Roots   

Roots are the first organs to emerge from the seed. They penetrate the soil and are responsible for the uptake of water and minerals from the rhizosphere. They also form various associations with soil microbes. Some of these have tremendous agricultural and ecological significance. Roots contain a tissue called Endodermis. The endodermis is one of the most important vegetative adaptations of terrestrial plants because it asserts biological control over water and mineral uptake in the root.

Roots form symbiotic associations with soil microbes. Virtually all plants have root symbioses. The most famous symbioses are those that involve nitrogen-fixing microorganisms. However, Mycorrhizal associations between plants and fungi are more universal and are extremely important.

Roots are the "Rodney Dangerfields" of the plant world because they are subterranean for the most part. Out of sight and out of mind. However, the root system may be as great as or greater than the shoot system in terms of biomass and complexity. This is especially true for the roots of desert plants. I am somewhat prejudiced because I studied strange Cycad roots for my Ph. D.

The roots of epiphytes are exposed directly to the atmosphere and have special adaptations to accommodate the consequent environmental insults that accrue from this. Prop roots help certain plants like Pandanus to remain erect. Because the Prop Roots of Pandanus originate from the stem (not from pre-existing roots) they are called Adventitious. Many vines produce adventitious roots which help them adhere to substrates. 'ie 'ie (Freycinetia arborea) is an excellent example of this. Its stem produces strong adventitious roots which encircle tree trunks, and allow the vine to grow to the top of the canopy where it can intercept maximal levels of light for photosynthesis. These roots are extremely fibrous and strong. Cell walls in the roots are heavily lignified. This makes them strong and resistant to decay plus mechanical damage. Ancient Hawaiians learned how to use these roots for fish traps and in house construction. 

Tissue Organization in Roots is extremely simple and that is why I start with Roots. Starting with the Epidermis and proceeding towards the center (centripetally) there is a series of concentric cylinders that contain Ground Tissue and Vascular Tissue with Xylem typically more central than Phloem. There are some special cases but this is the general case. The Endodermis is the innermost layer of the Ground Tissue and forms a boundary that surrounds the Vascular Cylinder or Stele. Stel can mean pillar or post in Latin.

Lateral Roots have an internal origin from the Endodermis or the outer part of the Stele. Consequently they are protected as they grow through the Ground Tissue and Epidermis. They also have a direct Vascular Connection from their inception. They are produced in regions that are no longer growing. Consequently they hare not damaged by sheer forces which develop from root growth through the soil.

The Primary Plant Body is produced by Apical Meristems. These are conveniently called the Root Apical Meristem (RAM) or Shoot Apical Meristem (SAM). These contain Meristematic Cells which produce cells that differentiate into mature Primary Tissues. Meristematic cells are Undifferentiated and serve as a reference point for Cytodifferentiation (Cyto = Cell). Meristematic Cells have the following traits. They have very thin primary cell walls. They are Isodiametric in shape, Densely Cytoplasmic and Microvacuolate. They have a large Nucleo:Cytoplasmic ratio. In other words the Nucleus occupies most of the cell's volume. Consequently, Meristematic Cells stain densely compared to most other cell types that have large central vacuoles. They have Proplastids which are very difficult to see with the light microscope. They are mitotic. 

In Stems and many Roots there is one Apical Meristem which produces the entire Organ (Root Cap & Root Body). However, some Roots have a separate Root Cap Meristem called the Calyptra. This is a broad, shallow zone of meristematic cells that produces the Root Cap. It is clearly distinct from the Apical Meristem that produces the Root Body. Such roots have a "Closed" organization. In cases where the Root Body and Root Cap are produced by one meristem the term "Open" is used to describe this type of meristematic organization. Closed roots generally have distinct Root Caps that can be removed without damaging the root apex. Scientists have used this to perform intricate experiments on geotropism and root meristem function. 

Apical Meristem Organization

Study the Root System of Water Hyacinth (Eichhornia crassipes) or Water Lettuce (Pistia stratiotes). 

These are aquatic plants and this makes it easy to see the entire root System. Both of these species are extremely invasive and have caused a lot of problems in Florida and other tropical areas.

Locate the Root Cap. Is it clearly discernable from the rest of the root?

Observe the many lateral projections along the root axis. What do you think these represent?

We will Explore this later in the Lab.

Study commercial slides of Corn Root Tips.

Locate the Root Cap. The red objects are Statoliths. 

Trace the central Root Cap Cells towards their Origin. Is this a small concentrated area or a wide region of Meristematic Cells?

Is the Root Cap clearly distinct from the Body of the Root. Does it have a Calyptra?

Does it have Closed or Open Organization?

Study the Demo slide of Salix (Willow) Root Apex and ask the same questions as above, and determine if it has Closed or Open Organization.

Study Demo slide of Musa (banana). Is it Closed or Open?

Study the Demo slide of Vanda which is an Orchid.

Find the Apex & Locate the Root Cap which is small and irregular compared to the other roots we have seen

Locate the Protoderm. This is the unicellular Primary Meristem that produces the Epidermis.  Cell divisions in the Protoderm are typically Anticlinal. The new cell wall is perpendicular to the surface when cells divide Anticlinally. This results in a layer that is one cell thick. These add length to a tissue.

