[R] Help about iterations

[Ronaldo Reis Jr.]

Hi,
Sorry by this newbie stat doubt, but with some brains in the list anybody can 
help-me.

I have a sistem composed by 3 quantitative variables.
The variables are:

The growth (Y in cm)
The nutrient (X1 in gram)
The water (X2 in ml)

Ok,
My doubt is:
Is correct or necessary make a iterations of the nutrient and water, or the 
iteraction is intrinsical of the model. 

glm(growth~nutrient+water)

or

glm(growth~nutrient+water+nutrient:water)

Working with one quantitative explanatory variable e one qualitative 
explanatory variable (with some levels), I understandy the iteration of 
quantitative x qualitative, for testing the curve inclination. But when I 
have some quantitative explanatory variables, the iteration generate a new 
vector by x1*x2 and the number generated by this vector is no sense 
(30g*2ml=60 and 2g*30ml=60, but is very different). 

I need to make iteraction or not?

A less important question:

How to make a plot with this and a curve of this?
the curve function use only one x, I have two or more x in same function.

Thanks and sorry again
Ronaldo
-- 
His mind is like a steel trap: full of mice.
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