiation. Resting, quiescent 

 tissue, prior to the induction of growth, is very vulnerable to radiation 

 at the point at which cell-division factors, such as those in coconut 

 milk, would otherwise intervene to induce growth by cell division. But 

 although this center of activity is knocked out by radiation, the cells 

 may still enlarge. By contrast, if growth induction by cell division has 

 occurred, the cells become surprisingly resistant to much higher dos- 

 ages of radiation (Table VI). Similarly, the pre-induction cells are 

 more resistant to cyanide and carbon monoxide than are the post-in- 

 duction cells, whereas the converse is true of susceptibility to inhibitors 

 that uncouple phosphorylation. This would seem to indicate that car- 

 bon monoxide, cyanide, and radiation all affect the cells at a point at 

 which the coconut milk stimulus normally intervenes to unleash the 



