Plant Support

Compare and contrast monocot and dicot support structures

Organisation of the plant body
Four basic tissued are identified in plants:

1.       Meristems- give rise to all other cells and tissues in the plant.

2.       Ground tissue- consists primarily of parenchyma cells that may function in storage, photosynthesis and secretion.

3.       Epidermis- one cell thick in most plants, forms the outer protective covering

4.       Vascular tissue- includes the xylem and the phloem

Plant tissue can also be divided into two groups:

1.       Simple tissue- consists of only one cell type

a.       Parenchyma

b.       Collenchyma

c.       Sclerenchyma

2.       Complex tissue- composed of more than one type of cell

a.       Xylem (contains 4 cell types)

b.       Phloem (contains 5 cell types)

Meristem

It is when growth in stems and roots is generated from specific regions of cell division and cell expansion. Meristematic tissue retain the ability to produce new cells indefinitely. Types of meristems:

1.       Apical meristem- at the tip of roots and shoots. Its division result in increase in length of stems and roots. It also results in embryo leaves and buds.

2.       Lateral meristem- increase the girth of stems and roots. Tissues developed from lateral meristem are referred to as secondary tissue as they are formed after primary tissue has matured.

a.       Vascular cambium- forms new xylem and phloem

b.       Cork cambium- locates exterior to the vascular cambium. Forms the outer bark of woody plants. Cell walls have suberin, a waterproof layer

3.       Intercalary meristem- grasses have neither vascular nor cork cambium but they have apical meristem called intercalary meristem located at the vicinity of nodes. These increase the length of stems

Plant growth take two forms:

a)       Primary growth- occurs trough mitotic division of apical meristem cells followed by differentiation of the resulting daughter cells. This occurs in young plants and the growing tips of roots and tips of older plants. It is responsible for increase in length and formation of specialized plant structures

b)       Secondary growth- Results from mitotic division of lateral meristem

Ground Tissue

This consists of Parenchyma, Collenchyma and Sclerenchyma

Parenchyma Cells

Structure- Parenchyma cells are roughly spherical with an average diameter of 25 mm. They tend to have large vacuoles and thin cellulose cell wall

Parenchyma cells function and distribution:

·       Packaging tissue between more specialized tissues as in the central pith of stems and outer cortex of stems and roots

·       Form the bulk of young plants

·       Supportive due to their osmotic properties. When turgid they become tightly packed and provide support. This is important in stems of herbaceous plants where they are the main means of support. During periods of water shortage these cells loose water and the plant wilts.

·       Metabolically active and therefore the site of many vital activities of the plant body which include photosynthesis (e.g. in potato tuber; edible part of fruit and vegetables).

·       Airspaces present between cells. These spaces interlink with the opening of the stomata and are involved in gas exchange in the plant.

·       Storage food or water. Water is stored in succulent plants. Starch is stored in cereal grains and potatoes. Proteins are stored in beans and peas while oils are stored in avocados and safflower

Specialised parenchyma:

1.       Mesophyll

a.       Package tissue between epidermal layers

b.       Photosynthetic parenchyma- chlorenchyma

i) palisade cells- elongated cells containing numerous chloroplasts

ii)  Spongy cells- irregular shaped with fewer chloroplasts Large intercellular spaces for gas exchange

2.       Aerenchyma- present in aquatic plants example water lily.  Contains large air spaces throughout to:

a.       Allow oxygen to diffuse to the submerged leaves

b.       Provide buoyancy

3.       Aqueous parenchyma- These cells are responsible for water storage in succulent plants. Water is stored in the large vacuole. When the cells use up water the cell wall is folded in

Characteristics of a hydrophyte:

1.       Aerenchyma for support

2.       Large sclereids for support

3.       Reduced vascular tissue as it is not required for support

4.       Stomata and a cuticle on upper epidermis only

Collenchyma

Structure: They are characterised by deposition of extra cellulose at the corners of the cells. The cells are elongated and can reach a length of 1mm.

Function:

·       Provide support in the organs in which it is found

·       Important in young plants, herbaceous plant and in leaves

Distribution:

·       Found below the epidermis in the outer region of the cortex and gradually merges into parenchyma towards the inside.

·       Sometimes instead of rings, collenchyma is deposited in bundles to form ridges as along the fleshy petioles of celery.

·       In dicot leaves it appears as solid masses running the length of the midrib, providing support for the vascular bundle. They have an important role in positioning the leaf for optimal light absorption.

·       It is absent in roots and in monocots

Sclerenchyma

Structure: Individual sclerenchyma fibres are strong but also elastic due to lignified secondary walls. Lignin has high compression and tensile strength. They are elongated with pointed ends. Their strength is increased by their arrangement into strands of tissue that extend for considerable distance. Their ends interlock, enhancing their combined strength. They only form in organs that have reached their full size as otherwise they cannot grow. They are indigestible by animals. Form protective layer. Pits are areas where primary wall is not covered by secondary wall. Pit membrane is two plasma membranes, two primary walls and a middle lamella. These can form conducting channels. There are two types of sclerenchyma:

a)       Fibres- elongated cells which are found in bunches

b)       Sclereids- usually roughly spherical found isolated in connection with other tissues or in bands or sheets

Function: The sole function of sclerenchyma is to assist in providing support and mechanical strength to the plant.
Distribution:

·       In xylem or phloem either individually or in groups

·       In the cortex below the epidermis of stems and roots

·       In the cap of vascular bundles of dicot stems

 Epidermis

·       One cell thick layer that covers the whole of the primary plant body.

·       Function: to protect the plant from desiccation, abrasion and infection.

·       During secondary growth it may be ruptured and replaced by a cork layer.

