Cambridge Secondary (7-9). Stage 9. Science. Learner's Book - Unit 1: Photosynthesis and the carbon cycle - Question 59

Tubes A and B contain water plant; C and D contain small animals; E and F contain both. A, C and E are kept dark; B, D and F receive light. Each tube has hydrogencarbonate indicator and the same small platform.

Suggest why you were asked to put a little platform into each tube, even if there were no animals to put onto it.