In this essay we will discuss about:- 1. Meaning of Cell Growth 2. Growth of Cell is Measurable 3. Phases 4. Growth Curve 5. Measurement 6. Growth Rate 7. Conditions.
Contents:
- Essay on the Meaning of Cell Growth
- Essay on the Growth of Cell is Measurable
- Essay on the Phases of Cell Growth
- Essay on the Growth Curve
- Essay on the Measurement of Cell Growth
- Essay on the Growth Rate
- Essay on the Conditions of Cell Growth
Essay # 1. Meaning of Cell Growth:
Growth is often referred to as an increase in size or weight (fresh or dry) of a cell, organ or organism. However, increase in size can occur without growth as absorption of water by a flaccid cell or regaining of turgidity by a wilted leaf. Similarly, during germination of a seed there is an actual fall of dry weight though the size and fresh weight increase.
Therefore, growth is defined as a permanent or irreversible increase in dry weight, size, mass or volume of a cell, organ or organism. Growth in living beings is intrinsic or internal.
It is in contrast to extrinsic growth observed in non-living objects like enlargement of a stone or swelling of a piece of wood placed in water. In living beings irreversible increase in size, mass or volume is an external manifestation of growth. It is also called apparent growth. Real growth consists of formation of new protoplasm.
This is possible only when the rate of synthesis of new proteins, carbohydrates and other protoplasmic constituents is higher than the rate of their breakdown. It occurs at the expense of energy. In plants, growth is accomplished by cell division, increase in cell number and cell enlargement. Therefore, growth is a quantitative phenomenon. It can be measured in relation to time.
Plant Growth is Generally Indeterminate:
In lower plants, growth is diffused as every cell can divide and enlarge. Higher plants possess specific areas which take part in the formation of new cells. These areas are called meristems. Meristems are of three types— apical, intercalary and lateral.
On account of the presence of meristems or growing points, plant growth is localised. The body of plants is built on a modular fashion (with discrete units or stages) where structure is never complete because the tips (with apical meristem) are open ended— always growing and forming new organs to replace the older or senescent ones.
Cells of the meristematic region have the capacity to divide and self perpetuate. They produce cells which lose the capacity to divide and enter G0 phase for undergoing differentiation to form particular tissues and organs. Root apical meristem (RAM) and shoot apical meristem (SAM) contribute cells for elongation of plant parallel to its axis. It is primary growth. Another meristem contributing to primary growth is intercalary meristem located above the nodes in grasses and related plants. In some cases, it is functional throughout the life of the plant.
However, intercalary meristem present in leaf and flower primordia has a short period of activity. It is consumed in the formation of the organs. The meristem which is consumed in the formation of an organ is called determinate meristem. The meristem which continues its activity throughout life of the plant is called indeterminate meristem.
Root apical meristem, shoot apical meristem, intercalary meristem (e.g grass) and lateral meristems are all indeterminate meristems. Lateral meristems contribute tissues for growth in girth.
It is secondary growth. Secondary growth occurs in dicots and gymnosperms. There are two types of lateral meristems which contribute to secondary growth, vascular cambium (forms secondary xylem and secondary phloem) and cork cambium (forms cork and secondary cortex).
Essay # 2. Growth of Cell is Measurable:
At the cellular level growth is due to increase in amount of protoplasm. However, it is difficult to measure increase in protoplasm. Increase in protoplasm leads to increase in cell, cell number and cell size.
This fact is used in calculating growth which, therefore, is a quantitative or measurable phenomenon. The parameters used for measuring growth are increase in fresh weight, dry weight, length, area, and volume and cell number. At the cellular level, growth occurs at a tremendous pace.
A single cell of root meristem of Maize produces more than 17,500 new cells every hour. In Watermelon, a newly formed cell increases in size up to 3, 50,000 times during its enlargement. Both increase in cell number and cell size constitute the basis of growth.
However, all structures do not show these parameters. A pollen tube grows only in length while growth of a dorsiventral leaf is measurable as increase in area. Rate of growth is growth per unit time. Rate of plant growth is slow in early stages.
It is called lag phase. It then increases rapidly during exponential phase followed by slowing down and then becoming stationary (for organs of limited growth) or steady (for organs of unlimited growth). The last phase is due to limitation of nutrients.
Essay # 3. Phases of Cell Growth:
Plant growth takes place in three steps or phases— formative, enlargement and differentiation.
1. Formative Phase:
It is also called the phase of cell formation or cell division. It occurs at root apex, shoot apex and other regions having meristematic tissue. New cells are produced by mitotic divisions of the meristematic cells.
