TOPIC 2: GROWTH AND DEVELOPMENT ~ BIOLOGY FORM 6
Growth is a fundamental characteristic of living organisms.
Growth – is defined as an irreversible increase in dry mass of living material.
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.
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.
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
Patterns of 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.
Isometric growth this occurs when organs grow at the same rate. In this situation, change in size of the organism is not accompanied by a change in shape, or external form of an organism.
This type of growth pattern is seen in fish and certain insects such as locusts
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.
The graph of perennial plants
Growth can be measured using different parameters e.g. Length/ height, mass (dry/ fresh), weight, volume, area.
These are graphs obtained when data obtained from different parameters of measuring growth are plotted against time.
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.
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.
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.
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.
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 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.
This dissolves the soft inner part, mean while the new cuticle soft first in secreted by epidermis.
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
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.
Insects that exhibit this type of metamorphosis are referred to as holometabolous insects. E.g. Butterflies, houseflies, Moths and bettles.
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 .
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.
The process of growth
The growth of a multicellular organism can be divided into three (3) phases.
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.
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.
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 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:
ii.S (synthetic phase)
iii. G2 (Gap two)second growth phase
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
This is the longest phase of mitosis division. Behavior of the chromosome is as follows;
3. Anaphase – is a very rapid stage.
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
. Significance of mitosis.
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.
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
Many animals and plant species are propagated by asexual method involving the mitotic division of cells alone .
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.
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.
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.
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.
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
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.
Types of germination.
There are two types of germination according to whether or not the cotyledons grow above or remain below it.
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.
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
dicotyledons the epicotyledons is hooked to protect the plumule tip .it immediately straightens on exposure to sunlight. In grasses which are monocotyledons, the plumule is protected by a sheath called coleptile. The first leaf
grow out through coleptile and unrolls response to light.
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.
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:
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.
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.
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.
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 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
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)
These are small groups of meristematic cells which normally remain dormant but retain the capacity to divide and grow at later stage. They form branches or specialized structures such as flowers. Lateral buds also form
underground structure such as Rhizomes and tubers.
They are under control of apical meristems. The lateral buds develop in the axis of the leaves and stem.
Primary growth in roots.
The growth region of the root is distinguished into:
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.
The zone of cells division is distinguished into the following:
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.
b) 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.
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
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 adventitious buds.
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.
2) 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.
Diagram of Early stages in secondary thickening of typical woody dicotyledon stem.
a. Primary structure of stem.
b. Cambium forms a complete cylinder.
c. a complete ring of a secondary thickening has developed.
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 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.
This refers to the outer region of the old trees consisting of recently formed xylem element. This is of light color and some living cells. This part of the stem performs the physiological activities such as conduction of water and nutrients, storage of food.
Cork and lenticels.
This refers to the tissue formed by activities of a secondary lateral cambium or the Cork cambium to replace the ruptured epidermal cells. It is immediately below the epidermis. The rupturing of the epidermis is a result of increasing circumference of the stem due to outward growth of the secondary xylem.
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 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.
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 .
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.
-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.
For Secondary dormancy.
SEED VIABILITY AND GERMINATION
a). SEED VIABILITY
Factors governing seed viability:-
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.
3.Storage condition of
4.State of health of the seed
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.