·       Epidermal cells secrete a waxy substance called cutin which forms the cuticle present only in aerial parts of root.

·       Contain no chloroplast except for the guard cells.

·       A number of specialized cells occur in the epidermis, including guard cells, trichomes and root hairs.

Guard Cells

·       Are specialized epidermal cells which occur in pairs, with a pore between them called the stoma.

·       The cell wall is not uniformly thickened – important to control stomatal aperture

·       The mechanism involved in opening and closing of the stoma involves unequal stretching of the inner and outer walls of the guard cells

·       The cell volume increases and the guard cells curves, causing the stoma to open.

Trichomes or Hairs
These are outgrowth from the epidermis- unicellular or multicellular

Functions:

·       In climbing plants e.g. goosegrass hooked hairs occur which prevent stems from slipping from their supports

·       Glandular trichomes can be used to excrete excess salt absorbed from salty soils by halophytes.

·       Some hairs retain moist air as in xerophytes to reduce water loss e.g Maram grass.

·       They may secrete scents as in lavender or enzymes as in carnivorous plants

Root Hairs

·       Increases surface area for absorption of water and mineral salts

·       Have a thin cell wall to facilitate absorption

·       The root hair region is referred to as the piliferous layer.

Xylem

Functions: It conducts water and salts and provide support.

Consists of four cell types:

1.       Tracheids

2.       Vessel elements

a.       Protoxylem and metaxylem

3.       Parenchyma

4.       Fibres

Tracheids

·       Are elongated and lignified single cells

·       Have tapering end walls that overlap with adjacent tracheids and fibres

·       Are dead with empty lumens when mature – for unobstructed passage of water

·       Water passes laterally through unlignified portions of cell wall/pits

·       Are the primitive, original water-conducting cells of vascular plants

·       Only conducting vessels in conifers

·       Angiosperms have both tracheids and vessels

·       Pattern of lignification of the walls resemble that of vessels

Vessels

·       Main water conducting units in angiosperm xylem

·       Are very long tubular structures, formed by fusion of several cells end to end in a row

·       Vessel elements/vessel member:

o   Is the cell which forms a xylem vessel

o   s short and wider than a tracheid

o   Is open at both ends unlike a tracheid which is closed

o   No overlap of vessel elements occurs

Protoxylem and metaxylem

·       The first vessels for the protoxylem, located in the apex just behind the meristem, where eleongation of surrounding cells is still occurring

·       Mature protoxylem can be stretched as lignin is not deposited over the entire cellulose wall, but only in rings or spirals

·       These act as reinforcement for the tubes during elongation of the stem or root

·       As growth proceeds, more xylem vessels develop and these undergo more extensive lignification, completing their development in the mature regions of the organ and forming metaxylem.

·       Mature metaxylem vessels cannot stretch or grow as they are dead

·       Metaxylem vessels show three basic patterns of lignification:

o   Scalariform

o   Reticulate

o   Pitted

Xylem parenchyma- functions in food storage, deposition of tannins, packing, gaseous exchange through the intercellular spaces.

Xylem fibres- are shorter and narrower than tracheids and have much thicker walls. They have pits similar to those in tracheids but are more similar to sclerenchyma fibres, having overlapping end walls. They do not conduct water, so they have thicker walls and narrower lumens than vessels. Their main function is mechanical strength of xylem.

Phloem

It is composed of living cells modified for translocation and has no mechanical function. It have 5 cell types present:

1.       Sieve tube elements

2.       Companion cells

3.       Parenchyma

4.       Fibres

5.       Sclereids

Sieve tubes and companion cells

·       Sieve tubes are long tube-like structures that translocate solutions of organic solutes like sucrose

·       Are formed by the end-to-end fusion of cells called sieve tube elements

·       The first phloem that forms is the protophloem and like protoxylem is produced in the zone of elongation

·       Metaphloem is that which matures after elongation has ceased

·       Sieve tube elements:

o   Have cellulose and pectic substances in their walls

o   Lack nuclei

o   Are alive and depend on adjacent companion cells

o   Have a sieve plate (perforations in end walls)

·       Companion cells have dense very active cytoplasm

·       Each sieve tube element and companion cell, together form a functional unit

Phloem parenchyma, fibres and sclereids

·       Phloem parenchyma and fibres are found in dicots but not in monocots

·       Phloem parenchyma has same structure as parenchyma elsewhere

·       Phloem fibres are exactly similar to sclerenchyma fibres

Root Structure

Four zones or regions are commonly recognised in developing roots:

1.       Root cap- protects tissues behind it

2.       Zone of cell division- mitosis goes on

3.       Zone of elongation- cells elongate

4.       Zone of maturation- cells differentiate like growth of root hairs.

Endodermis

It is a single layer of cells whose primary walls are impregnated with suberin. Suberin is a fatty substance that is impervious to water. The suberin is produced in bands called Casparian strips.

Function of endodermis:

·       Is to protect the centre of the root from harmful substances

·       To regulate the movement of water in the root

o   The Casparian strips are fused to the cell membrane of the endodermal cells and prevent water and dissolved minerals from passing through them

o   Mineral nutrients can only move passively within root cell walls until they reach the endodermis. At that point, they must be actively transported across a cell membrane to continue further into the root. This allows the plant to accumulate mineral nutrients in the stele

o   This has a regulatory effect: it can largely exclude harmful chemicals from the centre of the root

o   All tissues interior to the endodermis are collectively called the stele.

Pericycle

·       Roots possess a layer of parenchyma one to several cells thick called pericycle between the vascular tissue and the endodermis.

·       It is a region of cell division and produces lateral roots

·       No equivalent in the stems