Roots are the invisible part of the plant. Buried in soil many people never see them, yet their presence and functions are critical to the plant and to our own survival. Roots produce their own lateral branch roots to form an expanding network of cylindrical structures that penetrate soil, anchor the plant, mine the soil of minerals, and drain the soil of water for use by the plant.
The apex is where the root grows. Near the tip is where new cells are made and recently-made cells elongate. This growth in length by cell number and cell length is how the root makes its way through soil and may find untapped minerals and water. The expanded network of roots provides better anchorage too. So a discussion of roots generally needs to start with the apical growth of roots. Here is a micrograph of the longitudinal section of a root apex.
The root tissue at the apex is far too soft and delicate to penetrate soil without having evolved some amazing methods to ease the pathway. On the very tip of the root apex is the root cap, and this tissue provides the solutions to the penetration problem.
Friction is reduced in two ways. The root cap cells secrete mucilage. This is a polysaccharide polymer that absorbs moisture from the soil and expands into a slimy lubricant. If you remember mucilage as a gooey liquid in a bottle in grade school, you are on the right track. That material was flavored and perhaps some of your childhood friends liked to eat mucilage. Why was it flavored? This material is designed to be spread on the back of a piece of artwork or on a paper flap...then dried. Later when the artwork needs to be attached to something a person licks the area where the mucilage dries and sticks the artwork to the surface. The flap of the home-made envelope is also licked to seal it just before mailing. Yes, mucilage is the genius behind envelopes and stamps!
The second way that the root cap reduces friction is by sloughing cells from its surface. These slough cells leave the rootcap and are pressed along the soil channel as the root penetrates. These dead cells line the pathway and smooth out the rough edges of soil particles. To replace these slough cells, the columella (a central core of cells) in the root cap keep dividing by mitosis. Thus a replenishing supply of cells is always available under good conditions.
Just under the rootcap is sometimes a pad of cells, the quiescent center, but this is absent in some species. However just proximal to the root cap is a zone of cells that are actively dividing. We call this the zone of mitosis. Here we can observe the cell division process: mitosis. The actively dividing cells are called meristematic cells. Here is a link to a table showing you some of the phases of mitosis.
Again, moving just a millimeter up (proximally) in the root tip, we find the zone of cell elongation. Here the cells are rapidly elongating. The vacuoles grow tremendously inside the cells, so water-uptake is a large part of this function. Also, we begin to see more highly developed organelles in the cells. Mature mitochondria and plastids are found in the cells. The cells are converting from meristematic to parenchymatous. Cells right next to them may be going through apoptosis and achieving a mature dead state. Here is a view showing some living parenchyma cells right next to some dead sclerenchyma cells in a longitudinal section of mature root.
Sclerenchyma cells develop by first elongating or expanding to their final size. Then additional layers of cell wall, secondary wall, are laid down inside of the primary wall. This thickened wall material is also impregnated with lignin, a brittle polymer of polyphenolic substances made by the cell. The lignin hardens and waterproofs the cell. Cut off from water and mineral uptake, the cell dies and assumes support or transport functions that only dead cells can assume. Hollow cells with hard walls are great for support, for conduction, or for reducing herbivory.
As we continue to move away from the root tip, i. e. move proximally, the zone of elongation includes more and more cells achieving mature states. Obviously the elongation and maturation zones overlap considerably. The cells elongate and differentiate. We can distinguish developing different cell types. The cells become mature with time and these proximal cells are much older than those nearer to the apex. Epidermal cells a few millimeters or perhaps even centimeters away from the root cap are growing out among the soil particles as root hairs. These root hairs are quite numerous and greatly increase the surface area of a root:
The improved surface area provided by these root hairs permits better water and mineral uptake form the soil. As we shall see later in the course, these root hairs are critical in that they release hydrogen ions (H+) onto clay particles in the soil, exchanging them for mineral cations. This process is called cation exchange and is the "mining" operation for the root.
As we continue moving away from the root tip in the proximal direction, we finally arrive a mature root areas. Here all the cells and tissues are completely mature and we can distinguish many layers:
To the outside of this cross-section, we find the epidermis. And some of these epidermal cells will grow out into the soil as root hairs. As this area will be absorbing water from the soil in mature, but still young, parts of the root , it is not surprising that these hairs and epidermal cells will not possess cutin in the outer wall. Of course for most roots, surrounded by moist soil, desiccation is not much of a problem! As the epidermis is not cutinized, the epidermis does not have any need for stomata...guard cells are generally absent in root epidermis.
The first layer inside the epidermis is the cortex. In stems this layer would have collenchyma cells to provide support; roots are completely surrounded by soil and so have no need for collenchyma. In stems the cortex might do photosynthesis; roots are in the darkness of soil and survive exclusively by respiration. Chloroplasts never develop. The cortex of a root might serve as a storage area at least in younger sections of root. Large intercellular spaces suggest that gas exchange is critical here for respiration.
The innermost layer of the cortex is a layer one-cell thick, called the endodermis (endo-inner; dermis-skin). This is perhaps the most critical part of a root in terms of soil minerals. This layer of cells is tightly glued to each other forming a continuous, cylindrical sheet of cells:
There are no intercellular spaces in this sheet. Moreover, the transverse and radial walls of the cells in the sheet are impregnated with a waterproofing chemical, suberin. This material picks up the red dyes and so those walls appear coated with a red strip. This was named the Casparian strip in honor of its discoverer. Here is a diagram showing the Casparian strip:
All of this waterproofing means that to pass from the cortex to the interior of the endodermis, all chemistry must penetrate the outer and inner tangential walls of the cells. Water and minerals must literally pass through the endodermal cells! Of course this means they must penetrate the cell membrane of that cell. The membranes of endodermal cells have transport proteins embedded in them for uptake of useful minerals...but of course do not have transport proteins for "other" materials found in soil. Thus the endodermis is responsible for selective mineral uptake!
