TOPIC 2: GROWTH AND DEVELOPMENT | BIOLOGY FORM 6
GROWTH AND DEVELOPMENT
GROWTH AND DEVELOPMENT: Growth is a fundamental characteristic of living organisms. Development refers to an increase in complexity due to differentiation of tissues and organs (improvement in the functions of the body)
OR
Growth – is defined as an irreversible increase in dry mass of living material.
OR
Growth is a permanent or irreversible increase in dry mass of protoplasm due to synthesis of proteins.
Dry mass is mass without water.
Why dry mass?
By specfifying dry mass we can ignore the short term fluctuation in the water content of the cells for instance when the plant cells take in water by osmosis. The reverse process can take place when cell lose water.
Other definitions of growth.
1. Growth as increase in size. This definition is inadequate some organisms e.g. plants can increase in size as they take in water by osmosis, but this process may be reversible when they lose water.
2. Growth as increase in cells number
This definition is inadequate when the zygote divides repeatedly to form a ball of cells, they early embryo, there is an increase in cells number without increase in size of daughter cells.
In some cases you can increase in size without increase in cells number e.g.: in the region of elongation behind the root and shoot tips.
3. Growth as increase in number of individuals (population of the single- celled organisms e.g. Micro – organisms such as bacteria)
Development: The process of development is so closely linked with growth that the phase “growth and development” is common used to the process which are normally thought of as growth. Development refers to an increase in complexity due to differentiation of tissues and organs (improvement in the functions of the body)
Growth can be regarded as change in the quantity as development is the change in quality.
Factors that affect growth
There are both external and internal factors that control growth.
External factors Internal factors
- Food/ nutrients 1.Genes
- Diseases 2.Hormones
- Temperature 3.Enzymes
- Oxygen
- Light
- Space
- Toxins
- Soil
- Carbon dioxide(CO2)
Patterns of growth
1. Positive and negative growth
Positive growth – it occurs when synthesis of materials (anabolism) increase break down of materials (catabolism)
Example of positive growth in plants, the production of seedling which involve increase in cells number, cells size, fresh mass, length, volume and complexity of form as the seedling starts to photosynthesize and make up its own food.
Negative growth – occurs, when catabolism exceeds anabolism. Example increase in dry mass of germinating seeds
2. Allometric and isometric growth
Allometric growth occurs when organs grow at different rate. This produces a change in size of the organism which is accompanied by a change in shape of organism.
The pattern of growth is characteristic of animals. In almost all animals last organs to develop and differentiate are the reproductive organs. In man, the heart, brain and gonads all have different growth rate.
3. Limited and unlimited growth
Limited growth(definite/ determinate) – is the type of growth which shows a seization in growth when an organism matures and reaches a reproductive age. For example growth in annual plants.
The graph of Annual plants
Unlimited growth is a type of growth which occurs throughout the life of an organism. This growth occurs mostly in perennial plants. It is characterized by a series of sigmoid curves.
Measuring growth.
Growth can be measured using different parameters e.g. Length/ height, mass (dry/ fresh), weight, volume, area.
The curves show the overall growth pattern and extent of growth. It has found that growth pattern in many organism tend to be the same regardless the parameter used in measuring growth. In many cases if there is increase in measurable parameter is plotted against time a S- shaped growth curve is obtained. The shape of these curves is described as sigmoid, meaning S- shaped. The term sigmoid is derived from the Greek word sigma meaning letter S. A sigmoid curve is divided into four (4) parts or phases.
The sigmoid growth curve.
1. Lag phase
This is the initial phase during which little growth occurs (slightly decrease in growth).In flowering plants this phase show a slight decrease in growth. This is the result of loss of dry mass during seed germination.
In microorganism a few may die at the time of innoculation to the culture medium therefore showing decrease in number. Due to this phase the population of micro organism can grow rather slowly because they may have been in dormant state and time is required before their metabolism begin to work efficiently. Other reason for little growth for micro organism may be adjustment to the new diet.
2. The log phase or exponential phase.
This refers to the grand period of growth during which growth proceeds exponentially. During this phase the rate of growth is at maximum. The rate of growth is proportional to the amount material or number of cells or organism already present. In microscopic organisms this phase occurs when there is no limiting growth. Nutrients and oxygen are in plentiful supply umple space is available. In flowering plants is the period when green follicles increase in amount.
