WORKING OF A PLANT
Generally, the structure of a flowering plant consists of two systems:
- Root System
- Shoot System
- Root System:
It is the part of a plant that grows under the ground. It consists of the primary root and its branch roots. Through roots, a plant anchors in the ground. Root tips have many tiny root hair that provides a large absorptive surface for absorbing water and dissolved minerals from the soil.
INTERNAL STRUCTURE OF A
The outer protective layer of cells is called the epidermis.
The cortex consists of many layers of cells. It stores water and food.
- Vascular Bundles:
Vascular bundles consist of xylem and phloem. Xylem tubes transport water and phloem tubes conduct prepared food.
- Shoot System:
It is part of a plant that grows above the ground. Stem and leaves make up the shoot system. Flowers, fruits, and seeds are also formed in this part.
FOUR INTERNAL PARTS OF A SYTEM
There are three main internal parts of a stem. These are:
(i) Epidermis, (ii) Cortex, and (iii) Vascular bundles. The epidermis is protective in function while the cortex stores water and food for the plant. The phloem of each vascular bundle lies outer to the xylem.
INTERNAL PARTS OF A LEAF
Internal parts of a leaf are as below:
(i) Epidermis (ii) Mesophyll (iii) Vascular Bundles
The epidermis is a kind of protective layer of cells. A leaf has two protective layers of cells, an ‘upper epidermis’ and a ‘lower epidermis’. Numerous tiny pores are found mainly in the lower epidermis. These holes are called stomata (singular: stoma). Each stoma is present between a pair of ‘guard cells’ of a characteristic structure. The exchange of oxygen, carbon dioxide, and water vapors between the leaf and the air take place through stomata.
In a leaf, between the upper and lower epidermis mesophyll tissue is present. Chloroplasts inside these cells contain the green substance, chlorophyll. Therefore, mesophyll cells make food by photosynthesis. There are many intercellular spaces mainly between the cells of the lower portion of mesophyll, which contain air. These air spaces are connected with the stomata and play role in the movement and exchange of gases.
- Vascular Bundles:
The midrib and veins of a leaf are made up of vascular bundles. Xylem is found in the upper part while phloem in the lower part of each vascular ‘bundle. You will study in a later section of this chapter that the .structure of the leaf is well-suited to its function of photosynthesis. (Process of making food in plants)
TRANSPORT SYSTEM OF PLANT
The roots and leaves of a plant play the main role in the transport system of a plant.
Role of Xylem and Phloem Tissues:
Water and minerals from the soil are absorbed by plant roots. These are then transported upward to leaves and other parts by xylem tissues. Food (sugars) from the leaves is transported up and down by phloem tissue tubes. Xylem and phloem usually run side by side, forming vascular bundles. There is an extensive network of these tubes throughout the plant body. The midrib and the veins in a leaf are made of xylem and phloem tubes (vascular bundle). These tubes are continuous with those in the leaf stalk and, in turn, in the branches, main stem, roots, and branches. This network of xylem and phloem form the plant’s transport system.
Absorption of Water by Roots:
Water enters the plant through its root, mainly the root hair. The concentration of water molecules is more in the soil than in the root hair.
The movement of water molecules from lower to higher concentration is called diffusion.
Diffusion when involves cell
membrane, for movement of molecules, is called osmosis. Thus, water from soil moves into root hair through their cell walls and cell membranes. However, the cell membrane does not allow many substances to pass through it. The movement of water molecules through a cell membrane, from a region of higher concentration of water to a region of low& concentration of water molecules, is called osmosis.
Dissolved minerals from the soil also enter a plant through its roots. A part of this absorption is by simple diffusion while the rest takes place by active transport.
Active transport requires the energy of the organism while diffusion does not. Diffusion is the movement of many molecules or atoms from a region of their higher concentration to a region of their lower concentration. Water and dissolved minerals in the roots then move through the cortex from cell to cell by osmosis and diffusion to reach the xylem of the root. Difference between Diffusion and Active Transport:
- Active transport requires energy
while diffusion does not.
- Diffusion is the movement of any
type of molecules or atoms from a region of higher concentration to a region of lower concentration.
- Water in the roots then moves from
cell to cell by osmosis and diffusion in order to reach the xylem of the
The loss of water by evaporation in plants is called transpiration.
