Leopold an - D Hall - a Miodel of Transp ' Ort

The polarity of plant cells is a basic featuire required for orderly growth and(l differentiation. The elonigation of cells in the growth of a stem expresses a polarity; apical dominaince relies on the polarity of a growth-reguilating system. The transport of the atlxin indoleacetic acid is ulniquie in being the only knowin polar vector in ncatuiral growthregtulating systems, and a better knlowledge of it should eventutally leadl tus to a more initimate uinderstanoling of the polarity of cells and wNhole organism s. The fincling that the transport of autxinl is apparetntly driven by a secretive actioIn of the cells (f stetns or coleoptiles (5) permits oIne to conceive of polar transport as being a preferential secretion of auixin out of the lower encd of the cell as compareol with the uipper en(l. \Vith this concept, one miay theni ask how extensive the dlifference between the amotunits coming outt of each end woutld need to be for ani experimentally determinledI net polarity of tranisport in a stem. The article demonstrates how this (difference may lie quantitatively approximated by compuiter manipuilation of a mathematical model, even thouigh direct measuiremenlt at the cellular level is nlot possil)le.