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 particle. 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.
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 Butress 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 vertially 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 butress.
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 butress
into what are known as flying butresses today. These are more
aesthetically pleasing than the more gothic butresses.
