Auxins and shoottropisms-a tenuous connection?

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Abstract
The classical Cholodny—Went (C—W) hypothesis is discussed. This explains geo- and phototropic curvature in shoots in terms of a perception phase at the apex with a response at some remote site. The changes in growth rate which cause the curvature are said to be due to gradients of auxin between the two sides of the shoot. It is pointed out that it is often not possible to separate the site of perception from the site of response, thus removing the need for a chemical message. Experiments with half segments of stems show that the lateral transmission of a message is not involved. It is suggested that the measured auxin concentration gradients are not of sufficient magnitude o account for the observed changes in growth rate which occur during tropic curvature. If shoots are supplied with auxin exogenously so as to set up lateral gradients, then the bending which results is not always in the direction which the C-W hypothesis would predict.