PLANT CELL GROWTH AND NUTRITION 461 



accumulated the ions (bromide). No such system, responsive to the 

 morphogenetic effects of Hght on growth, can be held to be indifferent, 

 or unaffected, during the course of these relatively long experiments, 

 during which some cells became senescent and no doubt lost their 

 absorbed content ( chloride ) to the solution. Thus the first experiments 

 on Nitella focused attention upon the need of the accumulating cells 

 for an energy source which could be made available through metabo- 

 lism and applied specifically to the accumulation of ions in the cells. 

 But these experiments could equally well have drawn attention to the 

 fact that the further accumidation so achieved was essentially a func- 

 tion of growing, albeit slowly enlarging, cells. However, the emphasis 

 on the system that could grow, whether by cell division or enlarge- 

 ment, was to come in other ways. 



Although the early work of Hoagland and his collaborators 

 ushered in, as it were, a period of intense inquiry and investigation at 

 the end of the first quarter of the twentieth century, one may now see 

 that the second quarter-century did little more than extend our knowl- 

 edge of the kind of process, or processes, here involved. Different 

 schools of thought made their impatt upon the problem in ways 

 affected by, if not limited by, the experimental materials that were 

 favored and the experimental techniques that were adopted. For sum- 

 maries of the early works on slices cut from storage organs — with their 

 emphasis upon aspects of metabolism affected greatly by oxygen ten- 

 sion, by temperature, and by proximity to the outer surface of the disk, 

 and their emphasis upon cytological events that foreshadowed a later 

 stress on characteristics of cells associated with their ability to grow— 

 reference may be made to the account already cited (Steward and Sut- 

 cliffe, 1959). 



The use of excised roots in Hoaglands laboratory quickly pro- 

 duced an apparent paradox. The distribution of the accumulated ions 

 along the axis of unbranched and excised roots reflected the activity of 

 the accumulating cells in tlieir own growth and development. Cells 

 near the tip at the peak of their growth acquired high concentration, 

 those a\\'ay from the tip accumulated less (Prevot and Steward, 1936; 

 Steward, Prevot, and Harrison, 1942). But the parallelism with growth 

 seemed to be evaded bv the massive accumulation of ions which could 

 occur in a short time, though it was not maintained later, when the so- 

 called low-salt excised root systems were exposed under appropriate 

 conditions to the salt solutions ( Hoagland and Broyer, 1936 ) . The re- 

 lation to aerobic metabolism, and therefore the process as an active 

 non-equilibrium one, was again apparent from the role of oxygen ten- 

 sion, temperature, etc. But the paradox became clear when it was 

 shown that during the growth of the roots and the plant, supplied only 

 with limiting amounts of nutrients (NO3 especially), they accumulated 



