Plants, a detailed study.

1.        Importance of Plants

Basic needs of Plants

The basic needs of plants are energy, sunlight, water, nutrients, gas exchange,      protection from herbivores and suitable environment for reproduction.

Photosynthesis is the process of converting light energy received from sunlight to chemical energy.  The equation for Photosynthesis is:

 Carbon dioxide+ water –^{Solar energy}→ glucose + oxygen 

Glucose is a carbohydrate that consists only of carbon, hydrogen and oxygen atoms. The main source of chemical energy for the plant is from carbohydrates. In an environment that has enough sunlight, water and carbon dioxide; plants can meet their need for chemical energy through photosynthesis..

Herbivores eat plants mostly because they are rich in carbohydrates.  Since plants are not mobile and they need to protect themselves, they adapted changes according to their environment. Example: producing toxins and bad tasting substances, making outer layers tough, hairy and prickly.

Nutrients like Nitrogen, phosphorus and potassium are used to synthesize the proteins lipids and other components needed for plants. With the help of mycorrhizal fungi present in the roots, plants absorb these nutrients present in water. Water is also necessary for the growth and repairing of plant cells. If plants loose too much water or if there is no water, the plant will wilt and die, and if the plant is exposed to too much water also it will die.    

Plants exchange gases with the environment during the process of photosynthesis and cellular respiration. In vascular plants, leaves are used for respiration.

            The Vascular plant Body: Roots and Shoot

Vascular plants are a large group of plants with different shapes and sizes. But the body of the vascular plants has the same design. The body of the vascular plants includes an underground root system and a shoot system above ground. The call walls of these plants are mainly composed of cellulose and molecules–linked chain of glucose. Vascular plants have three main non-reproductive organs. They are the leaf, the stem and the root. These organs are made up of three main tissues. These tissues are dermal tissue, vascular tissue and ground

tissue. Dermal tissues are the outer most cell layers. They often have thicker cell walls. They are covered with a waxy cuticle. There are two tissue types: epidermis and periderm. The dermal tissues protect plants from injury, herbivores, disease, and water loss. The vascular tissue has two tissue types: xylem and phloem. Xylem is the thick walled cells that are dead at maturity and phloem are thin walled cells that are alive at maturity. The xylem and phloem transport water and nutrients and support plant body. The ground tissue has three tissue types. They are parenchyma, collenchymas and sclerenchyma. The parenchyma is the thin walled cell living at maturity. Collenchyma is the thick walled cell living at maturity. The sclerenchyma is the cells that have lignin in their cell walls and they are dead at maturity. Parenchyma and collenchyma support the growth and development of the plant. The parenchyma stores carbohydrates and starch. The collenchyma and sclerenchyma support and protect the plant body.

Meristematic tissue is also a part of the vascular plants. Meristematic tissues are also called meristem. Meristem cells are the actively dividing cells. They are found in the parts of plants where growth occurs. And they eventually develop into specialized cells and tissues.

            Updating the Phylogeny of Vascular Plants

There are three major groups of vascular plants. They are the lycophytes and pteridophytes, the gymnosperms, and the angiosperms.

 The seeds of angiosperms contain either one or two cotyledons.  Angiosperms were divided into monocots and dicots. Monocots have one cotyledon and dicots have two cotyledon. But recent DNA studies suggest that dicots can be again divided into Amborellales, nymphaeales, other early angiosperms and eudicots. Monocots are the most recently evolved. They evolved 100 million years a

2.        Leaves

Functions of Leaves

            Leaves play important roles in a plant’s growth. Leaves are the primary site of photosynthesis in most plants. Leaves also help in the gas exchange between the plants and the environment. Chloroplast present in the leaves help in absorbing sunlight. The chloroplast is the site of photosynthesis in plants. The chloroplast contains photo pigments that absorb particular wavelength of light. Leaves also protect plants by producing sharp spines, bad taste, irritating compounds etc.

Structure of Leaves

Most leaves have a flattened area known as blade. The blade is attached to the stem by petiole. The veins in the leaves contain vascular tissues. Venation is the arrangement of veins on a leaf. The venation of monocots differs from that of eudicots. Monocots have parallel venation and eudicots have branched venation.

