JK-TK/Newdata huggingface dataset:
Temperature
Most seeds require suitable temperature before they can germinate. Seeds will not germinate below 0°C or above 47° C. The optimum temperature for seeds to germinate is 30°C. At higher temperature the protoplasm is killed and the enzymes in the seed are denatured. At very low temperatures the enzymes become inactive. Therefore, the protoplasm and the enzymes work best within the optimum temperature range. The rate of germination increases with temperature until it reaches an optimum. This varies from plant to plant. Enzymes
Enzymes play a vital role during germination in the breakdown and subsequent oxidation of food. Food is stored in seeds in form of carbohydrates, fats and proteins which are in insoluble form. The insoluble food is converted into a soluble form by the enzymes. Carbohydrates are broken down into glucose by the diastase enzyme, fats into fatty acids and glycerol by lipase, and proteins into amino acids by protease. Enzymes are also necessary for the conversion of hydrolysed products to new plant tissues. Hormones
Several hormones play a vital role in germination since they act as growth stimulators. These include gibberellins and cytokinins. These hormones also counteract the effect of germination inhibitors. Viability
Only seeds whose embryos are alive and healthy will be able to germinate and grow. Seeds stored for long periods usually lose their viability due to depletion of their food reserves and destruction of their embryo by pests and diseases. Study Question 4
In an experiment to investigate the effect of neat on germination of seeds, ten bags each containing 60 pea seeds were placed in a water-bath maintained at 85°C . After every two minutes a bag was removed and seeds contained in it planted. The number that germinated was recorded. The procedure used for pea seeds was repeated for wattle seeds. The results obtained were as shown in the table 4,2,
(a)Using a suitable scale and on the same axes, draw graphs of time in hot water against number of seeds that germinated for each plant. Use horizontal axis for time and the verticalaxis for the seeds that germinated. Explain why the ability of pea seeds to germinate declined with time of exposure to heat. Explain why the ability of the wattle seeds to germinate improved with time of exposure to heat.
Use horizontal axis for time and the verticalaxis for the seeds that germinated. Explain why the ability of pea seeds to germinate declined with time of exposure to heat. Explain why the ability of the wattle seeds to germinate improved with time of exposure to heat. Practical Activity 3
To investigate conditions necessary for seed germination
Requirements
Cotton wool, seeds, water, six fiat bottomed flasks, 2 corks, 2 test-tubes, blotting paper, incubator, refrigerator, thermometer, pyrogallic acid and sodium hydroxide. Procedure
Prepare three set-ups as shown in figure 4.5. Leave the set-ups to stand for five days. Record all the observable changes that have taken place in the flasks hi each set up in a table form as shown
Study Question 5
Which condition was being investigated in set-up I, II and III? For each set-up explain the results obtained. What was the role of flask B in each set-up? Types of Germination
The nature of germination varies in different seeds. During germination the cotyledons may be brought above the soil surface. This type of germination is called epigeal germination. If during germination the cotyledons remain underground the type of germination is known as hypogeal. SET UP 1
Pyrogallic acid + NaOH
Cotton wool
Seeds Moist
Water
SET UP 2
Moist cotton wool
Dry cotton wool
SETUPS
Fig. 4.5: Set-up for investigating conditions necessary for germination
Epigeal Germination
During the germination of a bean seed, the radicle grows out through the micropyle. It grows downwards into the soil as a primary root from which other roots arise. The part of the embryo between the cotyledon and the radicle is called the hypocotyl. This part curves and pushes upwards through the soil protecting the delicate shoot tip. The hypocotyls then straightens and elongates carrying with it the two cotyledons which turn green and leafy. They start manufacturing food for the growing seedling. The plumule which is lying between two cotyledons, begins to grow into first foliage leaves which start manufacturing food. Hyopgeal Germination
In maize, the endosperm provides food to the embryo which begins to grow.
They start manufacturing food for the growing seedling. The plumule which is lying between two cotyledons, begins to grow into first foliage leaves which start manufacturing food. Hyopgeal Germination
In maize, the endosperm provides food to the embryo which begins to grow. The radicle along with a protective covering(c(?/eorfci2a) grows out of the seed. The epicotyl is the part of the embryo between the cotyledon and the plumule. The epicotyl elongates and the plumule grows out of the coleoptile and forms the first foliage leaves. The seedling now begins to produce its own food and the endosperm soon shrivels. This type of germination in which the cotyledon remains below the ground is known as hypogeal germination. Practical Activity 4
To investigate epigeal and hypogeal germination
Requirements
Tin or box, soil, water, maize grains and bean seeds. Procedure
Place equal amounts of soil into two containers labelled A and B. In A, plant a few maize grains.In B, plant a few bean seeds. Water the seeds and continue watering daily until they germinate. Place your set-ups on the laboratory bench. Observe daily for germination. On the first day the seedlings emerge from the soil, observe them carefully with
regard to the soil level. Carefully uproot one or two seedlings from each set. Observe and draw the seedlings from each set Label the parts and indicate the soil level on your diagram. On the fifth day since emergence, again uproot another seedling. Observe and draw. Indicate the soil level on your diagram.. Tabulate the differences between the two types of germination studied. Cotyledon Plumule
Radicle
Root
Cotyledon Hypocotyf
Fig. 4.6 (a): Epigeal germination for castor seeds
Plumule
Coleoptile
Scutellum
Coleorhiza
Primary and Secondary Growth
The region of growth in plants is found in localised areas called meristems as shown . A meristem is a group of undifferentiated cells in plants which are capable of continuous mitotic cell division.
