ROOT CAP: the thick layer of cells at the forefront of the growing tip. Cells secrete mucigel to
help them glide through the soil but the soil is just plain scratchy and these cells are
constantly being destroyed.
ZONE OF CELL DIVISION: Includes the apical meristem, which is a group of cells
specialized for the production of new cells, located at the farthest point of the tissue or
the organ produced (found in both shoots and roots). Very different from animal growth!
Animals have diffuse growth; plants have localized growth. It would be impossible for
the whole root to expand at once; the entire root would have to slide through the soil.
With apical growth, the tip is pushed through the soil. This zone also contains a region
called the “quiescent center”, where cells are not mitotically active. These cells are
resistant to harmful factors such as radiation and toxic chemicals, and they serve as a
reserve of healthy cells. They can form a new apical meristem if one is damaged by
sharp objects, burrowing animals, nematodes, or pathogenic fungi.
ZONE OF CELL ELONGATION: Cells enlarge here, pushing the apical zone forward.
ZONE OF MATURATION (= ZONE OF CELL DIFFERENTIATION): The vascular
tissues of the root become differentiated. Instead of forming bundles containing xylem
and phloem, as in leaves and young shoots, the xylem forms a solid mass in the center,
surrounded by strands of phloem. (See Figure 2.)
Root hairs are unicellular and extremely transitory. They only live for 4-5 days (but new
ones are always forming). Root hairs greatly increase the root’s surface area
(approximately double). They can also penetrate tiny pores between soil particles.
FIGURE 2. CROSS SECTION OF ROOT IN MATURATION ZONE.
A mature root in a woody plant will also have secondary xylem and secondary phloem,
produced by a vascular cambium. The vascular cambium arises between the primary
xylem and the primary phloem. Lateral roots will also grow.
Cortex cells transfer minerals from the epidermis to vascular tissue.
cortical cells differentiates into a cylinder called the endodermis.
contains a “Casparian strip” that controls the passage of minerals.
FIGURE 3. ENDODERMIS SHOWING CASPARIAN STRIP
The innermost layer of The endodermis The cell walls to the inside and outside of the endodermis are normal, thin primary walls, but the walls of cells touching other endodermis cells are waterproofed with lignin.
If minerals are traveling through the intercellular spaces or by diffusion through cell
walls, they are stopped by the Casparian strip. Minerals can flow freely as long as they
are going through cell cytoplasm (this means that they at one point, had to gain entry to a
cell by crossing the cell membrane).
FIGURE 4. DEVELOPMENT OF A LATERAL ROOT
Some cells just became mitotically active at the inside of the cortex. They make a new
apical meristem that comes crashing through the root, destroying the cortex and epidermal
cells. On the back of this page, you’ll see a diagram of two types of mycorrhizal.
