Question

many plant responses are controlled by \boxed{}, which are molecules that can effect plant growth.

Explanation:

In biology (a subfield of Natural Science), plant responses and growth are often controlled by hormones (or plant hormones). These are signaling molecules that regulate various aspects of plant growth and development, such as cell elongation, flowering, and responses to environmental stimuli. The description in the question matches the role of plant hormones.

Answer:

hormones (or plant hormones)