3. Stationary phase
This marks the period where the overall growth has ceased .The parameters under consideration remain constant In micro organisms it is the phase when the number in the culture stabilize besides they neither decrease nor increase i.e. the number of individual dying are approximately equal to the number of new individual formed.
4. Decelerating phase (decline phase)
This is the period in which growth become limited as the result of the effect of some internal and external factors, or the interaction of both. In many mammals including humans, this marks the period of negative growth. It is a period of senescence associated with increasing age.
In micro organisms which are grown in a confined environment, this is the period where the carrying capacity of the environment declines and it is unable to support the high density of organisms. Nutrients are decreasing and excretory products are increased in the medium. The rate of growth keeps on decreasing until all organisms die as a result of starvation, shortage of oxygen or presence of waste products in toxic amount.
Diagram showing a sigmoid curve
- Growth in arthropods
Growth in arthropods occurs in a series of stages (instars). These series of stage show sudden changes in weight or length. This pattern of growth is known as intermittent growth, or discontinuous growth. Each growth stage is called an instar.
Reasons for intermittent growth.
All arthropods have an exoskeleton made up of hard chitinous cuticle, which prevents overall growth of the whole body. The exoskeleton is shade periodically in the process called moulting or ecdysis to allow growth.
The new cuticle underneath in soft enough to allow growth to take place. The cuticle later hardens making growth impossible until cuticle is shed again. It is for this reason; growth occurs in spurts interrupted by series of moults.
Diagram showing growth in arthropods
Hormonal control of ecdysis or moulting in insects
Moulting or ecdysis is controlled by a moulting hormone (mH) or ecdysone which is released in response to a specific stimulus. The moulting hormone is secreted by the thoracic gland. The production of ecdysone by thoracic gland is stimulated by certain hormone produced by neurosecretory cells in the brain.
Moulting hormone is a steroid. It brings about shedding of the cuticle and growth of an insect. Growth of insects is accompanied by series of mouths. Moulting hormone cause the secretion of moulting fluid immediately beneath the cuticle.
Growth in insects.
The process of growth in insects involves changes in body for involving number of stages in their life cycle i.e. from the young to the adult form. The changes of forms from the young to the adults are referred to as metamorphosis.
Metamorphosis is found also in other groups of organisms such as amphibians, molluses, crustaceans, termatodes, cestodes and echinoderms to mention a few. In these organisms the term metamorphosis applies to those rapid changes which occur during the transition from larva to adult form.
Metamorphosis in insects.
These are two (2) types of metamorphosis
1. Complete metamorphosis
In this type, the life cycle of an insect passes through a series of four (4) stages i.e. Egg, larva, pupa and adult form.
2. Incomplete metamorphosis (hemimetabolous)In this type of metamorphosis, an insect pass through series of three (3) stages where the young resembles the adult. The insect passes into three (3) life stages i.e. the egg, nymph and adult.
Insects which exhibit this type of metamorphosis are known as hemimetabolous insects e.g. Cockroaches. Grasshoppers and mosquitoes.
Hormonal control of metamorphosis in insects
In insects, a successive moults lead to an insect to acquire either suddenly, or gradually, the features, characteristic of adults .The process of metamorphosis in controlled by two hormones .
- The moulting hormone.
- Juvenile hormone.
Metamorphosis is suppressed by the, juvenile hormone secreted by the gland called corpus allatum in the brain.
In the presence of juvenile hormone in the blood, epidermal cells under the influence of moulting hormone produce cuticle characteristic of juvenile stage. These are the nymph or larva as the case may be in order words, juvenile hormones inhibit metamorphosis and especially causes, the retention of larval characters in the suppress gene responsible for producing adult structure.
At metamorphosis the corpus allatum stop secreting its juvenile hormone and the moulting hormone in the absence of the juvenile hormone cause of epidermal cells to lay down the adult type of cuticle.
Summary
- For shedding of the cuticle the moulting hormone is required.
- Moulting hormone accompanied by juvenile hormone Cause epidermal cells to produce a larval cuticle.