Internal Leaf Structure

            Cells are arranged in leaves in a manner to support different cell functions happening in a leaf. Epidermal cells are the outermost layer which is covered by cuticle. Cuticle is a waxy coating that prevents water losing and be a barrier against bacteria, moulds and insects. Epidermal cells are transparent and they do not contain chloroplast. Chloroplast is mainly found on the cells of mesophyll. This tissue is specialized for photosynthesis. In most plants there are two types of mesophyll palisade mesophyll and spongy mesophyll. Palisade mesophyll are elongated and closely packed, and they contain most chloroplast. Spongy mesophyll is loosely packed with air spaces for the gas exchange. The vascular tissues in the vein are situated at the spongy mesophyll.

            In plants like water lilies which are aquatic the spongy mesophyll is replaced with aerenchyma. Aerenchyma is found in aquatic plants and it is composed of loosely packed parenchyma cells with large pores between them. Aerenchyma helps leaves to float on water. Most of the floating leaves have stomata on their upper epidermis, thus exposed to air. The leaves that do not float have no stomata.

Leaf Specializations            

            Plant leaves have adaptations to protect plants. Most leaves produce chemicals to repel herbivores. Some other species of plants protected themselves with the structural adaptations on their leaves.

            In cold regions that experience temperature below freezing level the leaves containing water freezes and will be killed. In order to save the water and nutrients plants with these kind of leaves protect themselves by losing their leaves in the fall.

Human Uses of Leaves

            Leaves provide important nutrients to humans and other living organisms and have a very important role in their diet. Leaves are also used to add flavor to food. Leafy vegetables that are dark green in colour contain important like calcium, potassium, iron and magnesium and also vitamins B, C, E and K. They also provide nutrients that protect our cells from damage. Leaves are also used for various religious ceremonies, industries etc.

Leaves and Chemicals

            Many plants protect themselves by producing toxins in their leaves. Many of these toxins are poisonous to humans. But now humans discovered different ways to use plants toxins in beneficial ways. Some plants toxins are used for treating cancer and heart disease.

Psychotropic Drugs   

            Some plants produce chemicals that affect the nervous system. These chemicals are psychotropic. Psychotropic means they alter perception, emotion, or behaviour.

3.        Stems

    Functions of stem

                Stems play a very important role in plant life. They connect the vascular tissues in the leaves to the vascular tissues in the root. This allows the transportation of water and dissolved substances. Stems also support the leaves and other parts of the plants. Help leaves to get maximum sunlight. Help in pollination by raising flowers in an ideal position. Stem is also used to store water in some species. Stems also help in protecting plant by using tough outer layer, thorns etc.

Stem Structure

                There are different types of stem among the plant species. The two types of stem commonly seen are herbaceous stem and woody stem. Herbaceous stem are stems that do not contain wood. Their stems are pliable and have chloroplast in it so they carry out photosynthesis and their epidermis is thin. Woody stem are stems that contain wood. Their stem is hard and has barks. All gymnosperms have woody stem. Most of the angiosperms that have woody stem are eudicots. Monocots do not have woody stem but some plants have stems with similar characteristics.

Anatomy of Herbaceous Stems

The figure above shows the anatomical structure of herbaceous stems of monocots and eudicots. In herbaceous stems, the vascular tissue is arranged in distinct vascular bundles in ground tissue. A vascular bundle is a long, continuous strand of vascular tissue that consists of xylem and phloem. From roots to leaves vascular bundles runs continuously. In the vascular bundle the xylem is always closer to the centre of the stem, and phloem is always closer to the outside of the stem. These vascular bundles are arranged differently in monocots and eudicots. In monocots vascular bundles are found throughout the ground tissue and in eudicots vascular bundles form a ring.

Anatomy of Woody Stems

                Woody stems have a more complex structure. They grew thicker over the years due to the presence of the vascular cambium. Vascular cambium is a layer of meristematic cells in the vascular tissue that divide to form new xylem and phloem. Every year vascular cambium produces a new layer of xylem and phloem.

