Back to: O level Biology NOTES Uganda syllabus

MODIFIED ROOTS

1.Storage roots

These are thick fleshly and succulent roots. They contain stored food like sugar and starch. The roots are modified as root tuberse.g. carrots, cassavaand sweet potato roots.

2.Breathing roots

These are found on some plants growing in swampy areas e.g. white mangrove. Its roots grow up through the mud to the air. The root parts above the mud are spongy and absorb air from the atmosphere.The main root of such plants bears branch roots.

3.Stilt roots

These roots develop from the main stem in certain plants such as red mangrovewhich grow in muddy areas. Stilt roots provide additional support to the plant.

4.Prop roots

These are found growing on plants such as maize, sorghum and sugar canes. They develop from the nodes of the stem close to the soil surface. They provide extra support by holding the plant firmly to the soil surface.

5.Buttress roots

These are large thick roots growing from the base of certain stems e.g. Mvule trees, silk cotton, etc. They provide extra support to the plant by anchoring it firmly in the soil

6.Clasping roots

These are roots growing from the nodes of climbing stems such as figs (mituba trees), vanilla and orchids. They secret a sticky substance which dries up in air. This helps such plants to cling on to other plants for support.

7.Epiphytic roots

These grow on certain plants called epiphytes. Epiphytes are plants which grow and get support from other plants. These roots hang freely in the atmosphere. They absorb moisture from the atmosphere.

8.Sucking roots

These are roots found growing on certain parasitic plants e.g. figs (mituba). They grow from the stem and penetrate the host plant. These roots absorb water, mineral salts and organic food compounds from the host plant.

INTERNAL STRUCTURE OF A ROOT (LONGITUDINAL SECTION)

In a longitudinal section through the growing end of a root, its parts are divided into 4 main zone or regions:

1. Root cap
2. Region of cell division (meristematic region)
3. Region of cell elongation
4. Region of cell differentiation (maturation)

1)Root cap

This is found at the tip of the rootand is made up of loosely arranged cells. It protects the tender apex of a root from mechanical damage as it makes its way through the soil. It’s absent in aquatic plants.

2)Region of cell division

This is the growing apex of the root lying just behind the root cap. The cells in this region undergo repeated divisionsto form new root cap and new cells that increase the length of the root.

3)Region of cell elongation

This is the region lying just above the region of cell division. The cells in this region absorb water and develop vacuoles, the cells being elastic, elongated and enlarged. This causes an overall growth in the length of the root.

4)Region of cell differentiation.

This is also called the region of absorption. The characteristic feature of this region is the development of root hairs; these are fine, delicate, unicellular hair like extensions of epidermal cells (periferous layer). They absorb soil water and dissolved mineral salts from the soil. The cells in this region acquire specific shapes and functions thus they are said to be differentiated or specialized.

NB: the region behind the zone of differentiation is the oldest part of the root. It has permanent tissues and is covered by a layer of cork which prevents the evaporation of water from the roots.

Transverse section of a root

2.Dicot root

The transverse or cross section of most young roots has two regions

• The outer cylinder (cortex)
• The central cylinder (stele)

1)Cortex

This is the outer most layer of a root which is wide, composed of many smaller layers of thin walled cells called parenchyma(for strengthening the root).

It is surrounded by the outer layer within thin walled cells called periferous layer through which root hairs rise. The periferous layer has no cuticle but the cells forming it have cellulose cell wall. This allows water and mineral salts to be absorbed from the soil by root hairs. As the root grows older, the cells die and periferous layer is replaced by cork cells which prevent water loss from the roots. The inner most layer of the cortex is called endodermis. The endodermis is made of a layer of barrel shaped cells which are thickened so as to allow free movement of water. The endodermis is a ring around the central cylinder (stele).

2)Stele

This is made of a pericycle, vascular tissue and pith (in case of monocots).The pericycle is the outermost layer of the stele. It’s made up of thin walled cells. It surrounds the vascular tissues of the root and it produces lateral roots. The vascular tissues are composed of xylem and phloem and may contain cambium and pith tissues. Xylemis the water conducting tissue through which water and mineral salts pass from the soil upwards to the stem and leaves.Phloemis the food conducting tissue that carries manufactured food from the upper parts of the plant mainly leaves and distributes it to various parts of the root.

Cambium: causes secondary thickening of the root.It adds all secondary xylem cells on its inner side and secondary phloem cells on its outer side by continuous cell division during growing season.

Pith: is a small area in the centre of the monocot root. It is composed of parenchyma cells for strengthening the root. It’s normally absent in most roots because the centre is normally occupied by the xylem. It also stores food and water for the plant.

STEMS

This is the ascending portion of the plant axis that develops from the plumule of the embryo. It has the following characteristic features;

• It bears leaves at the nodes.
• It has nodes and internodes.
• It has buds in the axills called axillary buds.
• It has flowers or fruits.
• Its terminal bud is located at the tip of the stem.

NB: the axill is the angle between the leaf and the stem.

Functions of stems

a)Primary functions

• They hold leaves in the best position for receiving enough sun light needed in the process of photosynthesis.
• They conduct water and mineral salts from roots to leaves and manufactured food from leaves to other parts.
• They hold flowers and fruits in good position so that they can be easily pollinated or dispersed.
• When stems are young, they carry out photosynthesis thus making food for the plant.
• Stems have lenticels (pores) that facilitate gaseous exchange.

b)Secondary functions

• Some stems may specialize in storing food and water e.g. stem tubers like corms, Irish potatoes, rhizomes and sugar cane.
• Protect a plant against browsers by forming thorns, spines or prickles.
• Vegetative reproduction or propagation through the stem cuttings e.g. cassava and sweet potatoes.
• They support the plant by climbing stem tendrils e.g. pasumpea (wild pea).

