The mechanism of the action of the growth substance of plants

It was first suggested by Sachs that stimulating substances of the type which would now be called hormones play an important part in cell growth. Experimental evidence followed much later along two lines, ( a ) the discovery by Haberlandt (1921) of a substance produced in wounded tissue, stimulating cell division, and ( b ) the gradual accumulation of evidence from many workers of the existence of a substance present in growing plants which stimulates growth by increase in cell size. It is with certain aspects of this latter phenomenon with which we are concerned. Since the earlier extensive literature has been reviewed by Stark (1926) and the more recent literature by Went (1928) and Nielsen (1930), it will be sufficient here to summarize briefly the properties of this growth substances in relation to plant growth, so far as they have been determined. Although the growth substances has been shown to be non-specific, we have limited ourselves in this work to consideration of the phenomena occuring in Avena coleoptiles. In these coleoptiles, at the age used for experiment, cell divisions do not occur, and growth takes place by extension only. On this account they have been the principal objects studied.