- Moulting hormone alone without the juvenile hormone cause it to produce adult cuticle.
The process of growth
The growth of a multicellular organism can be divided into three (3) phases.
- Cells division – an increase in cell number as a result of mitotic division and cell division.
- Cell expansion– is the irreversible increase in the cell size as a result of the uptake of water in the synthesis of living materials.
- Cell differentiation – the specialization of cells.
a) Cell Division
Cells are formed from pre – existing cells by the process of cell division. Cell division strictly is the process of division of the cell cytoplasm into two (2) daughter cells. The two (2) daughter cells share the same structures (organelles) which are duplicated before the cytoplasm start dividing.
The two (2) major events in the information of new cells include.
- Division of the nucleus ( nuclear division )
- Separation or distribution of the cytoplasm between the daughter cells. The division of the nucleus is known as karyokinesis and the separation of the cytoplasm is known as cytokinesis.
Nuclear division
There are (2) two types of nuclear division
1. Mitosis – is the process by which the cell nucleus divides to produce the two daughter nuclei containing identical sets of chromosomes to the parent cells.
OR
Mitosis is the type of nuclear division that maintains a diploid number of chromosomes in the daughter cells.
Mitosis occurs in somatic (body) cells. It leads to the formation of body cells.
2. Meiosis – is the process by which nucleus divided to produce four (4) daughter nuclei each containing half number of chromosome of the original nucleus.
Meiosis is alternatively known as reduction division since it reduces the number of chromosomes in the cells from the diploid number (2n) to haploid number (n)
Meiosis occurs in gonads. It leads to the formation of sex cells.
NB: nucleus division principally involves the distribution of chromosomes in the daughter cells. Chromosomes are the most significant structures in the cells during cells division since they are responsible for the transmission of the hereditary information from generation to generation.
The cell cycle
Refers to the sequence of events which occur between the formation of a cell and its division into daughter cells. The cell cycle has three (3) main stages.
Interphase
Interphase is the period of intense synthesis and growth. The cells produce materials required for its own growth and carrying out other functions. Interphase is further divided into:
i.G1 (Gap one) or first growth phase
G1 is a phase which characterized with:
- Production of mitochondria, chloroplasts (in plant ),lysosomes, ER, Golgi complex, vacuoles etc
- Formation of structural and functional; proteins.
- Production of RNAs
- Ribosomes are synthesized
- Metabolic rate of the cells becomes very high.
ii.S (synthetic phase)
- DNA synthesis takes place
- Production of histones that cover each DNA strand
- Chromosomes become as two(2) chromatids
iii. G2 (Gap two)second growth phase
- Centriole replicates
- Mitotic spindle start to form
- Energy store increases
- Intensive cellular synthesis (synthesis of RNA and protein)
- Mitosis (M) -is the process of nuclear division involving the separation of chromatids and their redistribution as chromosomes into daughter cells.
- Cell division – is the process of division of the cytoplasm into two (2) daughter cells.
Process of mitosis
Mitosis is a continuous process which occurs in four (4) active stages. These stages are the prophase, metaphase and telophase. An intermediate stage the interphase occurs between one cell division and another. The following are mitosis stage in animals
1. Prophase
This is the longest phase of mitosis division. Behavior of the chromosome is as follows;
- Chromosome appears as pair of chromatids joined by centromere.
- Nuclear membrane tends to disintegrate.
- Nucleoli start to disappearing
- Centrioles move to the opposite pole
- Microtubules radiate from centrioles called astars.
2. Metaphase
- formation of spindle fibres (asters)
- Pairs of chromatids attached to spindle at the centromere.
- Nuclear membrane and nucleoli disappear
- Chromosomes line up at the equator of the spindle.
3. Anaphase – is a very rapid stage.
- The centromere splits into two (2)
- Daughter centromeres are pulled to the opposite sides by spindle fibre.
- Separated chromatids are now called Chromosomes, are pulled a long behind the centromeres.
4. Telophase
- Chromosomes reach the poles of the cell.
- Chromosomes uncoil, lengthen and form chromatin network.
- Spindle fibres disintegrate. Each centriole then replicates
- Nuclear membrane reappears and nucleoli reappear
- Leads to cytokitnesis.
