These are all OK and mean much the same thing: " in order for it to grow" " in order to grow" " to grow" " for it to grow" " so (that) it grows" and " so (that) it can grow" are grammatically possible but seem a bit odd in this sentence. They seem to suggest conscious planning (rather than nature simply taking its course). " so (that) it could grow" is not right ("could" is the wrong tense/mood).
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for learningCould I say , for example: " I signed up for this course inIt's theoretically possible but it feels more awkward and redundant than your original sentence about the elephants. Usually one would find another way to say this, most simply "I signed up for this course to improve my drivingortherorder for me to improve my driving skills"?