In the very center of the stem is the vascular cylinder. A closer diagram of this cylinder is shown next:
Now, assuming the right combination of minerals is transported across the endodermis, the minerals now enter the outermost layer of the vascular cylinder. This layer is called the pericycle. This layer has intercellular spaces, so the minerals are again free to "go around" the cells. The pericycle itself is responsible for forming lateral (branch) roots and connecting them to the rest of the vascular tissues...in all plants. In woody plants, the pericycle also forms the periderm (bark) of the root. Here is a photomicrograph showing some branch roots developing from the periderm and growing out through the endodermis and cortex:
Next toward the interior is the phloem. The phloem is concentrated into two or more patches in the cross section. The cross section far above on this page is showing you four patches. Of course in three dimensions, these are long strips of phloem running the length of the root. They are bringing the sugar and amino acids down to the root tip from the stem and leaves. Here is a closeup of some phloem cells connecting end to end...you are seeing the sieve plate through which the sugars and amino acids travel to go from cell to cell to arrive at the root tip.
Between the phloem and the xylem in the center of the vascular tissues lies the cambium. This is a meristematic cell layer that divides to produce secondary phloem to the outside and secondary xylem to the inside. This will add layers to the bark and to the wood as a woody tree or shrub root grows. Herbaceous (non-woody) plants, such as grasses, lack a cambium layer.
Finally in the center of the vascular cylinder is a solid, if fluted, cylinder of xylem. This primary xylem has ridges between the valleys (where the phloem strips run). The example diagram above, shows the xylem in four ridges. In other species there can be two ridges of xylem (diarch), three (triarch), four (tetrarch), or more (polyarch). Generally dicot plants have fewer ridges of xylem than do monocot plants. On the tops of the ridges are what we call protoxylem. This kind of xylem laid down when the root is still elongating has annular (ring), helical (spiral), or reticulate (network) wall thickenings added to the primary wall. The cells in the core of the xylem are laid down after growth is complete and the walls are fairly evenly thick except for pits ("holes") through which water and minerals may enter to go up to the stem and leaves. Here is a longitudinal section through the xylem at the ridge of a root. You can see the various kinds of wall thickenings in protoxylem nicely here:
This diagram shows you a section of xylem in a woody plant...this is fully mature secondary xylem, so the walls are thick with just a few pits to interconnect them. Close inspection will show xylem parenchyma included between the conducting cells, as well as a range of other cell types:
The root system as a whole can take several forms depending upon the species of plant and the environment to which it is adapted. For a tall tree, a deep taproot is a great way to resist the lateral forces of wind. In this photo of the tea plant (Camellia sinensis) the tap root is excavated in a trench and the full-size man is standing next to it to give you an idea of diameter:
A tap root is great against lateral winds...like a deeply-set fence post. However as anyone who has pulled carrots or beets knows, a tap root gives very little resistance against uprooting! Plants that have evolved among grazing animals (grasses among bison on the prairie) have to resist more uprooting forces than wind forces. A fibrous root system is far better for this purpose. To uproot a grass plant, you literally have to lift a huge volume of fibrous-root-penetrated soil...no mean feat! Plants between grasses and tall trees in size usually have some balance between tap and fibrous systems.
Of course a root system needs to grow where the minerals are. In the tropics, the soils are impoverished. Most of the minerals are already tied up in the trees. The only new minerals are found at the surface when animals defecate, urinate, or the trees lose leaves, flowers, fruits, or limbs. To be effective in retrieving what is available, the trees of the tropics must keep their roots very shallow...a problem for big trees...and there are giants in the tropical forests! In this case the "blow-over" protection is provided by surface roots that expand vertically and expand right up the trunk. These are called buttress roots. Indeed these kinds of roots helped architects in the gothic world figure out how to keep the walls of huge cathedrals from bowing out and dropping the roof on the parishioners! The example shown here is a fig tree and the person is my wife...not a tiny person...to demonstrate how the roots run along the surface and grow up in ridges as a buttress.
Indeed trees with buttress roots can withstand hurricanes and typhoons with these supports. But other tropical trees evolved a different solution to the same problem. In Pandanus the tree develops adventitious or prop roots. These form in stem tissue above the ground, grow out a distance and then into the soil, branch and anchor out some distance from the trunk. They literally prop up the trunk of the tree. Again architects in the Renaissance period improved the look of cathedral walls by designing "flying buttresses"...arches. These roots were inspirational!
Since we are still tramping around in the tropics, here is one more example of root adaptation. This tree lives on the edges of shores in marshy areas with salty mud as the "soil." This is a harsh place and Black Mangrove trees are adapted to thrive here. The mud is anaerobic (no oxygen), so to keep respiration going in the cortex, phloem, and cambium, etc. the plant has evolved "snorkels." These are called pneumatophores (literally breathing stalks) that give a pathway for oxygen to get to the cells of the roots growing in the mud of the marsh.
Meanwhile closer to home, roots of maple trees are adapted for winter. Materials accumulated are stored in trunk and roots each summer. The leaves do not survive the winter so they are mined of all important "goodies." This happens in the autumn. Then in the spring the minerals and carbohydrates are returned to the buds to flush out the new growth. We tap that xylem sap to make maple syrup. This photo reminds us of the "mining" process of autumn.
Go to an Overview about roots.
This page © Ross E. Koning 1994.
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