The meristematic cells have thin cellulose walls, dense protoplasm and large nucleus. Plasmodesmal connections occur abundantly amongst the meristematic cells. Mitosis adds new cells to the body. The rate of mitosis is very high. Apical meristem of Maize root adds some 17,500 new cells every hour.
All the cells are genetically similar because in mitosis the chromosomes are replicated and divided equally both quantitatively as well as qualitatively. Mitosis is, therefore, also called somatic cell division. In higher plants formative phase occurs in meristems or growing points.
As the formation of new cells requires intense biosynthetic activity, the rate of respiration in the cells of formative phase is very high. Due to cell divisions the growing points show some increase in their size.
2. Phase of Enlargement:
The newly formed cells, produced in the formative phase undergo enlargement. Cell walls of the enlarging cell show plastic extension through enzymatic loosening of micro fibrils and deposition of new materials (intussusception). The enlarging cell also develops a central vacuole.
Growth due to cell enlargement is very high. Cells in Watermelon fruit increase up to 3, 50,000 times. Rate of respiration is high but less than that of the cells in the formative phase. The phase is found just behind the growing points and is mainly responsible for growth of plant parts.
Cell enlargement may occur in all directions as in isodiametric parenchymatous cells. In many parts cell enlargement takes place prominently in the linear direction so much so that this phase is also called phase of cell elongation. Maximum elongation occurs in conducting tissues and fibres.
3. Phase of Differentiation or Maturation:
The enlarged cells develop into special or particular type of cells by undergoing structural and physiological differentiation. Through structural differentiation a cell attains a particular shape, size, thickening and internal constitution.
In physiological differentiation a cell takes up a particular function, e.g., absorption by root hair, transfer of metabolites by transfer cells, photosynthesis by mesophyll cells, conduction by sieve tube cells, tracheids and vessels. Both structural and physiological differentiation produce various tissue and cell types. The diverse cell types observed in root are epidermis, cortex, vascular tissues, etc.
Experiment 1. To Study Phases of Growth (Fig. 15.3):
Apparatus:
Seeds of Pea or Bean, moist saw dust, water, petri dish, blotting paper, water proof ink, pen, scale.
Working:
Germinate a few seeds of Pea or Bean in moist saw dust. Pick up a couple of seedlings with straight radicle of 2-3 cm length. Wash the seedlings. Blot the surface water.
Mark the radicles from tip to base with 10-15 points at intervals of 2 mm with the help of water proof or India ink. As soon as the ink dries up, place the seedlings on moist blotting paper in a petri dish. Allow the seedling to grow for 1-2 days. Measure the intervals between the marks.
Results:
Depending upon the distance between successive marks four regions can be noted in the growing radicle. The first lies at its tip and has little growth. It is the region of cell formation.
The second part shows the maximum elongation. It represents the region of cell elongation. The third zone has lesser elongation and is region of cell differentiation. The last part of the root is the region of mature cells where growth has stopped.
Essay # 4. Growth Curve (Fig. 15.4):
It is the graphic representation of the total growth against time. The period of time, in which growth takes place, has been called grand period of growth by Sachs (1873). The rate of growth is not uniform during the grand period of growth.
If total growth is plotted against time, an S-shaped or sigmoid curve is obtained. It consists of four parts — lag phase, log phase (exponential phase), phase of diminishing growth and stationary phase (steady growth for organs or organisms of indefinite growth).
Growth is slow in the lag phase, rapid during log or exponential phase, slow again during the phase of diminishing growth. Growth stops completely during the stationary phase.
The various parts of growth curve correspond respectively to formative phase, phase of enlargement, phase of differentiation and mature state. Log phase is also called grand phase of growth. Sometimes, the rate of maximum growth of the log phase is maintained for some time. It is then known as linear phase. It appears as an upright line in growth curve.
Essay # 5. Measurement of Cell Growth:
Growth is measured through measuring:
(i) Increase in length, e.g., stem, root, pollen tube,
(ii) Increase in volume, e.g., fruits,
(iii) Increase in area, e.g., leaves,
(iv) Increase in diameter, e.g., tree trunks, fruits,
(v) Increase in fresh or dry weight. The various modes of measuring increase in length are:
1. Direct Method:
Growth in length is measured at intervals of a few days by means of a scale. This is not much used, as in this case, growth over short periods cannot be measured.
2. Horizontal or Travelling Microscope (Fig. 15.5):
It is a device for measuring growth more accurately than by means of a scale. A point is marked very near the tip of growing shoot by means of India ink. The horizontal or travelling microscope is focused over this point. After a definite interval the marked point is observed under this microscope.