Cotyledon Plumule
Radicle
Root
Cotyledon Hypocotyf
Fig. 4.6 (a): Epigeal germination for castor seeds
Plumule
Coleoptile
Scutellum
Coleorhiza
Primary and Secondary Growth
The region of growth in plants is found in localised areas called meristems as shown . A meristem is a group of undifferentiated cells in plants which are capable of continuous mitotic cell division. The main meristems in flowering plants are found at the tips of shoots and roots, in young leaves, at the bases of the inter-nodes, and in
Apicai meristem
Leaf primordium
Vascular tissues
" beginning to form
Epidermis xylem Phloem Pericycte
Cortex
Medulla Cambium
(c) Epidermis
Node
Phloem
Xylefti Epidermis
Cortege
Endodermis
Root cap
Zone of cell division
Zone of cell elongation (expansion)
Zone Trf cell
differentiation
Permanent tissues
Fig. 4.7(a) and (b): Longitudinal section of the root tip and apex Fig. 4.7(c) and (d): Transverse section of the stem and rooi
zone
vascular cambium and cork cambium. T
he meristems at the tips of the shoots and the roots are known as apical meristems and are responsible for primary growth. The cambium meristems are responsible for secondary growth. Primary Growth
Primary growth occurs at the tips of roots and shoots due to the activity of apical meristems. These meristems originate from the embryonic tissues. In this growth there are three distinctive regions, the region of cell division, cell ejpngarion and eel] differentiation. See figure 4.7. The regipn of cell division is an area of actively dividing meristematic cells. These cells have thin cell walls, dense cytoplasm and no vacuoles. In the region of cell elongation, the cells become enlarged to their maximum size by the stretching of their walls. Vacuoles start forming and enlarging. In the region of ceH differentiation the cells attain their permanent size, have large vacuoles and thickened watt cells. The cells also differentiate into tissues specialised for specific functions.
Vacuoles start forming and enlarging. In the region of ceH differentiation the cells attain their permanent size, have large vacuoles and thickened watt cells. The cells also differentiate into tissues specialised for specific functions. Primary growth results into an increase in the length of shoots and roots. Study Question
;:Mgure;4-S indkate the appearance of cells at different regions at the apical meristems.Nudeus -Cytoplasm
Fig. 4.8
Rearrange them into three regions:
Zone of cell division. Zone of cell elongation.; -,
Zone of cell differentiation. (jb) Name specialised tissues formed at tl
zone of cell differentiation. Region of Growth in a root
This is determined by taking a young germinating seedling whose radicle is then marked with the Indian ink at intervals of 2 mm. The seedling is left to grow for sometime (about 24 hours or overnight) and then the ink marks are examined. When the distance between successive ink marks are measured, it is found that the first few ink marks, especially between the 2nd and 3"1 mark above tip of root have increased significantly. This shows that growth has occurred in the region just behind the tip of the root. The difference between the length of each new interval and the initial interval of 2 mm gives the increase in the length of that interval during that period of time. From this the rate of growth of the root region can be calculated. See figure 4.9. „Increase in length . „_
Growth = ^ . .—^— X 100
Original length
Practical Activity 5 To determine the region of growth in roots
Requirements
Germinating bean seeds with radicle of about 1cm in length, cork, pin, beaker or gas jar, water, Indian ink, blotting paper or filter paper, marker and ruler marked in mm. Procedure
Bent wire
Ruler
Take the germinating been seed, andusing a blotting paper, dry the radicaltaking care not to damage the root. Using a marker and ruler make light inkmarks 2mm apart along the length ofthe root. See figure 4.10(a). 4. 5
Make a drawing of the marked root. Pin the seedling onto the cork and place it in the beaker containing a little water. See figure 4.10(b). Leave it overnight. Take out the seedling and examine the ink marks. 6.
