800 PLANT GROWTH lO 



characteristics of growth, or the action spectrum of the Hght effect, and the cor- 

 responding effects of temperature, or of different wavelengths of Hght, on produc- 

 tion of auxin. Of late so much attention has been given to the role of light in de- 

 stroying auxin, as a fancied explanation of the shortening of stem elongation by 

 light (an explanation for which actually there is no direct evidence) that the role 

 of light in the fonnation of auxin in leaves has been overlooked. 



In the second place there is equally little doubt that auxin is not the major 

 limiting factor in the growth of the lamina. Isolated disks or fragments of leaf 

 tissue, when floated on auxin solution, usually do grow somewhat, depending on 

 the plant used and the experimental conditions. In the author's experiments, 

 sections cut from Pelargonimn leaves, free from large veins, grew 21% in sucrose 

 alone and 45% with lAA, i mg/1, added. In the work of Bonner et al. (1939), 

 disks from radish leaves in sucrose showed no response to auxin, but when sources 

 of nitrogen, such as adenine, proline, or asparagine were added, some growth was 

 observed (up to 20%). The diffusate from peas soaked in water was about twice as 

 effective. However, not even this material caused appreciable growth of rye [Secale) 

 leaf primordia attached to the stem apex; the elongation of these young leaves on a 

 sucrose-salts medium was not increased by any of a large group of substances tried 

 (De Ropp, 1945-47). Only when the tissue produced adventitious roots did the 

 apex enlarge; it then produced 8 leaves. That the action of the roots could be due 

 to increased uptake of salts or even of water was made unlikely by appropriate 

 experiments, and it seems probable that some substance necessary for growth of 

 rye leaves is synthesized in roots. Cabbage apices behaved in the same way, the 

 leaves only developing appreciably after roots had been formed. Unfortunately 

 in similar experiments with disks cut from cabbage leaves, roots did not have any 

 such effect. It can only be concluded that the role of roots differs from plant to 

 plant and perhaps also with the stage of development of the leaf. 



Whole, rooted, bean plants grown in light with sugar but without CO2, make 

 good experimental material since they form only minute leaves (Wiedow and Von 

 Guttenberg, 1953). These plants form leaves of moderate size when treated with 

 a variety of substances, including thiamine, ascorbic acid, lAA, or yeast extract. 



For monocotyledonous leaves an important growth factor is gibberellin (pp. 760, 

 765). Leaves of rice plants infected with Fusarium moniliforme, or treated with gib- 

 berellin prepared from cultures of this fungus, elongate up to 50% more than normal 

 (Brian et al., 1954 and Japanese literature there cited). Many, but not all, dwarf 

 varieties of corn respond by elongating both leaves and internodes and as little as 

 o.ooi [jLg of gibberellin can be detected by placing it on a leaf (Phinney, 1956). 

 Isolated sections of monocotyledonous leaves will also elongate in gibberellin. 

 They will not generally elongate in auxin, and in fact, sections of the leaf sheaths of 

 rice, whose elongation is promoted by sucrose, are inhibited by auxin (Mvirakami, 

 1956). It is curious that other members of the grass family, notably sugar-cane, 

 although susceptible to the Bakanae infection, do not show the characteristic leaf 

 overgrowth, and it may be that the gibberellin is already present in these plants 

 in non-limiting concentrations. Indeed, gibberellin A3 has been extracted from 

 healthy plants (for review and discussion see Stowe and Yamaki, 1957, 1959), 

 and a related substance from beans has been identified as gibberellin A.;. 



