The chemical stimulus essential for growth by increase in cell number

Summary The development of this study has been a succession of steps each of which rests on the preceding. It falls naturally into three distinct stages. The first is that ofIdentification Here it was found that the lead precipitate present in the meristematic region of root tips grown in Pb-containing culture solutions is a combination of lead with sulfhydryl. In such tips mitosis but not growth by increase in cell size is inhibited. Also it was found that sulfhydryl is concentrated in the meristematic region of normal roots. Therefore the hypothesis was developed that growth by increase in cell number is specifically factored by -SH. The next stage was theTesting of Extracts Here it was found that acid extracts of the meristem of root tips accelerated root length growth when controlled by acid extracts of the next distal portion, while alkaline extracts similarly controlled showed no such activity. This proved that the root region of highest sulfhydryl concentration and mitotic activity contains a naturally occurring acid-stable, alkali-labile substance stimulative of root growth in length. These findings are thus physiologically and chemically consistent with the hypothesis. The next stage was theTesting of Synthetic Compounds Here the action of a variety of sulfhydryl compounds on mitosis in root tips and reproduction rate in Paramecium was studied, using the same compounds minus the sulfur moiety as controls. It was found that the -SH group stimulates cell division in both plants and animals. Cell size growth is not stimulated. Thus, through identification and testing of the identified group in natural and synthetic compounds, the conclusion is arrived at thatSulfhydryl is the essential stimulus to growth by increase in cell number.Credit is due to MissElizabeth Justice and MissJane Anderson whose conscientious and painstaking efforts made this study possible.