Cell division (cytokinesis.)
Cell division is a process of division of the cytoplasm into two (2) daughter cells. In preparation for division the cells organelles become distributed into the two (2) cells. After the nuclear division (karyokinesis.) the cytoplasm is divided into two (2) (more or less) equal parts. The cytoplasmic division differs in animal and plants cells.
Cytokinesis in animals.
The cells membrane begins to invaginate where spindle equal was present earlier. The cell membranes of opposite ends meet at the centre and cell divides into two (2) daughter cells.
Cytokinesis in plants
In plant cells the spindle fibres do not disappear at the region of equatorial plane, they increase in number and form cell plate across the equatorial plane. As the plate gradually become more distinct and develops into the new cell, it divides the cell in two (2).
Difference between mitosis in plant and animals
| plants | Animals | |
| 1 | No centriole present. | Centrioles present. |
| 2 | No aster forms. | Aster forms. |
| 3 | Cells plate forms. | No cell plate forms. |
| 4 | No furrowing of cytoplasm at cytokinesis | Furrowing of cytoplasm at cytokines. |
| 5 | Occurs mainly at meristems | Occur in tissues throughout the body. |
Significance of mitosis.
1. Growth and development
Mitosis is a basic component of growth as its leads to increase in number of the body cells.
Body repair -the worn-out cells are replaced by the formation of new cells by mitosis.
The newly formed cells by mitosis have opportunity of differentiation forming of complex body.
2. Genetic stability
Mitosis produce the nuclei which have the same number of chromosome as the parent cells more over since these chromosomes were derived from parental chromosomes by exact replication of their DNA,they will carry the same hereditary information in their genes.
In other words , the daughter cells are genetically identical to their parent cells and no variation in genetic information is introduced during mitosis .
3. Asexual reproduction
Many animals and plant species are propagated by asexual method involving the mitotic division of cells alone .
4. Regeneration
The ability of some organism to replace the lost parts of the body such as legs in crustacean is brought about by the action of mitosis.
5. Seed germination
Germination – is defined as the onset of growth of the embryo in seeds
Or Germination is the transformation of seed in to a seedling
Environmental conditions needed for germination.
1. Water
Water is required to activate the biochemical reactions associated with germination. Many biochemical reactions in the germinating seed take place in aqueous solution.
Water is also an important reagent in hydrolyzing the store food. Water enters the seed through the micropyle and the seed coat or testa by process called imbibition.
2. Temperature
For the seed to geminate there are minimum or optimum temperatures required. The temperature for seed germination range from 5 to 40’c. The temperature influences the rate of enzyme controlled reactions.
3. Oxygen
Oxygen is the required for aerobic respiration , the process where food material are oxidized to release energy in the cells .
In cases aerobic respiration can be supplemented with anaerobic respiration.
Physiology of seed germination
Seeds store food materials such as carbohydrates, lipids, proteins, mineral salts and vitamins. The large food reserves in seeds are the lipids and carbohydrates. Starch is the major food reserves of grasses and cereals. Legumes are very rich in proteins.
The food materials are stored in the endosperm in absence of the endosperm food in seeds is stored in the cotyledons of the embryo for this reasons we have endospermic seed and non – endospermic seed
In bean seeds, the cotyledons have been modified for food the storage of food. The stored food is used to provide energy and raw materials for building the tissue before the new seedling is able to photosynthesize.
The events leading to food germination can be summarized as follows
1. The water taken in by imbibitions and osmosis hydrates the food reserves which results into activation of enzymes of respiration. Other enzymes are synthesized possibly using amino acids provided by the digestion of stored proteins.
2. Digestion of food reserves hydrolysis .The soluble products of digestion are then translocated to the growth regions of the embryo.
3. Break down of food substrates ( food reserves and the products of digestion) to release energy used in both storage tissue and growing embryo . This involves oxidation of substrate usually sugar to carbon dioxide and water.
4. Respiration account for loss of dry mass in seeds due to the loss of sugars. Water is not counted in the loss of dry mass as water is excluded in accounting the dry mass. The respiration rates in both endosperms or other storage tissue and embryo are high owing to the intense metabolic activity in these regions . The loss in dry mass continues until the seedling produces green leaves and starts to make its own food.
