Well heredity and evolution with it favour genes that give an organism - here a plant - a greater chance of survival. Survival subsequently gives the organism a chance to spread those genes onto its offspring which will (should/most likely will) inherit those genes for IT to survive and spread ITS (the original plant) genes further and further, thus ensuring the species as a whole will endure and flourish.
It's not nessecarily because a father is a bodybuilder the son is 'born' a bodybuilder as well. The son is just more likely to have the genetic blueprint - the promise - to become very muscular; have genes that promote fast and rapid muscle growth.
Continued breeding with the same strain might indeed improve a certain trait generation after generation if you keep breeding with the individuals that show the most strength in that trait, but it's not foolproof. Faults happen such as mutation during gamete forming. So a healthy strong plant can produce a defect pollen grain that gives rise to a less strong offspring.
As I said before, in the wild that defect offspring might not survive and so nature eradicates the weak ones itself. In culture it's up to the breeder to recognize traits that he desires 'favorable', but which are not nessecarily so in nature. Example is long stems: nice to look at and for cut flowers, but not handy in a wind blown meadow
Bottom line of my overly long story: the strongest plants for a given trait are the most likely to pass on that strong trait - and improve -over generations, but it's not foolproof. It's up to you to pick them out.