Periclinal divisions have new walls that are parallel to the surface this produces multiple layers. and adds to the thickness of a tissue.

Follow the Protoderm towards the base of the root. Try to see where Periclinal divisions of the Protoderm occur. The produce a Multiple Epidermis called the Velamen.

Continue scanning towards the base. Note the Root Hairs also find the region where the Velamen cells are differentiating.

Study a cross section of an Orchid Root

Locate the Velamen. 

Try to relate this to the longitudinal section you just observed.

Observe prepared slides of Mature Roots.

Identify Proto & Metaxylem.

Determine whether the developmental sequence for the xylem is centrifugal (Endarch) or centripetal (Exarch)?

Locate the Phloem sectors.

Locate the Endodermis.

The Epidermis may be present or absent, because it is often poorly preserved, especially in older stages.

In some cases you will be able to see a Casparian Strip in the outermost layer of the Cortex, just beneath the epidermis. This is an Exodermis (or hypodermis). What is its function?

A wide Cortex of  Parenchyma cells of which usually contain starch.

Endodermis with Casparian Strips in younger roots with thick secondary walls and "passage cells" in older roots.

Single layer of thin-walled Pericycle cells. The Pericycle is a layer or layers of Parenchyma  that form a buffer zone between the Endodermis and the Vascular Tissues in the Stele.

The Primary Xylem. The term primary means all of the xylem, which develops directly from the Procambium. The Procambium is the Primary Meristem that produces Vascular Tissues.

The strands of Primary Phloem alternate with the Primary Xylem ridges.

What type of archy do you see? di - tri - tet - pent - poly?

 

Root of a Monocot

Smilax (cat-brier or carrion flower) is typically studied. This is a vine and these may be aerial roots. Thus some details of their anatomy, especially regarding the epidermis and hypodermis may be due to their aerial environment. These specimens are from a Mature region of the root where water absorption is not occurring.

The following details should be observed as you scan the tissues from the outside (epidermis) to the center (xylem).

Uniseriate Epidermis with relatively thick walls.

Exodermis beneath the epidermis.

Cortex, most of the cortex consists of starch-containing parenchyma cells, with numerous intercellular spaces.

The inner layer of the cortex is the Endodermis, the cells of which are in an advanced state of development and exhibit thick secondary walls which cover the primary radial and inner tangential walls.

A thick-walled Pericycle, two to six cells in depth.

A ???????arch Vascular Cylinder with 20 or more separate strands of primary xylem alternating with an equal number of primary phloem strands.

The Xylem strands are embedded in thick-walled Parenchyma similar to that constituting the Pericycle.

In each xylem strand the smallest, oldest tracheary elements occur next to the Pericycle. These elements constitute the Protoxylem.

The intermediate and large tracheary cells constitute the Metaxylem.

Similarly, the smallest Sieve Elements next to the Pericycle, compose the Protophloem, the larger sieve elements located farther inward make up the Metaphloem. This means that they have Exarch or Endarch maturation? Choose one!

The center of the root contains parenchyma which have developed thick secondary walls. This area is often called the pith. However, these cells arise from the Procambium and are part of the Xylem. In the case of stem pith, the parenchyma cells originate from the Ground Meristem. Big deal you might say! In corn (monocot) a large vessel member may occupy the center. This clearly demonstrates the origin of these cells from the Procambium.

Observe Commercial Slides of Corn (Zea mays) Prop Roots

Compare with Smilax.

Casparian Strip

Look at the Demo slide of Crinum or other specimens to see the pink-stained Casparian Strip. This is  a narrow band in the radial and transverse walls of the Endodermis. The Casparian Strip is one of the most important adaptations of land plants because it brings the flow of water and solute under biological control!

Origin of Branch Roots

Examine cross sections of Salix (willow) roots.

The branch roots originate in the Pericycle. In these slides the branch roots are so large that it may not be evident they originated from this layer, although their origin is clearly endogenous (from within). 

Examine Demos that show Lateral Root Origin.

Roots of Water Hyacinth (Eichornia crassipes)

This is a beautiful plant but it is also a dangerous pest. It thrives in tropical freshwater and brackish environments. It literally swamps out competitors by growing so rapidly that its decaying biomass makes the environment anaerobic so that most other species die. It s a major problem in Florida many tropical countries.

Observe the Entire Root System

Note the overall morphology of the roots. Pay special attention to the hair-like projections.

Locate the Root Cap and use forceps to gently pull it away from the apex. Is this likely to be a Closed or Open root?

Make Transverse Sections ABOVE the hairy region!

 Stain with Toluidine Blue.

Locate the Following.

Epidermis - Are root hairs visible???

Cortex

Endodermis

Pericycle

Xylem

Phloem

Pith 

Is this a Monocot or a Dicot? Does it have any Special Features ?

Make a Transverse Sections through the "Hairy Zone". 

Stain as above.

Where do the hairs originate?

What are the hairs?