                Bark includes all tissues found outside the vascular cambium. Bark also includes phloem, cork cambium and cork. The sugar is transported with the help id phloem. The cork cambium produces cork. The cork is a thick layer which protects the stem and prevents water loss.

Growth Rings

                Growth only happens in spring and summer in temperate regions. In spring the vascular cambium grow more rapidly and produce more xylem which has thin walls. This layer of wood which has a light colour is called spring wood. During summer less amount of xylem is produced with thick layer and the bark colour is dark. This kind of wood is known as summer wood. Together these layers are called growth ring.

Cell Types in Vascular Tissues

The xylem and phloem helps the vascular tissue carry out its functions of transport and support. There are two types of xylem cells; they are tracheids and vessel elements. Tracheid is a long cylindrical cell that has tapered ends and the vessel element is shorter and wider than tracheid with less tapered ends. There are three types of phloem cells: sieve cells, sieve tube elements and companion cells. Sieve cells has nucleus and other features found in most cells. Sieve tube elements have cytoplasm but do not have a nucleus. Companion cell is always linked with a sieve tube element. It has a nucleus and all other elements that sieve tube element lacks.  

4.        Roots

Functions of Roots

                Roots work as the anchor of the plant and keep it upright. The other main role of the root is to absorb water and nutrients other than carbohydrates. Some roots store water and carbohydrates produced by the photosynthesis.

Types of Root Systems

                There are two types of root system the taproot system and fibrous root system. Taproot system has a large thick root and has smaller branches called lateral roots and fibrous root system has smaller roots with root hairs.

General Structure of Roots

                Root tip contains root cap and the meristem. Root cap is a thick layer of cells that produce a slippery substance that help roots to penetrate deep into the soil. The meristem produces cells and increases the length of the root. Root hairs are found on the root tips which help in increasing the surface area of the root. Root cortex is a region of parenchyma cells under the epidermis. Cells in the root cortex store carbohydrates and helps in transporting water from epidermis to xylem. The root cortex ends at the endodermis. A wax like substance wraps the walls of the cells of the endodermis forming a continuous barrier called the Casparian strip. The vascular cylinder contains the vascular tissues of the roots. The arrangement of xylem and phloem contained in the vascular cylinder varies among the angiosperms group. In gymnosperms and eudicots, the center of the root contains xylem cells forming an “X” or star shape in the center of the vascular cylinder. The phloem cells are also found at the centre of root around the xylem. But in monocots the centre of the root contains parenchyma cells surrounded by rings of xylem and phloem cells. The anatomy of a woody root is as same as the anatomy of a woody stem.

Root specializations

                The specializations help root more efficiently absorb water and nutrients, anchor the plant and store carbohydrates. The primary role of the plant is to absorb water and nutrients and some micro organisms help in help them in this task. The roots of over 80% plants have mutual relationship with mycorrihizal fungi. They help in penetrating into more small space than a root system can go and can break down organic matter. The fungi exchange water and nutrients with the plant for carbohydrates. Another relationship is the link between roots and the nitrogen fixing bacteria. The bacteria convert the nitrogen present on the air to a form where plants can use it. The bacteria live within nodules in plant roots.

                Most of the roots eaten by humans and other organisms have specialized carbohydrate storage. In other species, the lateral roots are specialized for storage. Lateral roots modified for storage are called tuberous roots.

Human Uses of Roots

                We use roots mostly for food. Roots are also a source of useful chemicals. Most vegetables that we use are roots. Roots are also used to make beverages and to feed livestock. Roots are used for making dye in textile industry. The chemicals produced from roots can be used as pesticides. Roots are also used to make some medicines to.

5.        Transport in Vascular Plants

Overview of Transport in the Plant

                The water in an environment contains dissolved substances including nutrients. This water called soil water moves between xylem and phloem and is critical to the delivery of soil nutrients and sugar to all parts of the plant. Sugars are transported in the phloem and nutrients are transported in the xylem.

