the movements of a plant underwater

This is from Tim Parks' lovely, teasing new novel, 'Dreams of Rivers And Seas':

Elaine was kind to him, but busy. In the past, she had been the vulnerable one. Now she had her rehearsals, she had a place in the world. Practising her mime in the sitting room, she swayed round the sofa with staring eyes, arms waving languidly. 'After the explosion,' she said. 'I'm supposed to be looking for my baby. But how can I really know what it would be like after an explosion?'

John watched her, her arms and wrists in particular. They were the movements of a plant underwater, he thought.