It has to be raised for the same because due to growth the point on the shoot rises above the previous level. The distance, through which the microscope is raised, is a measure for the growth in length of the shoot during the interval.
3. Arc or Lever Auxanometer:
It consists of a pulley which is attached to a pointer or large needle. The pointer has two unequal arms. The long arm can move over a graduated arc.
The short arm possesses an adjustable large screw by which the weight of the pointer can be adjusted so that the long arm can move freely over the arc upside downwards. Arc auxanometer magnifies growth. The magnification is equal to the length of the long arm as measured from the centre of the pulley, divided by the radius of the pulley.
Unspun silken thread is tied to the stem tip of the plant, the shoot growth of which is to be measured. The other end of the thread is tied to a small weight. Pass the thread over the pulley so that the small weight hangs down freely (Fig. 15.6).
The small weight keeps the thread stretched. It also helps the pulley to move along with its movement. As the stem grows in length, the small weight moves downwardly. The pulley and the pointer also move. The distance through which the pointer moves is read on the graduated arc.
4. Automatic Auxanometer:
(Pfeffer). It consists of a double or compound pulley, a revolving cylinder with a smoked paper and a pointer. The revolving cylinder can register growth on a smoked paper with the help of the pointer. The double pulley magnifies growth. The magnification is equal to the radius of the larger pulley divided by the radius of the smaller pulley.
Unspun silken thread is tied to the stem tip of a plant. The free end of the thread is attached to a small weight. The thread is now passed over the smaller pulley in the direction opposite to that of the revolving cylinder. Another thread is passed over the — larger pulley. It bears small weights at both its free ends. This thread bears an horizontal pointer on the side of the revolving cylinder (Fig. 15.7).
The pointer is in contact with the smoked paper which is wrapped or pasted over the revolving cylinder. The smoked paper is prepared by passing plain paper over burning camphor or oil lamp. Graph paper can also be used instead of smoked one. In that case the tip of the pointer is inked.
The clockwork of the revolving cylinder is started. As growth occurs, the two pulleys will move with the downward movement of the weight attached to the other end of the thread connected to the stem tip of plant. If no growth occurs, the pointer will mark only an horizontal line on the smoked paper. A stair case like line will show the diurnal pattern of growth.
The automatic auxanometer is an improvement over the arc auxanometer. It can register total growth, rate of growth at specific time and overall pattern of growth.
5. Increase in Cell Number:
In bacteria, yeast and many algae, the rate of growth is estimated by the increase in number of cells.
6. Increase in Weight:
Both fresh and dry weights are used for measuring growth. Fresh weight is used for measuring growth in fruits, bulbs, corms, roots etc. Dry weight is used for actual measurement of growth. For this the organs are dried in an oven at 110°C for several hours.
7. Increase in Volume:
It is used in case of fruits. The fruit is dipped in water. Increase in level of water will indicate the volume of fruit.
8. Increase in Diameter:
The method is used in case of globular and cylindrical organs, e.g., fruits, tree trunk. Vernier callipers for small organs and measuring tapes for large organs are used.
9. Increase in Surface Area:
It is used for measuring growth in flat organs like leaves. Increase in surface area is measured by placing the leaf on a standard graph paper and drawing its outline at fixed intervals.
Essay # 6. Growth Rates:
Increase in growth per unit time is called growth rate. Growth rate may result in arithmetic or geometric growth.
Arithmetic Growth (Fig. 15.8):
It is a type of growth in which the rate of growth is constant and increase in growth occurs in arithmetic progression— 2, 4, 6, 8, 10, 12. Arithmetic growth is found in root or shoot elongating at constant rate. Meristematic cells at the growing point divide in such a fashion that one daughter remains meristematic while the other grow and differentiate. The process continues.
Mathematically arithmetic growth is expressed as:
Lt = L0 + rt
Lt = length after time t. L0 = length at the beginning, r = growth rate. On plotting growth against time, a linear curve is obtained (Fig. 15.8 B)
i. Geometric Growth (Fig. 15.9):
It is quite common in unicellular organisms when grown in nutrient rich medium. Here, every cell divides. The daughters grow and divide. The granddaughters repeat the process and so on. Number of cells is initially small so that initial growth is slow. Later on, there is rapid growth at exponential rate. It is called log or exponential growth.