The graph below summarize the changes in dry mass of the endosperm and embryo during germination
Types of germination.
There are two types of germination according to whether or not the cotyledons grow above or remain below it.
1. Epigeal germination
This is the type of germination when the cotyledons are carried above the ground. In dicotyledons, the part of the shoot axis or internode just below the cotyledon the (hypocotyl ) elongates carrying the cotyledon above the soil, in epigeal germination the hypocotyle remains hooked as it grows through the soil , meeting the resistance of soil rather than the delicate plumule tip which is further enclosed and protected by cotyledon. The hypocotyl strengthens immediately on exposure to sunlight.
2. Hypogeal germination
This is the type of germination where cotyledons remain below the ground. The internode just above the cotyledons ( the epicotyl) elongates and therefore the cotyledons remain below the ground. In hypogeal germination of
GROWTH OF THE EMBRYO
The first sign of the embryo growth is the emergence of the embryonic root, (the radical).This grows down and anchors the seed. The radical is positively geotropic.
Then it follows the emergence of the plumule which grows upward and it is positively phototrophic.
Primary and secondary growth in flowering plants.
With exception of the young embryo, growth in plants is confined to certain regions known as meristems .
Growth in plants is said to be localized i.e. confined to specific regions such as root and shoot tips.
Meristems
A meristem is a group of cells which retain the ability to divide by mitosis, producing the daughter cells which grow and form the rest of the plant body. The cells that have lost the ability to divide form the permanent tissue.
Meristems are also known as initials. There are three types (3) of initials. The classification is based on their location. These include:
1. Apical meristems
This is a type of meristem located in the root and shoot apex. They are responsible for primary growth, giving rise to primary plant body. The effect of apical meristem is to cause increase in length.
2. Lateral meristems (the cambium.)
These are laterally situated in older parts of plants parallel with long axis of organs E.g. cork cambium (phellogen) and vascular cambium. They are responsible for secondary growth. Vascular cambium gives rise to
secondary vascular tissues, phellogen gives rise to the periderm which replaces the epidermis and includes cork. The Effect of lateral meristem is to cause increase in girth.
3. Intercalary meristems.
These are found between regions of permanent tissues E.g. at nodes of many monocotyledonous plants in the bases of grass leaves. Intercalary meristems allow growth in length to occur in regions other than tips. This is
very useful if the tips are susceptible to damage or destruction. E.g. being eaten by herbivores. Branching from the main axis is not then necessary.
Types of growth in plants.
1. Primary growth
This is the first form of growth which results into the plant increasing in length. This is the only type of growth occurring in most monocotyledonous plants and herbaceous dicotyledonous. Primary growth is a result of the activity of the apical and sometimes intercalary meristems.
2. Secondary growth
This is the growth which occurs after primary growth as a result of lateral meristems characterized by deposition of new phloem and large amount of secondary xylem called wood. Secondary growth results into increase in girth. Secondary growth is a characteristic feature of trees and shrubs. A few herbaceous plants show restricted amount of secondary thickening.
Primary growth
Primary growth in shoots. The shoots apex can be distinguished into four (4) regions. These are the regions of cells division, region of cell elongation, region of cell differentiation and region of permanent tissue. The cells become progressively older as you move from the apical meristems.
The region of cell division.
The apical meristem is dome shaped. The meristems cells are distinguished into the protoderm, which give rise to the epidermis, the ground meristems which produce parenchyma ground tissues which form the cortex and pith in dicotyledons, and the procambium which gives rise to the vascular tissues, including pericycle, phloem, vascular cambium and xylem.
Characteristics of Apical meristems
- The cells are relatively small, cuboids with thin cellulose wall and dense cytoplasm content.
- They have few small vacuoles.
- They are packed tightly together with no obvious air space between cells.
- They lack chloroplasts’.
- When they divided by mitosis, one daughter cell remains in the meristem while others increase in size and differentiate to become part of permanent plant body.
Zone of expansion or cells elongation.
The daughter cells produced by initials increase in size mainly by osmotic uptake of water into these cells. Increase length of stems and root is mainly brought about by elongation of cells during this stage.