Transport of Water and Nutrients

                The transportation of water and nutrients from the surface of the root to the parenchyma cells in the leaves is a complex process. The same process takes place in the aquatic plant.

The transport of water and nutrients involves three stages: (I) From soil to root, (ii) from roots to the stem, (iii) from the stem to the leaves.

Transport into the root

                The transport of water and nutrients happens by two different processes, they are water entering by osmosis and the entering of nutrients by active transport. Osmosis is the diffusion of water molecules across a selectively permeable membrane, form an area of higher concentration to an area of lower concentration.

                The cytoplasm of plant cells has lower concentration of water molecules. So the plant cell membrane allows water molecules to cross freely. Therefore water molecules enter root cells through osmosis.

                The nutrients needed would not enter the root cells by diffusion since the concentration of cytoplasm is higher than the concentration of nutrients in the soil water. Instead, active transport is used to move soil water into the roots. When the needed substances are in the cytoplasm the outer root cells are moved through the cells of cortex toward the endodermis. Once the nutrients reach the root cell they will not have to cross another membrane until they reach vascular the vascular cylinder. When they reach the endodermis they need to encounter the Casparian strip. This wax like structure prevents all the substances passing through the spaces between endodermal cells. The main role of the Casparian strip is to prevent substance from leaking back to the cortex.

Transport into the Stem

                Once the water molecules and nutrients cross the Casparian strip the liquid finally formed is called xylem sap. Substance in the xylem sap then move towards the stem. These substances are moved by osmosis. Capillary action also helps in this process.

Transport into the Laves

                But the capillary action and osmosis cannot alone take these substances to the top of the tree. The main driving force is actually coming from the leaves. Plants release water vapor through stomata during transpiration. The water vapor evaporates through the stomata when they are open. Because of the attraction forces between the water molecules the water molecules in the xylem column moves up and also they pull the molecules behind it.

                If the plant dose not has sufficient water it will wilt. When a plant is stores enough nutrients and water in the vacuoles, the vacuole exerts pressure against the cell wall. This pressure is called turgor. The turgor helps in supporting the plant. When a plant is unable to take up water from the soil, water will move out of the vacuole and the plant will wilt.

Transport of Sugars

                Plants use glucose and other sugars as a source of energy. These sugars are made by photosynthesis. A cell with a higher concentration of sugar is called source. A sink is a cell with a lower concentration of sugar.

Direction of Sugar Transport

                Sugar transport is different from transferring other nutrients. Unlike them sugar transport can be in any direction. The direction is related to the location of source cells and sink cells. In general the sugars are transported from a source to sink. The location of the sink and source cell may change. In regions with four seasons the location of the source and sink cells change according to the seasons. The transport of sugar can be divided into three general stage; (I) transport of sugars from source cells to phloem cells, (ii) transport through the phloem and (iii) transport from phloem cells to sink cells.

From Source to Phloem

                After a sugar molecule is produced it must be transported from the source cells and phloem cells. The concentration of sugars in phloem cells is generally higher than the concentration in source cells. So the sugar transport to the phloem involves active transport across the cell membrane.

Translocation

                The transport of substances for long distances through the phloem is called translocation. Unlike xylem tubes, phloem tubes are not hollow and the substances on the phloem sap have to move between the living cells. But sugar molecules move faster through phloem cells. Translocation therefore moves sugars quickly enough to supply energy to cells throughout the plant.

From phloem to Sink

                The sink cells have lower concentration of sugar than phloem cells. So sugar cells move from phloem to the sink cells by passive transport. In angiosperms the sugar moves from the sieve tube elements to the companion cells and then to the sink cells. Since sieve tubes lose sugar water it has a less concentration of sugar. Water then returns to the xylem by osmosis. This maintains the low turgor of phloem cells near the sink and the recirculation of water back into the xylem.

1. Succession

                Succession is a gradual change over time in the species that form a community.

Primary succession 

                Primary succession is the succession in an area that has no plant, animals, or soil. This takes place in completely barren rock or mineral deposits. This may also occur on life less surface. Primary succession begins when organisms start living on these bare rocks or lifeless places. These first colonizers are called pioneer species. As the succession proceeds the community slowly changes the biotic and abiotic. As the biotic and abiotic factors change the environments becomes favorable to some organisms and less favorable to others.

