VII GROWTH OF SPECIFIC ORGANS 815 



enough to offset completely the inhibiting action of auxin, and in one experiment 

 the combination of adenine and lAA gave even better elongation than adenine 

 alone. Still more suggestive, however, is the powerful action of kinetin. Isolated 

 pea stems bearing one bud and supplied only with sucrose showed the expected 

 inhibition of growth of the bud by lAA, but when kinetin was added as well this 

 inhibition was entirely removed and the buds elongated as fast as the controls 

 (Wickson and Thimann, 1956, 1958). No interaction with roots or other growth 

 factors was needed, and the concentrations of kinetin effective were closely com- 

 parable on a molar basis with those of auxin. These experiments could, of course, 

 have the relatively simple explanation that kinetin somehow helps the tissue of the 

 bud to destroy auxin (perhaps by promoting synthesis of an lAA-destroying 

 enzyme), but they could, and with more attractiveness, be taken to mean that 

 bud growth depends on a supply of kinetin and that it is with this supply that auxin 

 interferes. For if auxin traveling down the stem were to combine with kinetin, 

 perhaps on a mole-for-mole basis, then the buds would be deprived of kinetin 

 which might well be essential for their cell division and hence for their growth. 

 At any rate these findings open up new avenues for study of this still obscure 

 problem. 



VIII. CONCLUSION 



This brief survey of plant growth has been conducted at four levels; those of 

 chemical growth substances, of cells, of tissues and of organs. At all levels much 

 knowledge has been accumulated and as a by-product valuable advances in ap- 

 plied plant science have accrued. Problems at the fifth level, that of the whole 

 plant, remain elusive and difficult, although it is easy to believe that the recent 

 discoveries of new growth substances will make some explanations possible. The 

 similarities and differences between plant and animal growth, while fairly clear 

 on the surface, remain none too clearly defined in physiological terms, and it may 

 be that interfertilization between plant and animal sciences will be the most 

 fruitful field for the future. 



Literature p. 8i6 