The expansion of cells in addition is due to thickening of the cell wall either by cellulose or lignin depending on the type of cell being formed.
Zone of cells differentiation
The process of differentiation is initiated from the procambium. This gives rise to the protoxylem in the inside and protophloem on the outside which are part of the primary xylem and primary phloem respectively. Between the xylem and phloem, there are cells that retain the ability to divide. They form the vascular cambium.
Diagram of shoot tip showing apical meristem
Formation of leaves and Lateral buds.
Growth and development of the shoot also includes growth of leaves and lateral buds. Leaves arise from small swellings or ridges containing groups of meristematic cells.
These swellings or ridges are called primordial. The primordial appear at regular interval, the side of origin being called nodes and the region between the internodes.
The nodes can be arranged in specific pattern or arrangement on the stem. E.g. As whort, singly or spirally. The primordial elongates rapidly, as a result, it soon encloses and protects the apical meristems both physically and by
heat they generate in respiration. They later grow and increase in area to form the leaf blade.
The Lateral bud (auxiliary bud)
They are under control of apical meristems. The lateral buds develop in the axis of the leaves and stem.
A. Primary growth in roots.
The growth region of the root is distinguished into:
1. The root cap.
The root cap forms the outside of the apical meristems. It is made up of the parenchyma cells. It protects the apical meristems as the roots grow through the soil. The cells of the root cap are constantly being worn away and replaced. The outer layer of the root cap has mucilage which makes it slimy in order to reduce friction. The root cap also has the important additional function of acting as gravity sensors.
2. The zone of cells division.
The zone of cells division is distinguished into the following:
3. The quiescent centre
This forms the very tip of the apical meristems. The quiescent centre is composed of group of initials (meristematic cells) from which all other cells in the root originate. The cells in the quiescent centre has lower rate of cells division in comparison with the surrounding daughter cells.
The protoderm ground meristems and procambium.
These are different types of the apical meristems which follow below the quiescent centre. The functions of these cells are the same as the in shoots.
The protoderm form the epidermis, the ground meristems form the cortex, including endoderm and the procambium which form primary phloem, vascular, primary xylem, pith and the pericycle if present. The procambium in roots is used to describe the central cylinder in roots.
1. The zone of cells Elongation.
As in shoot the zone of cell division is followed by a zone of cell elongation. Growth in this region is brought by cell elongation due to osmotic uptake of water in the cytoplasm and then into the vacuole. The zone of elongation cells extends to about 10mm behind the root tip. The increase in length of these cells forces the root tip down through the soil.
Diagram of Root tip showing apical meristem
2. The zone of cells differentiation
This is a zone where each cells became fully specialized for its own particular function. In this region the phloem sieve element begin to differentiate. The development of phloem is from outside inward and become progressively more mature further back from the root tip.Xylem starts differentiating further back in the same manner as phloem that is from outside inwards (exarch xylem). The first to differentiate are the xylem
vessels, starting with the protoxylem vessels which transform into the metaxylem and later into mature xylem. The xylem in roots spreads to the centre of the root in which case no pith develops. Further differentiation in this region includes the development of the root hairs from the epidermis .
Formation of lateral root of adventitious root and adventitiousbuds.
Lateral roots : These are roots that arise from the main root formed by the resuming of the meristematic activity of the pericycle cells . This is in contrast to the formation of the buds in the shoot.
In the root a small group of the pericycle cells in the zone of differentiation resume meristematic activity and forms a new root epical meristem which grows forcing its way out through the endodermis , cortex and epidermis.
Adventitious roots and buds – Adventitious are those growing in uncharacteristic position formed by a certain cells resuming meristematic activity . Examples are the adventitious buds and roots.
- Adventitious roots – they develop independently of the original primary root and form the main rooting system of monocotyledons arising from the nodes on stem.
- Adventitious roots are important in the propagation of plants by stem cutting. Some plants like Ivy use adventitious root to cling.
- Adventitious buds –adventitious buds may develop on roots, steam or leaves. In trees can develop new branches adventitiously from buds arise in the trunk.