Leave it overnight. Take out the seedling and examine the ink marks. 6. 7. Measure the distances between the successive ink marks and record. Make a well labelled drawing of the seedling at the end of the experiment and compare with the drawing of the. seedling at the start of the experiment. Study Question 7
What part of the radicle has the inkmarks moved further apart? Give an explanation for your answersin (a) above. What is the increase in length withineach interval? Work out the rate of growth for theroot
Secondary Growth
Secondary growth results in an increase in width or girth due to activity of the cambium. In secondary growth new tissues are formed by vascular cambium and cork cambium. In monocotyledons plants there are no cambium cell in the vascular bundles.The growth in diameter is due to the enlargement of the primary cells. Secondary growth in dicotyledonous pjants begins with the division of vascular cambium to produce new cambium cells between the vascular bundles. This forms a continuous cambium ring. These cambium cells divide to form the new cells that are added to the older ones. The cambium cells have now become meristematic. The new cells produced to the outer side of cambium differentiate to become secondary phloem and those to the inner side differentiate to become the secondary xylem. More secondary xylem is formed than secondary phloem. The interfascuiar cambium a/so cuts orTparenchymatous cells which form secondary medullary rays as seen in figure 4.11 (a), (b) and (c). As a result of the increase in the volume of the secondary tissues, pressure is exerted on the outer cells of the stem. This results in stretching and rupturing of the epidermal cells. In order to replace the protective outer layer of the stem, a new band of cambium cells are formed in the cortex. These cells, called cork cambium orphellogen originate from the cortical cells. The cork cambium divides to produce new cells on either side. The cells on the inner side of the cork cambium differentiate into secondary cortex and those produced on the outer side become cork cells. Cork cells are dead with thickened walls. Their walls become coated with a waterproof substance called suberin. The cork cells increase in number and become the bark of the stem. This prevents loss of water, infection from fungi and damage from insects.
Their walls become coated with a waterproof substance called suberin. The cork cells increase in number and become the bark of the stem. This prevents loss of water, infection from fungi and damage from insects. The corky bark is also resistant to fire and thus acts as an insulatory layer. The bark is normally impermeable to water and respiratory gases. Periodically the cork cells, instead of being tightly packed, they form a loose mass. This mass is known as Jenticel. The lenticles make it possible for
Transverse section of dicotyledonous stem
Epidermis Primary phloem
Secondary phloem
Secondary xylem
Primary xy/em
Cortex
Beginning of secondary growth in dicot stem
Primary phloem
Secondary ph/oem
Secondary xylem Primary xylem
Epidermis Medullary ray
Cambium ring
Epidermis
Cork cell Cork cambium Secondary cortex Primary cortex
Fig. 4.12: Section through a lenticel
The rate of secondary growth in a stem varies with seasonal changes. During rainy season, xylem vessels and tracheids are formed In large numbers. These cells are large, have thin walls and the wood has a light texture. In the dry season, the xylem and trancheids formed are few in number. They are small, thick-walled and their wood has a dark texture. This leads to the development of two distinctive layers within the secondary xylem formed m a year, called annual rings. See figure 4.13. It is possible to determine the age of a tree by counting the number of annual rings. Furthermore climatic changes of the past years can be infered from the size of the ring. Primary phloem Secondary phloem
Cambium ring Medullary ray Pith
Primary xylem Cortex
Fig. 4.13: Annual rings
Cork
Role of Growth Hormones in Plants
Plant hormones are chemicals produced in very small amounts within the plant body, and play a very important part in regulating plant growth and development. Most growth hormones are produced at the tip of a shoot and transported downwards to the root. The root tip produces very small quantities of the hormones. There are many different types of plant hormones and one well-known group is the auxins. Indoie acetic acid (IAA) is one best known auxin. Auxins are produced at the shoot and root tips.
There are many different types of plant hormones and one well-known group is the auxins. Indoie acetic acid (IAA) is one best known auxin. Auxins are produced at the shoot and root tips. Maximum influence on growth in plants occurs when auxins are produced simultaneously with other plant hormones e.g. gibberellins. Maximum growth response in stems requires more IAA than tn roots. Auxins are known to have various effects on the growth and development in plants. They stimulate cell division and cell elongation in stems and roots leading to primary growth. Auxins cause tropic responses, which are growth responses in plants due to external stimuli acting from a given direction. On the other hand IAA stimulates the growth of adventitious roots which develop from the stem rather than tbe main root. Cuttings can be encouraged to develop roots with the help of IAA. If the cut end of a stem is dipped into IAA, root sprouting is faster. IAA is also used to induce parthenocarpy. This is the growth of an ovary into a fruit without fertilisation. This is commonly u^ed by horticulturalists to bring about a good crop of fruits particularly pineapples. Auxins are known . to inhibit development of side branches from lateral buds. They therefore enhance apical dominance. During secondary growth auxins Play an important role by initiating cell division in the cambium and differentiation of these cambium cells into vascular tissues. Auxins in association with other plant hormones such as the cytokinins induce the formation of callus tissue which causes the healing of wounds. When the concentration of auxins falls in the plant, it promotes formation of an abscission layer leading to leaf fall. A synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D) induces distorted growth and excessive respiration leading to death of the plant. Hence it can be used as a selective weed killer. GibbereHins are another important group of plant growth hormone. GibbereHins are a mixture of compounds and have a very high effect on growth. The most important in growth is gibberellic acid. Gibbereilins are distinguished from auxins by their stimulation of rapid cell division and cell elongation in dwarf varieties of certain plants. Dwarf conditions are thought to be caused by a shortage of gibberellins due to a genetic deficiency. Gibberellins are important in fruit formation.
