PLANT CELL GROWTH AND NUTRITION 487 



The argument has thus come full circle. To understand so seem- 

 ingly specific a problem as the means by which a plant cell acquires 

 its accumulated potassium, one had to invoke the inherent ability of 

 the cell to grow, and it was seen that this process could not be ade- 

 quately understood without recognizing the role the organized cell may 

 play through both its operation and its self-duplication. The passage 

 of ions from one organ to another involves problems of growth cor- 

 relation within the plant body. But the lesson to be learned from free 

 cell cultures is that the diploid cell retains far more information in its 

 organization than merely how to accumulate ions or to synthesize pro- 

 teins or absorb water, for it may retain essentially the full attributes 

 of the whole organism. And when, as in the case of carrot cells, this 

 potentiality is to unfold, two things are important: first, the cells should 

 be freed from restrictions that may be imposed by the proximity of 

 neighboring cells with which they are in organic connection, so that 

 each becomes free to develop in accordance with its own intrinsic 

 properties; and secondly, the free cell should receive exogenously all 

 the known factors that will make it grow. To do this, one may need to 

 recapitulate the nutrients supplied in' early embryogenesis by the use 

 of the contents of liquid endosperms. 



Thus some of the salient problems of plant physiology extend 

 beyond the much-publicized molecular-biological ones— which would 

 seem to reduce botany to a branch of chemistry. These problems pre- 

 sent the equally challenging need to comprehend the ways in which 

 such a variety of chemical controls will regulate or modulate the 

 growth and behavior of the whole cell or organism, but within the 

 broad limits set by its genetic constitution. In plants this path through 

 time— or "creode," as Waddington terms it— from egg to maturity is only 

 confined within relatively broad limits. These limits comprise such 

 distinctive patterns of development as those of long-day and short-day 

 plants, high- and low-night-temperature plants, and a variety of other 

 morphogenetic responses. The environmental factors that "trigger off" 

 these responses must be mediated by similar "epigenetic" effects ex- 

 erted upon the cells of the growing regions. These and similar challeng- 

 ing problems are discussed from the standpoint of the substances and 

 factors that control growth by cell division and cell enlargement in a 

 forthcoming review by Steward and Mohan Ram. One may note, how- 

 ever, that again the solution to the problem is concerned not alone 

 with the nature and perception of the stimulus, not alone with the 

 chemical substance through which the stimulus is transmitted to the 



for it would recognize that there are a great many different ways, morphogenetic 

 and biochemical, through which the inherent potentialities of the organized, living 

 system are unfolded in a complex but integrated manner. 