B. Secondary growth
This is the growth which occurs after primary growth as a result of the activity of lateral meristems .Secondary growth results in an increase in girth. It is associated with deposition of large amount of xylem called wood. The wood gives charactestic feature of trees and shrubs. Secondary growth is brought about by two (2) types of lateral meristems the vascular cambium which give rise to new vascular tissue cork cambium or phellogen which arises later to replace the ruptured epidermis of expanding plant body .
The activity of the vascular cambium.
There are two types of cells in the vascular cambium these are;
a) The fusiform initials. These are narrow , elongated cells which divide by mitosis to form secondary phloem to the outside or secondary xylem to the inside . The xylem material produced exceeds the amount of phloem.
b) Ray initials
These are almost spherical in shape and divide mitotically to form parenchyma cells. Parenchyma accumulates to form rays between neighboring xylem and phloem.
Diagram of Fusiform and Ray initials
Secondary growth in woody dicotyledonous stem
Secondary growth or thickening in stem is brought about by deposition of large quantity of secondary xylem and lesser quantities of secondary phloem by fusiform initials of the vascular cambium.
The vascular cambium is originally located between the primary xylem and primary phloem of the vascular bundles. This is called lutoa fascular cambium. The vascular bundles of dicotyledonous stems are arranged in form of a ring. When the primary xylem and primary phloem are first differentiated, there is no cambium across the pith or a medullary ray which lies in between the edges of the cambium within the bundles divide accordingly and form a layer of cambium across the medullary rays.The newly formed cambial strip which occurs between the gaps in the bundles is called interfaxular cambium. I.e. the cambium in between the two (2) vascular bundles. The complete cambium ring is formed.
The formation of secondary xylem and secondary phloem.
The cambial layer consists of essentially one layer of cells. These cells divide in a direction parallel with epidermis.
Each time a cambial cell divides into two, one of the daughter cells remains merstematic, while the other is differentiated into permanent tissues. If the cell that is differentiated is next to the xylem, it forms xylem while if it next to the phloem it becomes phloem.
The xylem is formed towards the inner side while the phloem towards the outside of the cambium. The cambium cells divide continuously in this manner producing secondary tissues on both sides of it. In this way, new cells are added to the xylem and phloem, and the vascular bundles increase in size.As the stem increase in thickness the circumference of the vascular cambium layer need to increase. This is achieved by radial division of the cambial cells.
Formation of medullary rays.
Medullary rays are formed by ray initials. These are parenchyma four cells that run all the way from the pith or medulla to the cortex.
The pith or medulla forms the central region of the stem of the dicot plant and roots of monocots. The extension of the pith in form of narrow parenchymarous strips are called medullary or pith rays. In some stem the pith is obliterated to form hollow. The medullary rays extend between thee vascular bundles. These are primary and secondary Medullar rays. The primary medullary rays are produced by original ray initials and secondary Medullary rays which are produced by later ray initials.
Function of Medullar rays.
The rays maintain a living link between the pith and cortex. They help to transmit water and mineral salts from xylem and food substance from the phloem rapidly across the stem.
Annual rings.
Annual rings or growth rings refers to the concentric layers of secondary xylem in the stem of the perennial plants each one of which represents a seasonal increment or different phase in deposition of new xylem tissues.
In transverse section of the axis these layers appear as rings and are called annual rings or growth rings. They are commonly termed as annual rings because in woody plants of temperate regions and those of tropical regions, where there is annual alternation of growing and dormant periods each layer represents the growth of one year.
By observing the pattern of annual ring one can pin point the time during which the wood was growing. Dendrochronology is the dating of wood by recognition of pattern of annual rings.
Heart wood and sap wood
The heart wood
This refers to the central region of the old tree where the xylem tissue have ceased to serve as conducting function and become blocked with darkly staining deposits such as tannins, gums, resins and other substance which make it hard and durable. It looks black due to the presence of various substances in it.
Cork and lenticels.
Cork (phellogen)
As the cork cells mature, their walls become impregnated with fatty substance called suberin which is impermeable to water and gases. The cells gradually die and lose their living contents which become filled with either resins or tannins.The cork cells fit together around the stem to prevent dessical infection and mechanical injury.
