Question

how can be prove that plants take oxygen

Answer 1

Depending on the amount of sunlight, plants can give out or take in oxygen and carbon dioxide as follows.
Dark – Only respiration takes place. ...
Bright sunlight – Photosynthesis uses carbon dioxide and makes oxygen faster than respiration produces carbon dioxide and consumes oxygen.

Answer 2

plants in day time prepare it's food and stores it . The stored food is needed for cell division,growth and reproduction therefore this food is needed to be converted into energy and this can be achieved if we burn this food but inside plant not possible thus there oxygen is required.