A comparative histochemical study of changes in nucleic acids, proteins, and carbohydrates was performed on freeze-substituted samples of tobacco callus, cultured under shoot-forming and non-organ-forming conditions. The DNA (deoxyribonucleic acid) content per cell showed no apparent change, whereas denser staining for RNA (ribonucleic acid) and protein were observed in shoot-forming regions just before and during the initiation of organized structures. The most dramatic finding was the heavy accumulation of starch in shoot-forming tissue; this accumulation preceded any organ formation and was mainly in regions which ultimately gave rise to primordia.
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Thorpe et al. (1970) studied this question.