Growth is defined as the irreversible increase in size and dry mass of an organism. Growth of a multicellular organism can be divided into 3 i.e.;
- Cell division i.e. an increase in cell number of a result of mitosis.
- Cell expansion. An irreversible increase in cell size as a result of the uptake and assimilation of materials and water leading to the synthesis of the protoplasm.
- Cell differentiation- Specialization of cells and the cells don’t divide any more in this process.
Growth is usually accompanied by an increase in the complexity of the organism by the formation of new tissues and organs. This is known as development and all of above processes take place at almost the same time. It is difficult to determine where one stops and the other beings.
Growth can be estimated by measuring some parameter of the organism. E.g. measure of dry weight, height, S.A, girth and fresh weight.
Allometric is the growth of the different parts of the body at different rates higher or lower than the growth rate of the body as a whole.
- The log phase. This is a phase of little growth.
- Log phase. The period of the exponential increase in growth.
- Inflexion point. The exponential increase begins to decline at this point. The rate of growth begins to decrease.
- Decelerating phase where there is decrease in the rate of growth which could be result of the effect of some internal or external factors or interaction of both.
- Plateau phase or stationary phase- This is when overall growth has ceased. In this phase, synthesis of new cells is equal to degeneration of the old cells or death of the old cells.
Intermittent growth in arthropods
The smooth curve is typical of most animals but there is an exception i.e. the arthropods.
If the weight changes of an insect are plotted against time, the curve shown below is obtained. Instead of increasing smoothly, growth takes place in a series steps. These correspond to a service of stages or instars in an insect’s development.
the graph shows increase in weigh of an insect over a period of a number of days. The sudden weights increases are marked by arrows correspond to the maulting of the cuticle. If it is terrestrial, it swallows air. If it is an aquatic insect or arthropod, it swallows water therefore expending the new soft cuticle before it hardens.
To intermittent growth is made necessary by the hard cuticle, the exoskeleton which prevents overall growth of the whole body. Periodically the cuticle is shed and only then can growth take place while the new cuticle underneath is still soft enough to allow the body to expand. In some cases, rapid expansion is achieved by the insect swallowing air or water. The distention of the gut pushes the soft integuments outwards, the new cuticle then hardens after which further growth is impossible until the cuticle is shed again. The shedding process is known as maulting (ecdysis) it is dissentious growth and it takes place by the secretion of a maulting fluid immediately beneath the cuticle. This dissolves the soft inner part of the cuticle leaving only the hand outer part. Meanwhile, the new cuticle, soft at first is secreted by the epidermis. Protected from the enzymatic action of the maulting fliud by its protective surface, it becomes the hard cuticle of the next instar. The cuticle is composed of chitin, a complex nitrogen containing polysaccharide. Hardening of the outer part is achieved by the chitin being impregnated with tanned proteins (hardened proteins).
Water proofing of the article is achieved by the deposition of a thin layer of wax at the surface.
An instar is the stage of an arthropod after maulting has been occurred and no growth takes place during this stage.
This is the change which occurs during the transition from larval to adult form and involves re-organisation of the body and break down of the larval tissues.
Some insects undergo incomplete metamorphosis eg cockroaches, gasshoopers, tsetse flies, ear wigs such insects are known as homimetabolus insects.
The cycle involves the egg which grows into the nymph which is similar to the adult but lacks wings develops into an adult which lays the egg.
Insects which undergo complete metamorphosis eg housefly, mosquito, bee flies, are known as holomotabolous insects.
Egg develops into larva pupa adult which lays the egg.
Insect metamorphosis is regulated by environmental factors like temperature. Light intenoity, humidity etc, amount of food, which influence hormonal and nervous activity within the insect. Food availability and certain light and temp conditions influence the neural secretory cells in the brain to release a hormone known as prothoracicotrophic hormone (PTTH) which passes down the axons to be stored in a pair of glands known as corpora cardiaca.
The stimulus to mault may vary from species to species but in all cases, it causes release of PTTH into the blood. The PTTH acts on two other glands ie;
- Corpus allatum
- Prothoracic gland
The prothoracic gland secretes a maulting hormone (MH) also known as ecdysone.
