Regeneration in Shoots

A cleared and stained whole mount, showing strands of tracheary cells (arrow) that have regenerated around a wound (left) in a Coleus stem. (Jacobs 1952) 

The main advance that Jacobs made was to use larger sample sizes (than the few plants characteristic of many earlier studies by developmental anatomists), to show reproducibility, and to analyze results with statistical tests of significance. Because he used whole-mount clearing methods (rather than serial sections of paraffin-embedded tissue), it was feasible to process the large samples. The stained tracheary cells could be seen through the cleared outer tissue (Fig. 4.1). 

These first quantitative studies of cell differentiation under the influence of hormones are among the few cases in the literature where exact substitution of synthetic hormone for the normal hormone source has been demonstrated to give full and exact replacement of the hormone source in terms of a physiological effect. The prevention of abscission by lAA is another such case (Chap. 8 in Jacobs 1979). In both cases, the full effect of the leaves was replaceable by lAA alone. The results confirmed the hypothesis that, of all the chemicals undoubtedly required to differentiate a strand of regenerated tracheary cells, lAA from the leaves was the one that acted as a limiting factor in the process. Furthermore, the results with the polarity of the stem (Fig. 4.2) supported the view that the ability of the stem to transport lAA was a further check on how much lAA reached the regenerating area (Jacobs 1954). 

More precise determinations were made by Thompson and Jacobs (1966), who made the wound in one of the smaller vascular strands in the flat sides of Coleus stems (rather than in the large strand in the corners), so that the regeneration would be almost all in one plane and would consist of fewer cells. 

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