Its Role in Conduction of Water:
From the root, water (along with dissolved minerals) moves up the plant through xylem tubes to stem and leaves. In some trees, water has to reach a height of 100 meters or even more.
In fact, water is pulled up through the xylem when it is evaporated from the leaves. Water first evaporates from mesophyll cells inside a leaf. It then diffuses out through tiny holes, called stomata (singular: stoma) into the air. This loss • of water by •evaporation from plants is called transpiration.
Role of Temperature in Transpiration:
Temperature is an important factor in which the rate of transpiration depends. The higher the temperature, the more is the transpiration. Lower is the temperature, less is the transpiration.
Transportation of Food in Plants:
Food (sugars) is made in the special green cells (mesophyll cells) of the leaves, from water and carbon dioxide. Water is brought to the leaf by xylem and carbon dioxide enters through stomata. Sugars from mesophyll cells enter the phloem cells of the leaf veins. From here, the sugars may be transported by phloem cells to all other parts of the plant where these are to be utilized or stored. Like xylem, phloem forms a continuous pathway throughout the plant. Phloem runs from the leaves to every other part of the plant. The food may be transported by phloem in any direction within the plant.
Green plants can make their food themselves from simple materials and are hence autotrophs (self-feeding). Photosynthesis (photo = light, synthesis = making things) is the process by which plants make their food (sugar, glucose) in the presence of light.
This unique ability of plants is due to the presence of a green chemical substance, the chlorophyll, in their leaves. Plants use sunlight energy to make their food. Chlorophyll can absorb sunlight and convert it into chemical energy. the chemical energy is then used to combine carbon dioxide and H2O to make food. This energy is converted into glucose molecules and thus stored in the form of starch. Oxygen is also produced during this process. Some of this oxygen is used by the plants for respiration and rest of it is given out into the air through stomata. Photosynthesis is not just one simple chemical reaction. It is a series of reactions, however, we may summarize photosynthesis in the following simple word equation:.
Carbon dioxide+Water Chlorophyll > Glucose+Oxygen
ADAPTATIONS FOR PHOTOSYNTHESIS
Structure of Leaf:
The structure of the leaf is very suitable for photosynthesis because:
- Most leaves are flat to absorb enough light effectively.
- They have a thick layer of mesophyll cells in the middle. These cells have chloroplasts that contain chlorophyll. Mesophyll cells make food for the plant by photosynthesis.
- They have a rich network of veins. These carry water to photosynthesizing cells, and glucose away from them.
- They are thin so that carbon dioxide and light can reach inner cells easily.
- They have a large number of stomata on the lower surface (lower epidermis). Carbon dioxide can enter and oxygen and water vapors leave through these stomata.
PHOTOSYNTHESIS–FOOD AND OXYGEN FOR ALL
Plants convert excess sugar (glucose) into starch, proteins, and fats and store it in their stems, roots, fruit, and leaves. These parts of plants are eaten by animals which in turn are eaten by other animals. Photosynthesis, therefore, is the process that provides food and energy to all lifeforms on Earth. reproduction. Oxygen produced during
In the photosynthesis is used for respiration by all living organisms. Similarly, carbon dioxide produced during respiration is used by plants during photosynthesis. Due to these processes, the ratio of oxygen and carbon dioxide is maintained in the air.
HETEROTROPHIC (VARIOUS MODES OF TAKING FOOD) NUTRITION IN PLANTS
Most plants are autotrophs. Only some are heterotrophs. Heterotrophic plants lack chlorophyll and thus cannot make their own food. Few of them get their food from some other living things called their hosts. They are known as parasites. For example, Cuscuta (dodder) is a parasitic plant.
Insectivorous Plants: There are some exceptional plants that occasionally feed on insects as well. Their leaves are modified for trapping insects. These insectivorous plants have chlorophyll and make their food (glucose).themselves. But they live in places where the soil is poor in certain nutrients.
Respiration in Plants: The process during which energy is produced in the body from food. is called respiration. Plants do not possess organs of breathing or respiration. But the process of respiration in plants is a continuous process that occurs all the time (day and night). During day time, the plant releases 02 and takes in CO2 but during night time, the plant releases CO2 and takes in 02. This exchange of gases during respiration and photosynthesis takes place through stomata.