An embryo initially shows geometrical growth in cells but later it passes into arithmetic phase (Fig. 15.10).
ii. Sigmoid Growth Curve:
Geometric growth cannot be sustained for long. Some cells die. Limited nutrient availability causes slowing down of growth. It leads to stationary phase. (There may be actually a decline). Plotting the growth against time will give a typical sigmoid or S-curve (Fig. 15.11).
S-curve of growth is typical of most living organisms in their natural environment. It also occurs in cells, tissues and organs of plants. However, S-curves of individual cells, tissues, organs and the organisms may not be synchronous because one cell after differentiation may enter the stationary phase while a second one after being formed may be in lag phase.
Similarly, some leaves of a branch growing exponentially may be just formed, others showing lag phase, log phase and mature phase. Some leaves may be in the process Of abscission. Most plants also show seasonal growth. The S-shaped growth curve would exhibit small stationary phase at the end of each annual growth (Fig. 15.12).
Law of Compound Interest (Exponential Growth). Growth is dependent on three factors— initial size (W0), rate of growth (r) and the time interval for which the rate of growth can be retained. It is just like depositing money in a bank.
The money will grow at compound interest. Growth will depend upon the initial size (amount of money deposited), rate of growth (rate of interest) and the period for which it is sustained (period of time in the bank).
W1 = W0 ert
Here W1 is the final size, W0 is initial size, r is growth rate, t is time of growth while e is the base of natural logarithms (2.71828). The magnitude of r or rate of growth has been called efficiency index by Blackman (1919) as the organs and organisms with higher r value will outperform others with low r value.
Quantitative comparisons between growths of various systems can be made by measuring their absolute and relative growth rates.
iii. Absolute Growth Rate:
Absolute growth curve is the actual growth curve obtained by plotting growth against time. It is commonly S-shaped. Absolute growth rate is the total growth per unit time.
A graph plotted for absolute growth rates for various times of grand period of growth appears bell shaped. The peak is formed when the growth rate is the fastest. The period of increasing growth is depicted by the first part of the curve while the period of decreasing growth rate is shown by the second part of the curve (Fig. 15.13).
iv. Relative Growth Rate:
It is growth per unit time per unit initial growth.
Growth in Given Time Period/Measurement at Start of Time Period
Suppose two leaves have grown by 5 cm in one day. Initial size of leaf A was 10 cm2 while that of leaf В was 15 cm2. Though their absolute growth is the same, relative rate of growth is faster in leaf A because of initial small size (fig.15. 14). It decreases with time (Fig. 15. 15).
Essay # 7. Conditions for Cell Growth:
Growth involves synthesis of more protoplasm, cell division, cell enlargement and cell differentiation. It is, therefore, influenced by all those factors which influence biosynthetic machinery, availability of water, oxygen, optimum temperature, optimum light, minerals and absence of stress conditions.
1. Nutrients:
They are raw materials for synthesis of protoplasm as well as source of energy. It is seen that rate of growth is proportional to size of bulb, tuber, rhizome, etc. It is called law of mass growth.
Nutrients should be rich in nitrogenous components for increased synthesis of protoplasm and carbohydrates for energy and cell wall synthesis. All types of micronutrients (micro-essential elements) and macronutrients (macro-essential elements) must be available for proper growth.
2. Water:
It is required for cell elongation, maintenance of turgidity of growing cells and providing medium for enzyme action. Even slight deficiency of water reduces growth. It may, however, promote differentiation. Water stress completely stops growth.
3. Oxygen:
It is essential for aerobic respiration and hence availability of energy for biosynthetic activity.
4. Light:
It is required for tissue differentiation, synthesis of photosynthetic pigments and photosynthesis. Its absence results in etiolation. Light also influences certain stages of growth. The phenomenon is called photoperiodism.
5. Temperature:
A temperature of 28°-30°C is optimum for proper growth in most plants. Higher temperature above 45°C hinders growth due to excessive transpiration, denaturation of enzymes and coagulation of protoplasm. Lower temperature inactivates enzymes as well as increases density of protoplasm.
6. Gravity:
Vector of gravity determines the direction of shoot and root growth. Direction of light also determines the orientation of leafy shoots.
7. Other Factors:
Excess of salt, mineral deficiency and other stress factors have a detrimental effect on growth.
Differentiation, Dedifferentiation and Re-differentiation:
Differentiation:
Growth is invariably associated with differentiation. For example, when a seed germinates, it does not simply increase in size but forms a seedling. Differentiation is a permanent localised qualitative change in size, biochemistry, structure and function of cells, tissues or organs, e.g., fibre, vessel, tracheid, sieve tube, mesophyll, leaf, etc. The exact trigger for differentiation is not known.
