PLANT ORGANS: Root, stem and leaf.
THE ROOT
Know: apical meristem, region of elongation and region of maturation and their relative positions. Also, root cap and mucilage!
Root in cross section: see diagram.
From outermost layer to innermost:
1. epidermis (with root hairs)
2. cortex (starch storage)
3. endodermis (selective membrane composed of cells with a band of suberin to prevent INTERSTITIAL ENTRY of water (or other substances) into the stele. The belt of suberin around every endodermis cell is called a CASPARIAN STRIP (after the person who first described those belts).
Water and dissolved minerals can pass through the endodermis. Most complex organic molecules (sugars, proteins, etc.) cannot.
4. pericycle - a secondary/lateral meristem that gives rise only to side branch roots.
5. (pith) composed of parenchyma, and surrounding the...
6. phloem (anywhere from three to many bundles surrounding a central core of ...
7. xylem
THIS ANATOMY IS TYPICAL OF AN HERBACEOUS ROOT.
Once a root becomes woody, it is characterized by rings of xylem, a relatively thin outer circle of phloem and an outer covering of bark, similar to that found in a stem.
Note that woody roots do not participate in water
uptake. This is done mainly at the very tips of the growing areas,
which are always herbaceous and have the type of anatomy we just
described.
ROOTS --
There are many different types of roots, including...
1. food storage roots - used by the plant to store
starch for metabolic activities later in the season. Typical
examples: carrot, beet, sweet potato.
2. water storage roots - found in arid regions,
these are roots that collect large amounts of water during rainy
season for the plant to use during dry season.
3. propagative roots - have meristematic regions
from where new, genetically identical plantlets can grow.
4. Pneumatophore - gas exchange surfaces on root
tips protruding from water-logged soil (e.g. - mangroves)
5. aerial roots - typical of epiphytes such as orchids
(in which these roots are called velamen, with a spongy outer
surface very good at absorbing and holding water) and bromeliads.
6. buttress roots - wall-like extensions off the
base of the trunk which provide support against physical assault
from high winds.
(e.g. - Ficus spp., Royal Poinciana, etc.)
7. contractile roots - pull young plant under the
soil where it is protected from weather & elements.
8. Haustoria - parasitic plant roots that invade
the tissues of a host plant and transfer nutrients from host to
parasite. (e.g. - Dodder--the orange, spaghetti-like parastic
plant found on trees in Southern Florida (and elsewhere).)
Two tissues layers present in roots,
but not in stems are:
1. endodermis (since water is not generally absorbed
through the surface of the stem!) - This single-celled layer forms a
cylinder around the root stele, with each cell bearing a band of waxy
SUBERIN, which prevents interstitial entry of water into the root. The
endodermis thus causes the plasma membranes of its component cells to act
as a selective filter for what gets into the vascular tissue of the root.
2. pericycle (no branch roots from a stem!) - This is another
meristematic layer found only in roots. It lies just under the endodermis,
and gives rise to lateral roots that branch off the central taproot.
Root Symbioses
1.
mycorrhizae - fungus roots.
An association (beneficial to both species) in which
a fungus penetrates the cortex of a plant root. The fungus receives
sugars and amino acids from the plant. The plant receives hugely
increased surface area for water and mineral absorption from the
fungus, which fans out for meters into the soil, reaching areas
not accessible to the plant alone.
(Possible additional benefit: fungus, being a decomposer,
exudes digestive enzymes into the soil, breaking organic matter
into inorganic matter, thus supplying the plant with readily available
inorganic nutrients.)
2. root nodules - housing nitrogen-fixing bacteria
These are found primarily in plants that are members
of the pea family (Fabaceae). The root nodules house special
species of bacteria which can take atmospheric, gaseous nitrogen
from the soil (not usable by plants) and convert it into ammonia,
nitrite and/or nitrate (which are usable by plants to make DNA,
RNA and protein).
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To understand where roots live, let's have a look at this profile of
SOIL and its
components.
A bit more on roots....
