ABSTRACT Sectional model of the tree's system branches, which was published earlier, was extended in the range (0,1) for model's allometric parameter μ, binding magnitude of the green biomass of a tree and its size (for spruce μ ≈ 1.83). The model has shown presence of a biomass of branches in this range that indicates, according to fractal geometry, on the placement of a green biomass in the form of set of "points". In turn according to contemporary notions about endosymbiotic character of appearance of organelles in eukaryotic cell this was interpreted as begin of endosymbiosis of cyanobacteria and a host (protist) and has been modeled by the groups placement of growing number of points on an interval under initial limitation the number of points per group. The limitation is connected to the initial absence in protist an infrastructure which is need for the growing number of cyanobacteria. It has been demonstrated that initially in course of increasing total number of points the parameter μ decreases from 1.0 to 0.25 under the fixed number of points per group and then increases to 1.0 under the fixed number of groups. This gives an initial deceleration of growth of proto-plant's size (a characteristic geometrical size of the point set) and then the exponential growth after deliverance from limitation of point number per group in course of host evolution. Similar initial deceleration of growth can be seen now also in higher plants. Morphological analogy of situations of beginning the plant embryogenesis and of seed germination with the situation described by the model of proto-plant appearance is discussed.