Young, cross-cut roots and stems show that they are formed by a gathering of tissues.
The arrangement of these tissues is specific to each organ and constitutes a typical primary internal structure of each organ.
A more complex secondary structure can be seen when an increase in stem and root diameter occurs.
Primary Root Structure
If we follow a meristematic cell that has just emerged by mitosis at the end of a root, we would see that it is lengthening as it distances itself from the end as new cells emerge. The highest growth rate in root extension will therefore occur in the region just above the meristematic region, called the distention zone.
After growing the cells begin their differentiation. In the innermost region, for example, the differentiation of conductive tissues will begin, while in the outermost region parenchyma and lining tissues will be differentiated.
The most peripheral region of the young root differ in epidermis, tissue formed by a single layer of flattened and juxtaposed cells. In the region below the epidermis, called cortexdifferentiates the cortical parenchyma, consisting of several layers of relatively unskilled cells.
The inner part of the root is the central cylinder, composed mainly of conductive elements (protoxylem and protofloema), fibers and parenchyma. The central cylinder is delimited by the endoderm, a well-adjusted cell layer with special reinforcements on the walls, the Caspary stretch marks. These stretch marks are like cellulose straps that firmly join neighboring cells, completely sealing the spaces between them. Thus, to penetrate the central cylinder, any substance must pass directly through the endodermal cells, since the caspary striae close the intercellular interstices.
Just below the endoderm is a thin-walled layer of cells called the pericycle, which delimits the central cylinder, where the xylem and phloem are located. The way conductive tissues are arranged in the central cylinder is one of the criteria for distinguishing dicotyledonous from monocotyledonous.