Answer:
The plant used everything around it in order to continue growing.
Step-by-step explanation:
Although nothing was added, the plant might have had a rich soil, so with that rich soil it was able to grow on its own. The plant might have been placed near a window, so it had enough sunlight in the day. And, the classroom might have had a air-conditioned room flowing, with enough space for the plant to grow in.
Hope this helps :D