ROOTS

Roots are the first organs to emerge from the seed. wpe9.jpg (28354 bytes)They penetrate the soil and are responsible for the uptake of water and minerals from the rhizosphere. 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 consequently more important in the biosphere.

Roots are the Rodney Dangerfields of the plant world because they are subterranean for the most part. Out of sight … 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 their substrate. 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 resistant to decay and mechanical damage.

Ancient Hawaiians learned how to use these roots for fish traps and in house construction. It is a mystery how they got the roots to be so straight. I have some ideas about this and I think this would be a good undergraduate research project.

Apical Meristem Organization.

Longitudinal Section of Musa (banana) Root: Identify the major regions.


Interface of the Root Cap and Root Body of Banana: Note the convergence of cells towards one focal point in the Root Body. See the BOX in the lower image. What term is applied to the cells at this focal point?
Cells in the Root Cap do not converge on the same area as cells in the body. The Root Cap Meristem (Calyptrogen) is clearly separate from the body and is spread over a wide area.
What type of root organization is this?


Isolated view of the interface between the Root Cap and Root Body. I added the black line which separates the two regions. Why did I do this?

It is not important for you to see the identical number of initial cells indicated below. The most important distinction is that between open and closed apex organization & whether there are only a few, or many Initials. Do not sweat the details!!!!!

In Salix three groups of "initials" are also present, one giving rise to the central cylinder, one to the cortex, and one to the epidermis and root cap. The distinction between the latter two groups is not especially clear.

The organization of the apical meristem in the two orchids is different from those above.

There are apparently two groups of initials, one giving rise to the central cylinder, and the other to the rest of the tissues.

Which meristems have the "open" type of organization, and which are "closed"?

Root Tip of Hyacinth: Note the Convergence of cell Files towards the Apex

Can you find a distinct Calyptrogen? Is this Open or Closed organization?

 

Last but not least a Cycad Root Tip!

Does it have an Open or Closed Organization?

Follow the Lines which overlap cell files that pass through the Meristem

Root Tip.

Note especially the root cap, the apical meristem, and the development of root hairs.

Be careful with these slides. The mounts are very thick and can be broken easily if you use high magnification objectives.

Examine the intact root systems of Water Hyacinth (Eichhornia crassipes) or Water Lettuce (Pestia stratiotes) & compare with the grass seedling.

Can it be easily removed with forceps?
Does this suggest its apical organization (Open vs Closed)?

Water Hyacinth is one of the worst pest plants in tropical fresh water environments. It grows like wildfire and crowds out other aquatic species. Its decaying biomass causes eutrophication. The net result is paradise for the water hyacinth but perdition for all other plants and associated biota. Evidently, Lake Wilson can have large water hyacinth blooms from time to time.

Root of an Herbaceous Dicot

The classic example of such a root is found in the genus Ranunculus (buttercup).

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

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, the 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 primary xylem. The term primary means all of the xylem, which develops directly from the procambium.

The strands of primary phloem alternate with the primary xylem ridges.

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

Vascular Cylinder of Ranunculus Root: The Endodermis has stained positively for Lignin.

Vascular Cylinder of Ranunculus Root seen with crossed polarizers: The Endodermis and the Xylem are birefringent!

Root of a Monocot

Smilax (cat's brier) is a vine and these may be aerial roots. Thus wpeB.jpg (14833 bytes)some details of their anatomy, especially regarding the epidermis and hypodermis may be due to their aerial environment.

The following details should be noted.

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, wpeE.jpg (33441 bytes)the cells of which are in the tertiary 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 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 wpeD.jpg (19259 bytes)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 *arch.

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 occupies the center. This clearly demonstrates the origin of these cells from the procambium.

Casparian Strip:

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).

 

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.

Epidermis - Are root hairs visible???

Cortex

Endodermis

Pericycle

Protoxylem - Metaxylem (Diarch ...?)

Primary Phloem

Pith ?????

Is this a Monocot or a Dicot?

Special Features ?

Where do the hairs originate?

What are the hairs?

Roots of Freycinetia arborea (ie’ ie’)

This is a vine-like epiphyte. It grows on large trees in the forest. It has subterranean roots but it also produces aerial roots, which attach it to tree trunks and branches. The roots are very strong and resist decay. They were used by ancient Hawaiians to make fish traps, baskets and cordage. They were also used as the foundation of feather helmets (manhole).

Do not place your thumb directly wpeA.jpg (19366 bytes)behind the path of the razor blade!!!!!!!

Is this a monocot or a dicot????

What are its **archs?

Where are the strengthening cells located?

What tissue or cell type accounts for the strength of these roots?

Why would they resist decay??? Hint Hint Stain Stain

 

 

 

Root Symbioses

The roots of most plants have symbiotic relationships with soil microbes. Some of these are unspecialized but others are extremely refined and involve the differentiation of elaborate root modifications. We will examine a few cases in this lab.

Legume-Rhizobium Symbiosis

Roots of many legumes develop symbiotic relationships with species of the bacterium Rhizobium. The bacteria fix nitrogen from the atmosphere which becomes available to the legume. There are two kinds of nodules.

Indeterminate nodules have apical meristems and grow for months. These tend to be cylindrical with a slightly pointed apex.

Determinate nodules have a short growth period and live for a few days. These tend to be globular.

     

Cycad - Nostoc Symbiosis wpeF.jpg (10232 bytes)

Cycads produce upwardly growing tumescent roots. These can be inhabited by nitrogen-fixing cyanobacteria (bluegreen algae). The bacteria trigger an unusual kind of mitosis which results in programmed cell death. Cell lysis produces a nice home for the cyanobacteria. This is called the "algal" zone. The bacteria are protected from desiccation, and they get "free" carbohydrates as well. The Cycad gets nitrogen which is usually the most limiting essential element in the environment.