Some living thing's bodies are weaker than others. For the species to live, the strong ones of the species will not get sick, reproduce and create strong offspring. The weak will die out.
That the oxygen levels in the test tube increased and carbon dioxide levels decreased because plants produce oxygen using carbon dioxide
Because plants go through a cycle(the cycle is seed, seedling, plant, flower and tree.) and it takes time for the plant food, water, and nutrients to go though the plant.