Ground meristem gives rise to cells that will specialize to become cortex or pith and fill in between the epidermis and the vascular bundles in the stem. Just below the apical meristem there is a zone of cell elongation that adds to the length of the plant by the elongation of cells. Vacuoles inside the cells in this region fill with water and stretch the cells out in length. In the region below the zone of cell elongation in stems there is the zone of differentiation . There is a special type of dividing cells develop inside young leaves and extend into the internodes and will give differentiate into vascular tissues that occur in the veins or vascular bundles.
Vascular tissues arise from a thread of dividing cells that by differentiation.
This thread of cells is known as the procambium or procambial thread. These cells in zone of differentiation develop into mature primary xylem and phloem cells. Two basic types of Vascular tissues Xylem cells, that conduct water from the roots to the stem , may be recognized from other cells types based on their position, their thickened cell walls that they usually stain red and which at functional maturity are dead cells. In a mature xylem all that remains is their cell wall. The primary cell walls are reinforced with a secondary cell wall material known as lignin.