Roots

Root Functions

Water Uptake: Roots take water from the capillary spaces between soil particles. This function is carried out by the young portions of the roots at the location of minimal cutinization of the epidermis and at maximum surface area. This location is found in the root-hair zone just proximal from the growing root tip. Thus roots take in their water through very fine roots located at the drip-line of the plant's canopy.

Mineral Uptake: Root hairs are responsible for initiating and maintaining cation exchange relationships with microscopic soil particles. Here the root hair secretes hydrogen ions onto the soil particle, exchanging them for mineral ions (calcium, magnesium, iron, etc.). Then the root removes those minerals from the soil water surrounding the soil particle. You can guess what the effects are if an area has acid rain, as we do in Connecticut. Again, as this function occurs in root hairs, fertilization of trees is best accomplished by placement near the drip line of the canopy.

Conduction of water and minerals:Roots contain xylem to conduct water from the soil up the plant and out through the leaves. These xylem tracheids and/or vessels are connected to others in an end-to-end design allowing soil water and minerals to be lifted up to the leaves. The evaporation of water from the leaves is the major pull of water through the xylem, but roots can also develop "root pressure" osmotically when the soil is well-watered and the plant has sufficient reserves. Evaporation of water from the soil/mineral solution bathing the leaf mesophyll cells concentrates the minerals for use by the plant as enzyme cofactors, etc.

Roots also contain phloem to conduct photosynthate from the leaves to the root tips. The metabolism of roots growing in the dark of the soil is essentially dependent upon respiration. This process requires carbohydrate or other organic molecules as fuel. It also requires a supply of oxygen, which is why soil needs to drain well for good plant growth.

Anchorage of plant: Roots can develop secondary xylem and/or can branch profusely to provide steady anchorage for the shoot system. The architecture of a most root systems is some compromise of two fundamentally-different designs.

Tap root: The primary root of the plant develops in length and girth with very little lateral branching...a carrot is a good example of a tap root. In trees this woody "post" provides excellent resistance to blowdown of the tree in wind storms. However a tap root provides little resistance to uprooting.

Fibrous roots: The primary root of the plant branches profusely interdigitating a network of fine roots in a large volume of soil particles. This design allows a plant to blow down easily but provides fabulous uprooting resistance. You can imagine that grasses that are subjected to intense grazing pressure (from bison in the prairies for example) and have evolved fibrous root systems.

Again, some plants will exhibit mostly tap roots, other will have primarily a fibrous system. Yet other plants will have a compromise between a major root and several large lateral "feeder" roots with extensive fibrous roots associated with them. You can imagine that this latter plant is neither tall tree nor short herb.

Specialized root architecture designs include Buttress and Prop root systems:

This tropical Ficus root system is adapted for supporting large tropical trees through hurricanes and typhoons in spite of the poor soil found in the tropics. To gather minerals for such a large tree, the root system must cover a large, thin surface of soil, but to hold up this big tree, a fibrous system would not work. So the woody roots expand vertically forming radial walls extending out from the sides of the trunk. The photo here includes my wife for comparison...she is about my own size, so these "planks" of woody root are indeed huge! This tree is cantilevered against the wind. Human architects observing this method of support in plants adapted it for use in holding up the long walls of cathedrals; the architectural structure is known as a buttress.

This tropical Pandanus shows good development of prop roots. These diverge from stem nodes as adventitious roots, arch out over the soil, and take root and branch out in the soil some distance away form the trunk. These woody arches then support this tree against hurricanes and typhoons. Again, architects inspired by such plants modified the cathedral buttress into what are known as flying buttresses today. These are more aesthetically pleasing than the more gothic buttresses.

This tropical Avicennia or black mangrove is demonstrating a specialized root structure known as a pneumatophore. The black mangrove lives in a swamp of anaerobic mud sediments. In order for its root system to survive here, it must provide a pathway for oxygen to supply respiration in the roots. These pneumatophores extend above the sediments; lenticels in their bark provide entry of oxygen and interior aerenchyma provides an internal pathway for oxygen to get all the way to the root tips. In a sense, then, pneumatophores are similar to a snorkel.

Storage of nutrients: The cortex of a root is a parenchyma tissue that can store large amounts of starch, sugars, minerals, and other biomolecules for long periods of time. During the late summer and fall, trees sense the changing daylength signalling the approach of winter. They degrade the polymers in their leaves (senescence) and return the amino acids, sugars, and essential minerals, etc. to the root via the phloem. These remain in the trunk or root system all winter and are returned to the shoot in the spring sap flow. We tap this sap from maple trees, boil it down (40:1) and produce maple syrup.