The living things, i.e., animals and plants produce their next generation by the process of reproduction.
Types of Reproduction:
There are two types of reproduction:.
- Sexual Reproduction
- Asexual Reproduction
- Sexual reproduction: In sexual
reproduction, sex cells, i.e., sperms, and eggs are involved. The sex cells fuse to form a zygote from which ultimately a young plant 1r animal is formed. Most of the plants and animals reproduce their kind through the sexual
process of asexual reproduction, sex cells are not involved. New individuals are formed by direct cell division at sortie parts of the same organism as in bacteria and amoeba. Each part grows up to form a new individual which eventually separates from the parents. Asexual reproduction is more common in plants and is the only way by which unicellular organisms can reproduce.
FLOWER-THE ORGAN OF SEXUAL REPRODUCTION IN PLANTS
The flower is the organ of sexual reproduction in plants. The floral parts are found in the form of whorls around an axis. Generally, a flower consists of the following parts:
- Pedicel: It is the stalk by which the flower is attached to the shoot. Some flowers are without pedicel.
- Calyx: This is the outermost whorl of the flower which consists of green leaflets. It protects the flower during the early stages when it is in bud form. Each one of these leaflets is called a sepal.
- Corolla: This is the most prominent part of the flower consisting of a whorl of colored and fragrant petals. The butterflies and other insects are attracted to the flower due to the corolla. When these insects visit a flower, pollen grains from the male part of the flower become attached to their body; and when they visit another flower, those pollen grains fall on the female part (carpel) of the flower. In this way, an important step of sexual reproduction is accomplished which is called
- Androecium-the Male Part of Flower: Inner to the petals, a whorl of club-like structures is present which is called Each club-like structure is called a stamen. The upper sac-like part of a stamen is known as anther. Within the anther, pollens are formed.
- Gynaecium-the Female Part of Flower: This is the central part of the flower. It consists of carpels. Each carpel is flask-shaped and has three parts: (i) The upper stigma, (ii) a tubular portion called style, and (iii) the lowermost swollen part, the ovary. The stigma receives the pollens coming from the anther. The ovary is the most important part of the flower. The seed is formed in the ovary, and ultimately the ovary changes into a fruit. In fact, the basic function of the flower is seed formation.
Pollination is of two types:
If the pollen from anther falls on the stigma of the same flower, or on another flower of the same plant, this is called self-pollination.
On the other hand, if the pollen grains from the flower of one plant fall on the stigma of the flower of another plant, this is called cross-pollination. Cross-pollination is thought to be better than self-pollination because it may lead to the formation of new varieties because of the combination of characteristics of two plants.
The fusion of the male gamete with the egg of females to form a zygote is called fertilization.
As a result of pollination, the pollen reaches the stigma of the flower. Its outer wall ruptures and a tube grows out which is called a pollen tube. The pollen tube has two male gametes in it. A female gamete which is called ovum or egg is present in the ovule. When the pollen tube reaches the ovule, it releases both the male gametes. One male gamete fuses with the egg to form a zygote. It
is the first cell of the new plant. It divides repeatedly to form the young plant called an embryo. The ovule forms the seed and the ovary ripens into a fruit.
STRUCTURE OF SEED
, Seeds depending on their size, color and form are of various kinds. However, they are not much different in their internal structure. The seed has the following basic parts:
- Testa: This is the outer hard covering of the seed which protects the internal
- Tegmen: It is a membrane present beneath the testa for the protection of the embryo.
- Endosperm: In some seeds endosperm stores food which is used in the germination of seed.
- Embryo: This is the most important part of the seed. This is a tiny plant whose parts grow to form a new plant. The embryo following parts:
- Plumule: This is the part of the embryo which forms the shoot, that is stern and leaves.
- Radicle: This part of the embryo develops into the root of the plant.
- Cotyledons: They may be one or
two in number. These receive the
food from the endosperm and supply
it to the growing tiny plant.
GERMINATION OF SEED
The purpose of deed formation is the production of a new plane. The next step after seed formation is the development of a new plant. When suitable conditions i.e., (i) suitable supply of water, (ii) suitable temperature, (iii) oxygen, are provided the seed starts germination, forming a new plant. The process of development continues up to the maturation of the plant.