Gibbereilins are distinguished from auxins by their stimulation of rapid cell division and cell elongation in dwarf varieties of certain plants. Dwarf conditions are thought to be caused by a shortage of gibberellins due to a genetic deficiency. Gibberellins are important in fruit formation. They induce the growth of ovaries into fruits after fertilisation. They also induce parthenocarpy. Gibberellins also promote formation of side branches from lateral buds and breaks dormancy in buds. This is common in species of temperate plants whose buds become dormant in winter. In addition, this hormone also inhibits sprouting of adventitious roots from stem cuttings, it retards formation of abscission layer hence reduces leaf fall. Gibberellins also break seed dormancy by activating the enzymes involved in the breakdown of food substances during germination. Cytokuuns also known as kinetins, are growth substances which promote growth in plants when they interact with auxins. In the presence of auxins, they stimulate cell division thereby bringing about growth of
roots, leaves and buds. They also stimulate formation of the callus tissues in plants. The callus tissue is used in the repair of wounds in damaged parts of plants. Cytokinins promote flowering and breaking of seed dormancy in some plant species. They also promote formation of adventitious roots from stems and stimulate lateral bud development in shoots. When in high concentration cytokinins induce cell enlargement of leaves but in low concentration they encourage leaf senescence and hence leaf fall. Ethylene is a growth substance produced in plants in gaseous form. Its major effect in plants is that it causes ripening and falling of fruits. This is widely applied in horticultural farms in ripening and harvesting of fruits. It stimulates formation of abscission layer leading to leaf fall, induces thickening of stems by promoting cell division and differentiation at the cambium meristem. But it inhibits stem elongation. Ethylene promotes breaking of seed dormancy in some seeds and flower formation mostly in pineapples. Abscisic acid is a plant hormone whose effects are inhibitory in nature. It inhibits seed germination leading to seed dormancy, inhibits sprouting of buds from stems and retards stem elongation. In high concentration, abscisic acid causes closing of the stomata. This effect is important in that it enables plants to reduce water loss. It also promotes leaf and fruit fall.
In high concentration, abscisic acid causes closing of the stomata. This effect is important in that it enables plants to reduce water loss. It also promotes leaf and fruit fall. Another hormone, florigen is produced in plants where it promotes flowering. Apical Dominance
Although auxins, particularly IAA are important stem and root elongation, they are known to exert profound effects on other aspects of plant growth and development. If an apical bud which normally contains high concentrations of auxins is removed, it is
observed that more lateral buds lower down the stem sprout, producing many branches. This shows that high concentrations of auxins have an inhibitor}' effect on sprouting of lateral buds and therefore hinders growth of many branches. This forms the basis of pruning in agriculture where more branches are required for increased harvest particularly on crops like coffee and tea. The failure of lateral buds to develop in the presence of an apical bud is due to the diffusion of auxins from the shoot apex downwards in concentrations higher than that promoting lateral bud development. Practical Activity 6
To investigate apical dominance in plants
Requirements
Tomato seedlings growing in a tin. Procedure
Cut off the terminal buds from 3seedlings in the tin, leaving the otherseedlings with the terminal buds intact,
Leave the seedlings to continuegrowing for five more days. Study Questions 8
list the differences noticed betweenthe two groups of seedlings? Explainhow the differences come about. From your observations, explain thebasis for pruning tea and coffee. Growth and Development in Animals
In higher animals, most cells with the exception of the nerve cells, retain their power of division. Thus, there is a continued breakdown and replacement of cells. Animal cells undergo rapid cell division and cell differentiation but, unlike plant cells, they undergo very little cell enlargement. In most animals growth occurs through: their life till they die. This type of growth called continuous growth. Arthropods e.g. insects show rapid growth immediately after moulting with periods when no growth increase occurs. This is called discontinuous growth. Insects exhibit two types of reproducti processes. In some insects, the ova in t female are fertilised by the spermatozoa frc the male. This is a typical example of sexi reproduction, common in butterflies ai moths.