Root Apical Meristem

The root grows from its tip and here we see two longitudinal sections of a young root tip:

The Root Cap: The very tip of the root ends in a thimble-like covering, the root cap. This cap has a column of cells in its interior that are meristematic... they divide rapidly to make more cells. The derivative cells of these divisions are pushed outward by additional divisions and ultimately will slough off from the root cap surface. These slough cells assist in reducing friction and coating the rough surfaces of soil particles through which the root must grow. In addition, the root cap cells secrete mucilage. This chemical is strongly hygroscopic (attracts and locks up moisture) forming a gel. This mucilage acts as a lubricant for penetration in the soil. Humans use mucilage for the adhesive on stamps and envelope flaps...yes you have licked the mucilage the plants use to penetrate the soil.

The Zone of Mitosis: Immediately proximal to the root cap is a cluster of cells that do not actively divide. This pad of cells is often called the quiescent center. These cells probably represent a reserve of cells to be recruited later in time for the meristem. As such they serve as corrections for proliferating somatic mutations. Just proximal to the quiescent center are cells that divide rapidly by mitosis, adding new cells to the length of the root. This is of course just one contribution to elongation of the root. Just for your review, here is a table of mitosis:

Meristematic Cells Divide by Mitosis

Interphase

 

Prophase

 

Metaphase

Anaphase

Telophase

 

The Zone of Elongation: Just proximal to the zone of mitosis is a zone of cell elongation. In this part of the root the newly created cells expand in their long dimension to push the meristem and root cap through the soil. The addition of the cells and their elongation are the tandem contributors to root elongation. This elongation involves resculpting the wall, growth of the cell within, a coalescence of the vacuoles to form a single large vacuole, and maturation of the organelles in these cells.

The Zone of Maturation: As we keep moving proximally (away from the root apex), we find that the cells that are elongating are also differentiating. They are becoming distinguishable from each other. Some are destined to be typical parenchyma cells, while others will mature to be sclerenchyma cells. Here is a view of adjacent cells in a root that have become differentiated... the bluish parenchyma cells lay right next to some reddish sclerenchyma cells:

The difference between these cells is a matter of how much division occurs to make the cells, how much elongation occurs after the cells were made, and then how the interior of the cells matured. In the case of sclerenchyma cells the primary wall (picks up bluish dye) is joined by additional secondary cell wall layers that become lignified (pick up the red dye). This incorporation of lignins into the wall material makes the cell wall exceedingly hard and even brittle. Eventually the cytoplasm is programmed for senescence and death, leaving behind an empty, hollow cell, with just the wall. As these cells are stacked end to end along the plant and the end wall degenerates, they form a kind of plumbing for the plant. These tracheary elements in the xylem will conduct water and minerals from the soil to the leaves. Yes, even dead cells can serve important functions. In addition to water and mineral conduction, the layers of xylem represent substantial support in the form of wood for tree trunks. From a human point of view, what dead cells are critical in human physiology? Think skin!

On the exterior of the root in the zone of maturation, the epidermal hairs elongate out into the soil particles as root hairs. Here is a photo of part of the root-hair area in the zone of maturation:

These root hairs increase the surface area of the root tremendously. They assist in soil water intake. They are also critical for secreting acids onto soil particles to initiate cation exchange of soil minerals. Thus the uptake of materials occurs in these very fine young-root areas. People who do not understand this attempt to move large trees without taking the soil ball with them. They will knock off the soil or hose it out, etc. to lighten their transplanting burden without realizing that they have doomed all the root hairs to desiccation. The tree will suffer a strong set-back shock and may even perish because of this. It helps if the moves are made in the cool of late fall or early spring.

Mature Root Cross Section

Below is a cross-section of a "typical" dicot root.

This is a list of the tissues within that root:

Root Hairs - cell extensions of epidermis increase absorptive surface
Epidermis - water and non-selective mineral intake via root hairs
Cortex - storage parenchyma (starch, sugar, etc.)
Endodermis - selective mineral pump (concentrates particular minerals as it pumps them into xylem area)
Pericycle - origin of lateral roots in young areas, bark on older roots of woody species
Vascular Cylinder

Phloem - conducts nutrients from leaves
(Cambium - makes wood-woody plants only)
Xylem - conducts minerals and water up

Now we shall consider each of these layers of the root in turn.

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:

In the view of the mature root cross section (well above) you might notice there are four protoxylem poles of the xylem area... these make this example tetrarch. The xylem/phloem arrangement is also clearly radial... with phloem on alternate radii with the xylem.

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:

Here is a diagram to assist you in identifying the structures:

A monocot root has a similar structure, but of course it has some important differences. One is that the pericycle does not produce periderm (bark) in the typical case. Typically there is no vascular cambium between the xylem and phloem to produce secondary xylem or phloem. The xylem of the monocot root typically has many ridges of protoxylem (is polyarch). And the stele is a siphonostele (with pith) rather than a protostele (solid vascular tissue with no pith). These differences mean that monocot roots typically are small in diameter and monocot plants typically have fibrous rather than tap-root systems.