Originally Posted by BioSci

Responding to a stimulus is not the same as consciousness.
Consciousness means actually thinking. Plant responses to the environment are of a wholly different nature than conscious actions or thoughts. Plants are not "aware" - but they can be "programmed" by metabolic and genetic mechanisms to respond to day length, temperature, and seasons.
Plants respond to the sun primarily by differential growth (commonly demonstrated in beginning biology classes) - the "movement" to gravity or light is a result of greater (or less) cellular expansion on one side, thereby making the plant change its orientation. Root growth is simply faster where moisture and nutrients are located.
"maximize its existence" is a bit too personified. Plants come with genetic instructions and their environment dictates how well they are executed - no thinking, planning, or consciousness involved.