Laboratory Exercise 4 the Root

LABORATORY EXERCISE 4

THE ROOT

Guide Questions:

1. Are all root tips of plants provided with root cap regardless of habitat where the species is growing? What are some functions of the root cap?

Answer:

                No.  The root cap is absent in some parasitic plants and some aquatic plants, in which a sac-like structure called the root pocket may form instead. Root pockets are also the root caps present in hydrophytes. The only difference between a root cap and a root pocket is that root pockets have the lost the protective function due to non-use as roots in hydrophytes are free floating. They need not penetrate the hard soil. Root pockets do not have the ability to regenerate. Root pockets are present in aquatic plants like Pistia and Water hyacinth.

                The root cap is a special layer of tissue which protects the tip of a growing root and helps to anchor the plant. At the very tip of the root is a loose mass of parenchyma cells which continually slough off and regenerate with the growth of the root system. In addition to protecting the interior of the root, the cap secretes a mucilage which stabilize water content of the surrounding soil, ensuring longer-lasting nutrition to the root system and making for easier root probing.  Finally, the root caps contains statocytes, which are specialized cells that help the plant to sense gravity and grow accordingly. These cells are full of starchy organelles which settle at the lowest part of the cell and encourage growth in that direction. If the root cap, with these statocytes, is removed, plants may grow in random directions because it has lost the ability for gravitrophism.

1. What is meant by quiescent center as part of the apical meristem?

Answer:

It is observed that in the root-promeristem of higher plants the initials divide much less frequently than other cells. This part of root-promeristem, which contains the initials of epidermis, cortex and stele and divides less frequently, is referred to as quiescent centre (QC). Quiescent centre is situated at the pole of cortex and stele and consists of four cells (e.g. Arabidopsis) or as high as 600 cells that have very low mitotic activity.

Quiescent centre is unusual in shoots, but it is universal in roots of higher plants. Though the occurrence of QC was reported in vascular cryptogams (e.g. Equisetum arvense) later investigations have established that the entire root apex of vascular cryptogams is mitotically active. Quiescent centre maintains a strict boundary in closed meristem. As a result, the derivatives of QC also maintain a strict boundary in the apical zone.

Quiescent centre consists of central cells only or central cell and part of its peripheral zones. It does not include the mother cells of columella. A separate cap meristem exists and it forms root cap. In open meristem the size and position of QC change.

1. Name the three primary meristems and the primary tissues they give rise to. Distinguish between monocot and dicot species based on this condition.

Answer:

        The apical meristem produces the three primary meristems, protoderm, procambium, and ground meristem, which develop into dermal tissues, vascular tissues, and ground tissues respectively.

        The dermal tissue includes the epidermis of the stem and leaves witch are covered by a cuticle.  The cuticle is composed of a waxy material that keeps the underlying tissues from drying out.  The epidermis of stems and leaves may contain hairlike extensions called trichomes.  In leaves the epidermis may have stomata, small openings that are controlled by the action of guard cells.  Guard cells contain chloroplasts, while epidermal cells and trichomes do not. Monocots and dicots both contains dermal tissues. However, secondary growth occurs in a dicot root but not in a monocot root which means dicot root have an outer covering which is called periderm that replaces the epidermis.

The ground tissue of a stem is divided into two regions, the cortex and the pith.  The cortex is located to the outside and/or around the vascular bundles, while the pith is located in the center of the stem.  Both the cortex and pith are composed mainly of parenchyma cells.  Support is given to the stem by collenchyma cells which may occur just under the epidermis in the cortex.  This collenchyma cell layer may eventually develop into sclerenchyma.  Monocots usually do not have a defined cortex and pith like that found in the Dicots. The layers of ground tissue are differentiated in dicot because their vascular bundles are distinctly found in the center surrounded by the cortex.

The primary vascular tissue forms longitudinal vascular bundles which run the length of the stem, and form veins in leaves.  In monocot stems the vascular bundles are usually scattered, but in others (wheat, rice, oats) may form two rings.  The scattered vascular bundles are interconnected and can form an intricate meshwork.  In dicots and gymnosperms the vascular bundles form a ring around the pith.  Vascular bundles branch off from the stem into the leaves of the plant.  These branch points are called leaf traces, and occur at the base of the node.  From the leaf trace, the vascular bundles subdivide to produce the veins of the leaf. Monocot stems have most of their vascular bundles near the outside edge of the stem. The bundles are surrounded by large parenchyma in the cortex region. There is no pith region in monocots. Dicot stems have bundles in a ring surrounding parenchyma cells in a pith region. 
In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.

1. In what groups and in what conditions are root hairs absent in root systems of plants? Why are root hairs not formed at region of elongation and after region of maturation?

Answer:

Plant that do not have roots are Bryophytes. These plants are small and lack true roots, stem and leaves. The body is thalloid. Although rhizoids are present that act as mineral and water conducting organ for plant. But still they lack true roots, which means that their root hairs are absent. Because they don't have roots and stems to transport water, mosses dry out very quickly, so they are usually found in moist habitats.        

Root hairs are found only in the zone of maturation, and not the zone of elongation, possibly because any root hairs that arise are sheared off as the root elongates and moves through the soil. Root hairs are attached to the root within the zone of maturation. Root hairs are tiny hairs used to absorb water and minerals from the soil by increasing the surface area. Root hairs are critical to a plant's development. If you are going to transplant a plant or tree, it is imperative to move the soil ball, to protect the tiny root hairs. The cells in the zone of elongation are not yet fully developed so they don’t need water and minerals to absorb.

1. How is absorption and anchorage taken care of in rootless like Psilotum?

Answer:

        Psilotum (pronounced "zi-lo-tum") is a low-growing plant devoid of any roots or leaves. Instead it has branched rhizomes under the soil surface, and forked green photosynthetic stems above which bear small leaf-like or scale-like appendages. Laced terminally on very small branches are thick-walled homosporous sporangia which release the plant's spores.

        Because P. nudum's primitive system of absorbing nutrients and water through rhizomes is terribly inefficient, the plant forms a relationship with mycorrhiza (a type of fungus). The fungus has access to the plants rhizoidal hairs - structures which act as absorption points for nutrients and water on the rhizomes. The fungus is better at nutrient and water uptake than the Psilotum, and so its relationship allows both organisms to exist in arid environments.