1. The chemotaxis of bracken spermatozoids involves their precise orientation in a gradient of bimalate or a few other similar ions. When a voltage gradient is established in a sperm suspension containing bimalate or other chemotactically active ions, a similar orientation is observed, causing the spermatozoids to swim towards the anode.

2. Photographic records of sperm responses reveal a linear relationship between turning rate and the component of the gradient perpendicular to the direction of swimming.

3. Estimates have been obtained of the ratio of the magnitudes of the two types of gradient required to produce an equal tactic response.

4. The results suggest that the sensory elements of the spermatozoids adsorb bimalate ions. The reversible adsorption of bimalate ions on ‘bimalate-combining sites’ on the anterior end of a spermatozoid might fully explain its tactic behaviour, without requiring any modification of flagellar activity by bimalate.

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