I.  Hanstein (1868, 1870) proposed that a precise relationship existed between initials in distal zone of apex and mature tissues in proximal portion of organ.

    A.  Dermatogen (Grk. skin, to bring forth) -> Epidermis
    B.  Periblem (Grk. clothing) -> Cortex
    C.  Plerome (Grk. that which fills) -> Inner tissues
    D.  Now generally accepted that there is no obligate relationship between such histogens and mature tissue

II.  Haberlandt (1914) proposed generalized terminology for histogenic regions of meristems that in general give rise to categories  of mature tissues.

    A.  Note that direct cell lineage is not implied with this terminology that is most widely used today.
    B.  Promeristem -> Initial group of cells that give rise to the following
    C.  Protoderm -> Epidermis
    D.  Procambium -> vascular tissue + vascular cambium
    E.  Ground meristem -> Cortex and Pith regions

III.  Meristem cell population is a dynamic one that could be modelled or described with the following equation:

    N = [1- (c-d)] * No      where

        N = number of meristematic cells in current population
        No = number of initial meristematic cells
        c = rate of cell division
        d = rate of cell loss from population due to differentiation

    Note when c = d  -> no change in number of cells in meristem
                       c > d  -> meristem population increases
                        c < d -> meristem decreases