The elongation of roots of wheat (Triticum aestivum L. cv. Diamant II), flax (Linum usitatissimum L. cv. Concurrent) and cucumber (Cucumis sativus L. cv. Favör) seedlings in the dark was strongly inhibited by various native and synthetic cytokinins (kinetin, benzyladenine, isopentenyladenine, zeatin and their corresponding 9‐ribosides). An inhibition of 50% was obtained for wheat roots with 3 · 10−9M zeatin and for flax roots with 6 · 10−9M isopentenyladenine. The ribosides were in all cases less inhibitory. The inhibition was reversed by various types of ‘antiauxins’ and ‘antiethylenes’ (such as structural auxin analogues, uncouplers, specific inhibitors of ethylene synthesis, free radical scavengers, inhibitors of ethylene action). These substances as a rule counteract also inhibitions caused by auxins. Auxins and cytokinins stimulate ethylene production synergistically, and the similar inhibitory effects of these two types of hormone can be understood if it is assumed that their effect is at least partly mediated through ethylene. The cytokinins must be considered as possible natural inhibitors and regulators of root growth.

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The effect of some flavonoids on the polar transport of auxins was investigated in hypocotyl sections of dark grown seedlings of cucumber, Cucumis sativus L., by means of '*C labelled auxins. In experiments of 4-6 h duration quercitrin, morin, dihydroquercetin, nadngin, sulfuretin and ferulic acid increased the polarity of the transport of indol-3yl-acetic acid (stimulation of basipetal, inhibition of acropetal transport). Naringenin, genistein and pinobanksin, on the other hand, decreased the polarity of this transport. For NAA no increase in the polarity of the transport could be observed, but all the substances tested inhibited the basipetal transport. There was no simple correlation between the effects on the polar transport and the effects on IAA oxidase.

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