The corpus allatum produces another hormone known as the juvenile hormone or neotonin. The simultaneous release of both hormones influences the formation of a new cuticle from the epidermal cells of the larva or the nymph and also influences formation of the larval or nymph body form. As metamorphosis proceeds, the amount of ecdyson released falls. At a certain level of the juvenile hormone, pupal ecdysis occurs and at the time of the next moult, no more juvenile hormone is released and ecdyson causes development of the adult form. It has been demonstrated tat ecdyson influences a particular stages of development by acting directly on the genetic transcription.
The juvenile hormone exerts its effect by modifying this response according to its relative concentration with respect to ecdyson.
Growth and development in plants
Primary growth and differentiation of tissues in the stem.
This is the result of the apical moristem and within the apical moristem are 2 regions of cells. The outer one is known as the tunica and it is the coat of the meristem. The inner one is known as corpus and it is the body of the meristem. Cell division in the tunica results in the growth of the epidermis while that in the corpus results in the growth of the internal tissues. Cells in the corpus after vacuolation and expansion differentiate to form the parendyma, collenchyma and sclerenchym which make up the bulk of the stem i.e. the cortex and the pith. Others don’t vacuelate but they differentiate into the procambial strands where the cells retain the ability to divide.
After the procambial cells divide, on differentiation, these cell form the vascular tissue. Those on the outside produce the protophloem while produced by the tunica and the corpus of the meristem, these cells elongate and further away from the stem apex where the stem is nolonger elongating, further differentiation of the cells produced by the procambium produces the metophloem and metaxylem. Near the apex, both the tunica and the corpus result in production of leaf promodia, at regularly spaced intervals as the stem elongates. It is these that grow into leaves later. Within the leaf axil is a small axillary bud that produces lateral branches down the stem near the stem apex, secretion of the auxin by the apical meristem usually prevents lateral growth of the branches and results in apical dominancy.
Primary growth of the root
At the very tip of the apical meristem is a quiescent centre, a gap of initials from which all the other cells in the root can be traced but whose rate of cell division is much slower than the daughter cells in the apical meristem around them. To the outside, the cells of the root cap are formed and these become large parenchyma cell which protect the apical meristem as the root grows through the soil. They are constantly being worn away and replaced. They also have the important function of acting as gravity sensers since they contain large starch grains which act as statoliths.
A statolith is one of a number of large starch grains which lie on the lower most side of some plant cells.
The precambium cells divide and give to the 10 phloem, vascular cambium and the 10 xylem. The phloem on the outside and the10 xylem on the inside.
It has a quiescent centre which has the epical initials. Apical initials are merismatic cells. All other cells are formed from them.
Just behind the quiescent centre, the root apex shows 3 merismatic regions. Similar to the ones in the shoot apex.
(i) The protoderm
Gives rise to the epidermis and it even has the calyptrogen which gives rise to the root cap.
(ii) The brown meristem
Gives rise to the parenchyma of the cortex and the endodermis.
(iii) The procambium
Gives rise to the pericycle, the 10 phloem xylem as well as the vascular cambium if the plant is to undergo 20 growth.
In the shoot apex, the protoderm is the tunica, the corpus is made of the groundmeristem and the pracambium.
The root also has regions of cell division, cell elongatin and cell differentiation.
Some plants like the conifers and woody dicots last for several years ie they are perennial. Since such plants grow in length each year, their mass increase beyond that can be supported by their 10 tissues alone. They therefore overcome the problem by increasing their girth as their length increases and they are able to do this by producing 20 tissues in a process known as 20 thickening or 20 growth. 20 growth therefore is the formation of additional 20 vascular tissue by the activity of the cambium with accompanying increase in the diameter of the stem and root as well as providing addition conducting and supporting tissue for the growing plant.
20 growth is brought about by the meristematic cells of the cambium which are able to divide mitotically. The cambium is found in between the xylem and phloem.
First of all, groups of the cambium cells divide radially to from a cylinder or ring of cambium tissue known as the vescular cambium. It separates the xylem from the phloem.
When the ring has been formed, it can be known as the interfacicular cambium whose cells are known as the fusi form initials cells divide tangentially to form 20 xylem tissue on the inside and 20 phloem tissue on the outside. In between the adjacent vascular bundles, there is a type of vascular cambium cells known as the ray initials. These divide to form traps of parenchyma cells which form the 20 phloem and xylem and transport water and nutrients between them. Usually, more 20 xylem is formed than the 20 phloem with the result that the phloem and the cambium ring get pushed outwards.