All the cells of an individual have the same genetic information. They are influenced by similar external factors. Depending upon the location inside the plant and internal cellular mechanism, some genes are repressed (not allowed to express their effect) while others are allowed to show their effect.
This causes the cells to behave in a particular fashion during growth and after maturation:
(i) Enlargement, lignocellulose wall thickening and emptying in case of tracheids,
(ii) Widening, some enlargement, wall thickening, emptying and loss of end wall in case of vessel elements,
(iii) Loss of nucleus, vacuolisation and perforation of end wall in sieve tube members,
(iv) Development of abundant chloroplasts in mesophyll cells,
(v) Deposition of suberin in cell walls, tannins in protoplasts and then death of cork cells,
(vi) Deposition of silica in epidermal cells of grasses,
(vii) Differential wall thickening, small vacuoles, formation of a few chloroplasts and small size in guard cells,
(viii) Free nuclear division, a central canal and secretion of latex in laticifers,
(ix) Secretion of mucilage in root cap,
(x) Elongation, thickening and emptying of sclerenchyma fibres,
(xi) Development of uneven pectocellulosic thickening in collenchyma,
(xii) Cutinisation of trichomes for preventing transpiration and formation of stationary air layer, and
(xiii) Development of schizogenous interspaces to form aerenchyma in aquatic plants.
Not only the plants are open-ended, their differentiation is also open. The same apical meristem cells give rise to different types of cells, e.g., xylem, phloem, parenchyma, sclerenchyma fibres, collenchyma, etc. The reason for the formation of different types of cells and tissues from the same type of meristematic cells is commitment or determination.
It is generally due to location and reception of particular signals. For example, cells distal to root apical meristem form root cap, while on the periphery they form epiblema followed by cortex, endodermis, pericycle, vascular tissues, etc.
Dedifferentiation and Re-differentiation:
The process of de-specialisation of differentiated cells so that they become undifferentiated and able to divide is known as dedifferentiation. It involves activation of certain genes which not only reverse differentiation but also stimulate cell division. Cork cambium, wound cambium and inter-fascicular vascular cambium are always produced through dedifferentiation.
Cell culture experiments are based on dedifferentiation of cells and formation of mass of undifferentiated cells called callus. Normally cells produced by dedifferentiated cells mature and form re-differentiated cells, e.g., secondary xylem elements, secondary phloem elements, cork cells.
Development:
Development is the sequence of events that occur in the life history of a cell, organ or organism which includes seed germination, growth, differentiation, maturation, flowering, seed formation and senescence.
The term development is also applied to changes in phases of life, e.g., vegetative to flowering leaf initiation to leaf expansion. Development occurs even at the subcellular level, e.g., appearance of chloroplasts in cells exposed to light. The last phase of development is senescence. Senescence or old age leads to death.
Sequence of events occurring during development of cells of higher plants is as follows:
Different structures develop in different phases of growth as well as in response to environment. The ability to change under the influence of internal or external stimuli is called plasticity. The intrinsic plasticity is found in juvenile stage of many plants, e.g., Cotton, Coriander, Larkspur, Ivy. Environmental plasticity is best seen in emergent hydrophytes like Buttercup (Ranunculus flabellaris).
In both cases plants show heterophylly and a number of other morphological features. Heterophylly is the occurrence of different types of leaves on the same plant habitually in different growth phases or under different environmental conditions.
In case of environmental plasticity shown by aquatic Butter cup Ranunculus flagellaris, the submerged leaves are highly dissected while the emerged leaves are broad and lobed. In Larkspur, the juvenile leaves are broadly lobed while the mature leaves become pinnately divided with lobes becoming linear in the region of flowers.
In Hedera helix (Ivy) the juvenile plant is root climber having alternate palmately lobed leaves while the adult plant has bushy habit with opposite entire leaves. It bears flowers and fruits. The fruits contain seeds for formation of new plants. Annuals, biennials and perennial monocarpic plants become senescent after the formation of fruits and hence die.
Perennial polycarpic plants continue to grow indefinitely and bear flowers and fruits annually after attaining maturation. However, growth of a perennial plant is not uniform throughout the year. In a calendar year, it shows periods of active vegetative growth, flowering, fruiting, senescence and dormancy. The different aspects or appearances of plants in different seasons of a year is called phenology.
Development includes growth and differentiation. It is under control of both intrinsic and extrinsic factors. Intrinsic factors include genetic factors and growth regulators. Extrinsic factors are light, temperature, water, oxygen and nutrition.