PLANT CELL CKOWTH AND NUTRITION 485 



are now known to contain powerful stiimili to cell di\ision and to all 

 those physiological processes that "follow the lead of growth." These 

 same stimuli— alone or in some cases in conjuction with synergists such 

 as 2,4-D, NAA, certain phenylacetic acids, etc.— will actually restore 

 even mature cells to active growth by division, a characteristic of the 

 embryonic state ( Steward and Shantz, 1959; Steward and Mohan Ram, 

 1960). In these circumstances the resultant growth, with all its con- 

 sequences for renewed protein synthesis, water absorption, and solute 

 absorption, is released b\' the exogenous stimuli, which unleash or 

 e\oke latent capacities for growth and dexx^lopment that were inherent 

 in the organization of the mature cells. The salient point for this discus- 

 sion is simply this. 



Extrinsic and intrinsic factors. Freed carrot cells, removed from 

 their neighbors in situ, display a \ariety of morphogenetic responses 

 when stimulated by coconut milk (Steward. Mapes, and Smith, 1958). 

 Since the stimulus to grow is in relative excess and comes from w-ith- 

 out, the cells respond in a \ariety of ways which suggest that, freed 

 from extrinsic controls, they now give expression to intrinsic capacities 

 for growth. B\- contrast, when the sam'e cells are part of an organized 

 mass, limitations upon their behavior are imposed by their position in 

 the mass, and they are thus confined to a rigid pattern of growth which 

 is dictated by the milieu. Thus a carrot tissue explant, free of cambium, 

 grows as a proliferated cellus and \irtually never organizes, but, in 

 sharp contrast, the cells freed from it produce a great \ariety of form 

 and responses by cell division, for they also organize readily, forming 

 roots, shoots, and even whole plants ( Steward, Mapes, and Mears, 

 1958). In other words, a single carrot cell from the secondary phloem 

 of the root, even after passage through many transfers in liquid culture, 

 still contains all the coded information necessary to recreate a carrot 

 plant. The nutrients that e\()ke this growth response, interestingly 

 enough, are just those nutrients that normalK' noiuish immature em- 

 bryos. Thus a given cell has its nature determined by its genetic con- 

 stitution, but its nurture is still a potent factor in its growth and de- 

 velopment. 



Totipotcncy of cells. A most striking point is that the freed cells of 

 carrot (cf. Figure 8), suspended in a liquid mediiun which can make 

 them grow {i.e., one containing coconut milk), are now seen to re- 

 capitulate in the first sequences of their division many stages that are 

 very reminiscent of the early embryogen\- of the carrot plant ( Figure 

 15; cf. Steward, 1958). Indeed, there are now good reasons to regard 

 a zygote as but a particular diploid cell which can grow when in an 

 internal medium fully competent to mak(> it grow. If the role of the 

 natural nutrients that normalh' nourish the embr\o is in large part re- 

 placed in the culture medium by the use of coconut milk or morpho- 