These are slit- like openings containing mass of loosely packed walled dead cells lacking suberin found at random intervals in the cork. The lenticels are produced by the cork cambium and have large intercellular air space allowing gaseous exchange between the stem and the environment.In the absence of lenticels it would be difficult for gaseous to take place in the stem as the cork which surrounds the stem do not allow air to pass.
Other tissues formed from cork cambium.While the cork (phellem) is produced to the outside of the cork cambium, in the inside one or two (2) layers of parenchyma are produced. These are indistinguishable from the primary cortex and form, the phelloderm or secondary cortex. The phellogen (cork) and phelloderm together comprise the periderm.
Diagram of lenticel.
The Bark.
The term bark is used to refer either to all the tissue outside the vascular system or strictly to those tissues outside the cork cambium, in either primary or secondary state of growth.
The bark cover the woody stem, peeling bark from a tree generally strips tissue down to the vascular cambium. The bark is composed of dead cells together with the cork layers.
SEED DORMANCY
Refers to a condition where seed will not germinate despite the presence of those environmental conditions for germination.
Causes of seed dormancy
1. Immaturity of the embryo
Newly harvested seed need some period of time for the embryo to become mature .The seed undergo some internal transformation before it can be able to germinate .This period where the seed undergo internal changes for maturation is called the after ripening. To terminate this type of seed dormancy allow the seeds to have enough period of time before they can be sown again .
2. Hardness of the testa
Hard seed coat or testa makes it impermeable to water and oxygen or being physically strong enough to prevent embryo growing.
How to break this type of dormancy.
- physical damage or scarification to the seed coat
- By action of micro organism such bacteria.
-soaking for a long period of time and by chemical action in the soil.
BREAKING OF SEED DORMANCY
The mechanism of breaking seed dormancy depends on the type of dormancy under consideration.
For primary Dormancy.
- The seed need to be stored for long periods until the embryo matures
- Wetting of seeds with appropriate solvents such as water and gibberellins so as to remove growth inhibitors.
- Removal of the testa by mechanical secretion or by weakening chemicals such as enzymes so as to enable the emergence of the radical and plumule.
- Using organic solvents to dissolve the waxy coating over the testa rendering it permeable to gases and water.
- Soaking the seeds in water for sometime soften the testa and therefore makes it permeable to water and oxygen
For Secondary dormancy.
- The seeds have to be supplied with appropriate temperature. This may be by chilling in deep freezers / refrigerators or by temperature shocking in which the seed are either boiled for a while or washed in hot water.
- The seeds have to be provided with specific light intensity so as to rise the levels of gibberellins to work (activated)
- They have to be provided with enough moisture and oxygen.
SEED VIABILITY AND GERMINATION
A. SEED VIABILITY
Viability is the capacity of the seed to remain capable of germination when conditions are not favorable.
A viable seed is that which is capable of germinating when all causes of dormancy are broken.
Seed viability ensures that the seed will germinate once supplied with all conditions necessary for germination and all causes of seed dormancy are broken.
Factors governing seed viability:-
1. Seed maturity
Immature seeds die, thus when sown they never germinate since they are inviable due to the fact that their embryos are not completely formed after seed formation, seeds need time to completely form their embryo so as to be able to carry active growth.
2. Activity of enzymes
Since germination is an enzyme controlled process, depends on enzymes for its take off.
In the seeds where enzymes are inactive, they cannot be activated, germination is impossible and the seeds are therefore inviable
3. Storage condition of
4. State of health of the seed
Temperature: Optimum temperature is acquired for the seed in the store for them to remain viable. Extremely high temperature denatures the enzymes in the seeds. This results into failure of seeds to germinate
Moisture: high moisture content of storage site can cause seed to hence lose their viability. Dry atmosphere is the preferred one.
Aeration: Enough supply of oxygen is required to cater for the minimum metabolism in seeds. Lack of aeration will hinder metabolic reactions in the seed and seeds may lose their viability
This diseased seed may lose its viability as its embryo may be infected by fungi or bacteria.
5. Time of storage of seeds.
This varies from seed to seed and from species. Most of the annual plant seeds lose their viability in a period of one year.
However there are other seeds e.g. those of cassia bicaspsularis and cassia maltijuge retain their viability for about 115 and 158 years respectively.
Also long storage may result into seeds being destroyed by insects.