Re: why have nervous systems evolved in animals but not plants?

Evolution works through the selection of traits that give an organism a 
reproductive advantage over its competitors. In any population there will 
be differences between individuals which have an impact on their ability 
to compete and procreate. Through the process of natural selection, those 
traits which confer an advantage to individuals in their ability to 
reproduce will be selected for. In the case of nervous systems as it 
applies to plants the question from an evolutionary point of view would be 
how would it give a plant an advantage? Nervous systems serve to provide 
information about the world through the senses and control movement 
(among many other things). This is important for an animal because to 
survive they must eat other living things, thus they need to sense those 
other organisms (through eyes, ears, noses etc.) and then be able to 
coordinate movement to catch the other organism (or if herbivore simply to 
find edible plant material). Plants do neither. They do not need to eat 
other living things (venus flytrap notwhitstanding) and do not move. So a 
nervous system instead of being an advantage would be a disadvantage 
because precious energy would be spent on something useless.

hope this helps,
gabriel vargas md/phd 
References
Biology 
by Neil Campbell