TYPES OF STEMS

1)Erect stems

These can support themselves in an upright position. They may be woody or herbaceous.

Woody stems: These have a high content of lignin and are hard. They are found in shrubs and trees.

Herbaceous stems: These contain no or less woody materials e.g. tomatoes, rice. The herbs are shorter than grass

2)Weak stems

These can’t support themselves upright but either creep or climb for support.

3)Underground stems

These are modified stems which remain permanently underground. They are often swollen and serve as food storage organs.

NB:

• Annual herbs only live for one year
• Bi –annual herbs live for two years
• Perennial herbs live for many years

MODIFICATION OF STEMS

Weak stems

1.Twinning stems (twinners)

These are stems that grow ascending spirally around a support. They are usually long and slender e.g. Dutch man’s pipe and lianas.

2.Climbing stems

These are stems that grow clinging to the support of other plants by means of tendrils. Tendrils are thin wire-like spirally coiled branches of certain stems. They may be modified at axillary buds e.g. in passion fruit plants or terminal buds.

3.Creeping stems (creepers)

These are long thin stems which grow along the surface of the ground, giving off roots at certain intervals of the nodes. Four types of creeping stems are;

a)Runners

This is a slender trailing stem lying flat on the ground possessing long internodes. A runner arisesas an axillary bud and creeps some distance away from the mother plant and grow into another plant e.g. oxalis.

b)Offset stems

This is a horizontal thickened short stem. It originates from the axil of the leaf and grows flat on the ground. It producesmany leaves above and a cluster of roots below e.g. water hyacinth and water lettuce.

4.Sucker

A sucker is a creeping stem that grows obliquely upwards, directly giving rise to a leafy shoot. E.g. banana, pineapple, sisal plant, etc

Underground stems

There are four types of underground stems namely:

1. Rhizome
2. Corm
3. Stem tuber
4. Bulb

1.Rhizomes

This is a horizontal thick underground stem having adventitious roots growing from the lower side of the nodes. It has terminal buds which develop into aerial shoots. It bears buds in axils of the reduced brown leaves called scale leaves.Rhizomes store a lot food for the plant. Some also act as organs for vegetative propagation e.g ginger, canalily, couch grass and Solomon’s seal.

2.Stem tuber

This is a short, fleshy underground stem swollen with large amounts of stored food. It has scale leaves and axillary buds which form the “eyes” e.g Irish potato, yams.

3.Stolon

A stolon is a horizontally growing stem that roots at the nodes and develops buds that grow into new plants. E.g. straw berry.

4.Bulb

A bulb is short conical-shaped underground stem comprising of thick fleshy leaves arranged in concentric circles. The thick fleshy leaves store food for the plant and are protected by outer dry brown leaves called scale leaves.

A terminal bud lies at the top of the stem and give rise to the aerial shoot. Axillary buds are situated between the leaf bases. Onions, garlic, tuberose, etc. are bulbs.

Structure of a bulb (onion)

5.Corm

A corm is a swollen fleshy underground stem that grows in a vertical direction. It is round-shaped and somehow flattened from the top to bottom. It has a terminal bud lying at the top of the stem and has scale leaves a rising from the nodes. Its roots grow randomly from the stem. Examples of corms are cocoyams crocus and yams.

INTERNAL STRUCTURE OF STEMS

Transverse section of a dicot stem

Internally stems have 3 main tissues;

1.Epidermis

It comprises of a single layer of cells which are brick-shaped. The outer wall of these cells is thickened by cutin, a waxy material which forms the outside skin of a stem called cuticle

• It protects the stem against water loss.
• It also protects the inner tissues of the stem from mechanical injury.
• It prevents entry of bacteria and germs into stem.

2.Cortex

This is the part of the stem between the epidermis and the vascular bundles. It’s made up of collenchyma, parenchyma and endodermis.

i)Collenchyma

This is the outer tissue of the cortex. It’s 3 or more cells thick. The cells are small, tightly packed and thickened at their corners. They offer mechanical support, hence strengthening and giving rigidity to the stem.

ii)Parenchyma

This is made up of large thin walled cells. These cells have air spaces between them called intercellular spaces. The spaces provide passage for water vapour and gases in the stem. Parenchyma cells offer support to the stem when filled with water and store some food.

iii)Endodermis

This is a single layer of rectangular shaped cells. It contains starch usually, and its main function is storage of food.

3.Vascular bundles

These are conducting or transporting tissues of a plant. They consist of xylem and phloem. The phloem lies externally and the xylem lies internally in each bundle.

Phloem:

The phloem conducts and transports manufactured food. It is made up of three main cells:

i)Sieve tubes

These are cylindrical tubes arranged end to end in long rows. Their cross-walls have many fine pores forming a sieve plate. They conduct manufactured food in the stem.

ii)Companion cells

These are smaller than the sieve tubes. They are filled with a dense cytoplasm and have nucleus. They control the activities of the sieve tubes.

iii)Phloem parenchyma

It stores some food in the stem. They are the first to be formedin the vascular bundle.

Xylem:

Xylem is water and mineral salts conducting tissue. It comprises of 2 types of cells i.e. vessels and tracheids. These cells have their walls thickened with a substance called lignin.

The xylem also provides mechanical strength to the stem due to the presence of lignified dead cells.

The lignified dead cells formed between the endodermis and phloem is termed as sclerenchyma.