Shortly after germinating and producing their first few leaves, many seedlings start to produce foliage that is indistinguishable from the foliage that they will produce for the rest of their lives. Other plants might initially be outfitted with leaves that are smaller, thicker, or somehow slightly different from what will appear later. Then there are those that produce juvenile growth that is completely distinct from later adult growth.
There are many reasons for juvenile growth. It might be a competitive advantage for plants that live in dense forests. For others, it might deter grazing animals. Ivy actually has three phases, with juvenile ‘ground cover’ growth that creeps over the forest floor in search of a vertical support, adolescent ‘vine’ growth that climbs the support, and adult ‘tree’ growth that blooms and seeds when it gets to the top of the support.
Eucalyptus trees produce juvenile growth that is more pungently resinous than adult growth, in order to deter animals that would otherwise eat it. Adult growth develops when the main trunks have grown beyond reach of most of the threatening animals. Vigorous new growth that develops in response to breakage or pruning later in life is also outfitted with juvenile foliage so that koalas and other climbers leave it alone.
Avocado trees grown from seed produce no obvious juvenile growth, but without wasting their effort on blooming and fruiting, they grow very fast and lanky to compete with a dense forest canopy, whether real or imagined. For their first several years, they need to be pruned for structure and containment. They eventually produce adult branches, and start to bloom and produce fruit (although the fruit might be variable.)
Avocado trees obtained from nurseries are grafted so that they are genetically identical to a specific cultivar (for conformity of fruit), and so that they can start to produce right away. Their ‘scions’ (upper part of grafted trees) are obtained from adult growth of stock trees, so do not take several years to mature. The same technique works for citrus, which otherwise produce fruitless and wickedly thorny juvenile growth.
Sometimes, juvenile growth is preferred. Ivy gets pruned out of trees and off of walls to preserve the juvenile growth as an appealing and efficient ground cover plant, while also eliminating the potentially destructive climbing vines.