The cambium cells divide radially in order to keep pace with the ever increasing circumstance of the plant stem or plant root. In temperature regions, growth in restricted to spring and summer. In the tropical and subtropical, it is throughout except in those areas which have the dry seasons. In the spring, the 1st formed xylem contains a high proportion of large vessels with relatively thin walls to carry the spring flow of the transpiration stream.
There is narrowing of the vessels and thickening of their walls as the summer progresses. Also, an increasing number of thick walled sclerenchyma fibres are formed. The result of this is a harder and denser wood being formed in summer which is different from the whiter and softer wood which is formed during spring. The consequence of this seasonal growth is the formation of a series of concentric annual rings which can be counted if the steam is severed. This gives an accurate method of estimating the age of the trees e.g. counting these annual rings in conifers in California has shown them to be over 3000 years. The increase in girth of the stem the surface tissue of the epidermis and eventually raptures them. But before this happens, just beneath the epidermis, there is a layer of the cells called the cork cambium. It can be known as the phellogen. This divides tangentially to form new surface tissues. Those cut off on the inside of the cork cambium form the 20 cortex (phelloderm). Those out off to the outside form a layer of cork cells known as the phellem. The 3 layers, the cork cambium, cork cells and 20 cortex are known as the periderm. The periderm and phloem are commonly referred to as the bark. That is why in ringing experiments translocation of food is inhibited because the bark has to be removed. The wall of the cork cells are impregnated with suberin. A fatty material which renders them impermeable to water and respiratory gases. Respiratory gases are exchangeable only at the lenticels. A lenticel is a loose muscle of cells formed in the corky layer of cells.
This is an inactive period of the life of an animal or plant during which growth slows or completely ceases. Physiological changes associated with dormancy help the organism survive advance environment conditions.
Annual plants survive the winter as dormant seeds while perennial plants survive as dormant seeds, tubers, bulbs, rhizome, corms etc.
Hibernation and aestivation are forms of dormancy in animals that enable them survive extremes of cold and high temperatures respectively. In these states, there is lowering of body temperature, slowing down of feeding, respiration, movement and other bodily activities.
This is when a viable seed fails to germinated even when all the conditions necessary for germination are provided. In this state, its metabolic activity is greatly reduced and its moisture content is very low.
Causes of seed dormancy
(i) Seed coat
Could be hard and impermeable and prevent the entry of water and 02.
(ii) Growth inhabitation
Some seeds may contain growth inhibitators eg abscisic aicd, coumarin, phenolic acid and short chain fatty acids in the embryo.
Endosperm and other tissues may also prevent germination.
(iii) A specific light requirement
In certain seeds, dormancy may be due to lack of red light e.g. in some lattuee plants, even if the seeds are soaked in water.
In certain seeds, e.g. phacelia and nigella will remain dormant when exposed to light.
Some seeds require a cold period before germination e.g. cereals and roses. This is because the cold period brings about a change in the gibberellins levels.
(v) Immaturity of the embryo
The embryo could be immature and thus unable to synthesize gibberellin.
(vi) Lack of water, 02 and a suitable temperature.
Water is required to activate bio-chemical reactions like digestions of food reserves after imbibitions e.g. starch is digested by amylase to form maltose. Maltose is digested by maltase to form glucose. Proteins are digested by protieses to form amino acids; lipids are digested by lipases to form fatty and glycerol.
02 is needed for aerobic respiration
The suitable temperature influences rate of enzyme controlled reactions. When all these conditions are absent, all the xns won’t take place and the seed will remain dormant.
Methods to break seed dormancy
- Weaken the seed coat by soaking it in water and dilute acids or artificially remove the testa be scratching it or rubbing it with sand paper.
- Apply growth promoters like gibberellins and cytokinins
- Expose the seeds to their optimum light requirements depending on the seeds e.g. if it is lattuce, phacelia or nigella.
- Expose sees to cold temperature e.g. cereals and roses.
- Allow the seeds to undergo the after ripening period to allow the embryo to mature.
Provide water, O2 and the right temperature
Advantages of dormancy
- Ensures that the seed germination under suitable conditions.
- Allows time for seed dispersal.
- Enables the plant to survive advance conditions.
Sormoneeds can be stored for a long time when they are still viable and thus ensure the survival of the species.