<P> Juvenility or heteroblasty is when the organs and tissues produced by a young plant, such as a seedling, are often different from those that are produced by the same plant when it is older . For example, young trees will produce longer, leaner branches that grow upwards more than the branches they will produce as a fully grown tree . In addition, leaves produced during early growth tend to be larger, thinner, and more irregular than leaves on the adult plant . Specimens of juvenile plants may look so completely different from adult plants of the same species that egg - laying insects do not recognize the plant as food for their young . The transition from early to late growth forms is referred to as' vegetative phase change', but there is some disagreement about terminology . </P> <P> The genetics behind leaf shape development in Arabidopsis thaliana has been broken down into three stages: The initiation of the leaf primordium, the establishment of dorsiventrality, and the development of a marginal meristem . Leaf primordium is initiated by the suppression of the genes and proteins of the class I KNOX family (such as SHOOT APICAL MERISTEMLESS). These class I KNOX proteins directly suppress gibberellin biosynthesis in the leaf primodium . Many genetic factors were found to be involved in the suppression of these genes in leaf primordia (such as ASYMMETRIC LEAVES1, BLADE - ON - PETIOLE1, SAWTOOTH1, etc .). Thus, with this suppression, the levels of gibberellin increase and leaf primorium initiates growth . </P> <P> Flower development is the process by which angiosperms produce a pattern of gene expression in meristems that leads to the appearance of an organ oriented towards sexual reproduction, the flower . There are three physiological developments that must occur in order for this to take place: firstly, the plant must pass from sexual immaturity into a sexually mature state (i.e. a transition towards flowering); secondly, the transformation of the apical meristem's function from a vegetative meristem into a floral meristem or inflorescence; and finally the growth of the flower's individual organs . The latter phase has been modelled using the ABC model, which describes the biological basis of the process from the perspective of molecular and developmental genetics . </P> <P> An external stimulus is required in order to trigger the differentiation of the meristem into a flower meristem . This stimulus will activate mitotic cell division in the meristem, particularly on its sides where new primordia are formed . This same stimulus will also cause the meristem to follow a developmental pattern that will lead to the growth of floral meristems as opposed to vegetative meristems . The main difference between these two types of meristem, apart from the obvious disparity between the objective organ, is the verticillate (or whorled) phyllotaxis, that is, the absence of stem elongation among the successive whorls or verticils of the primordium . These verticils follow an acropetal development, giving rise to sepals, petals, stamens and carpels . Another difference from vegetative axillary meristems is that the floral meristem is "determined", which means that, once differentiated, its cells will no longer divide . </P>

What is the process called when a tree grows new leaves