In this sentence, what's being carried out, or on, is a plant's life cycle.
By its nature, this has a beginning, a middle, and an end;
and restarts to do the same over again.
So there is one series that the plant can complete, and this allows carry out:
- With enough light, plants carry out a normal life cycle.
(i.e, with enough light, plants can sprout, grow, and reproduce)
But plants do reproduce, and they occur in groups with different cycles,
so there is also a continuity that allows the use of carry on.
- With enough light, plants carry on a normal life cycle.
(i.e, with enough light, plant life continues)
Since we're talking about plants here, this is not exactly a coincidence.
The distinction between carry on and carry out is designed for active
verb phrases with agent subjects, not metaphors of evolution.