Our Gemara on Amud Beis notes ways in which the Esrog tree is comparable to a fruit tree but also comparable to vegetables, and the implications for laws of tithing. Sod Yesharim (Succos 48) offers a mystical interpretation of this phenomenon. 

The Esrog Tree represents a hybrid of qualities. A fruit tree has an advantage in that it is semi permanent and does not wither from season to season, unlike vegetables. Yet vegetables have a quality that they renew each year because they are dependent on a constant flow of water. Apparently, in this way, the Esrog tree is like a vegetable and therefore the tithing is counted in the way that vegetables are, from when they are cut. 

These two qualities metaphorically represent two qualities: renewal and constancy. The perfect fruit, that is the perfect person, has both the stability of permanence as well as the fluidity of renewal. 

What are real world practical applications of this idea? As I have mentioned in other psychology of the dafs, Jewish philosophy seems to honor a tension between two states. There are numerous examples of this: Rachamim (mercy) and Din (strict law), Ahava (worship out of love) and Yir’ah (worship out of fear), and perhaps even the Talmudic dialectical method of deriving meaning through resolving apparent contradictions is also an encounter of opposites. In any case, as chapter three in Koheles points out in regard to temperament and behaviors, there is a time and place for everything. Thus, we strive to be grounded and rooted in tradition while also open to discovering and learning new things. This is true in our relationship with God as it is with our spouses.