So...are plants not natural or are they not part of the environment? (Or both?)
Plants are found within the natural environment therefore, by all means plants are natural in that sense. Though some of the materials (dna for example) of which they are comprised of, are by all means unique to the environment.
It seems that you are saying that plants are natural and part of the environment, but plant DNA is not natural or part of the environment. This makes little sense to me.
Would this also mean that plants are not 'designed', but their DNA is?
No, it would mean that plant is there as a result of an intelligent designer. DNA isn't found in the environment in the sense that you cannot look within a rock formation, river or air and say hay look I found dna here. DNA is found within a plant, yes, the plant is found within the environment, yes, however dna cannot be found outside the organism, or rather it isn't found within the natural environment.
Simply saying dna is natural because plants are natural does not cut it as that would be a, truism, it goes much deeper than that.
No, I would say that DNA is natural because it is made in nature - by nature. If that is a truism, then why are you disagreeing? Maybe it would help if you could tell me how you define 'natural'. This might help clear up the confusion.
Take a clay cup for example....Clay can be found within in a lot of places in the environment, not only within the cup, but outside it as well. (a mountain for example)
Take a clay mountain for example....Clay can be found within a lot of places in the environment, not only within the mountain, but outside it too. (a cup for example). I am really not seeing how you are differentiating between a cup and a mountain, apart from your premise that "cups are designed". But that would make your argument circular.
Intelligent agents usually create new (or unique) materials in an environment (metal, silk etc.) and they may use these new materials to create objects, a car & spider web (silk) for example.
Because the plant also fits within the scope of a designer (intricate & interconnected parts, unique materials etc) it is highly likely that it was created by an intelligent agent.
Or rather it is highly likely that an intelligent system was ordained to produce such creations, as plants depend on other life forms to survive.
Since these rules may only sometimes apply, how do you tell when they are applicable? What do you measure and how do you measure it? (The answer to 'How do you measure it?' is vital.)