Secondary succession

                Secondary succession is the succession in an ecosystem that has been disturbed by a natural event or human activity. Natural events consist of forest fires, floods, storms etc. Human activities include deforestation.

2. Asexual Reproduction in Seed Plants

Structure Involved in Asexual Reproduction

                Plants have a various types of structures for asexual reproduction. Some has modified stem others have modified leaves and some have modifications on their roots.

Costs and Benefits of Asexual Reproduction

·          If a plant a particular trait to take advantage of a particular environment, all of its offspring have the same trait.

·          The plants don’t need to produce specialized reproductive structure.

·          Production needs less energy

·          Only one plant is required

·          Offspring have a high survival rate

3. Sexual Reproduction in Seed Plants

Seed Function and Structure

               The seed mainly has two functions: to protect and nourish the enclosed embryo and to carry the embryo to a new location. Dispersal can move a plant's seed to locations where there is less competition.    

                The figure below shows example of the general structure of seeds in monocots and eudicots. These seeds contain an embryo, a nutritive tissue to support embryo growth, and a seed coat for protection.

Costs and Benefits of Sexual Reproduction                   

·          Offspring produced have a high level of genetic diversity

·          The products of sexual reproduction are seeds. Seeds can be dispersed away from the parent plant so that the seedlings may get better growing condition or less competition for resources.

·          Seeds remain dormant for a long time and germinate in favorable condition thus increasing the chance of survival.     

                    Sexual Reproduction in Gymnosperms

                                The reproduction of gymnosperms has great importance in our day to day life. Conifers produce both male and female cones.  Haploid cells called microspores are produced by meiosis within male cone and each of the microspores develops into a pollen grain containing a male gametophyte. Meiosis in female cones produces megaspores that give rise to the egg-producing female gametophyte.

Pollination and Fertilization

                The transferring of pollen grains from anther of one plant to the ovule of the same or another plant is called pollination. When a pollen grain lands near the ovule a hollow tube grows out of the pollen and carries the pollen nucleus to the female sex cell. The pollen produces two sperm nucleus. One sperm nucleus fertilizes the egg and the other sperm nucleus degrades. It takes about 13 months for the egg to get fertilized. After fertilization the ovule develops into the various structures in the seed and the zygote develops into the embryo.

Sexual Reproduction in Angiosperms

                The products of the sexual reproduction in angiosperms are seeds inside the fruit. Fruit is the matured ovary. These seeds and fruits are important to many organisms, since they contain energy and nutrients.

                Flowers are the key organs in sexual reproduction of angiosperms. The figure below shows the general structure of the flower. The stamens make up the male reproductive flower parts. Stamen consists of anther and filaments. The anther produces pollen grains. The filament raises the anther above the female organs. Carpel includes the female reproductive parts. The stigma is a sticky surface that acts as a landing site for pollen grains. Below the stigma is the style, a tube-like structure that leads down the ovary. The ovary contains one or more ovules, each of which forms a seed when it is fertilized.

                There are distinct differences between monocot and eudicots flowers. The petals and stamens of monocot flowers are always in multiples of three. In eudicot flowers, petals and stamens are in multiples of four or five.      

                The figure below shows the changes that take place in flower structures during the life cycle of typical angiosperms.

Pollination and Fertilization

                In angiosperms, pollination happens by wind or by animals. Pollinators are the animals that transport pollen grains. In angiosperm most species cross pollinate. In cross pollination the pollen grains are transferred from one individual to another. But some plants can self pollinate. In self pollination pollen can transfer from one flower to another on the same plant. Plants are capable of both self and cross pollination. Pollination occurs as soon as a pollen grain sticks to the stigma. When conditions are right the pollen grains begin to form a pollen tube. The pollen grows down the style till ovary. As in gymnosperms the angiosperms also carries two haploid sperm nuclei. When the sperm nucleus reaches the ovary the, both sperms are involved in separate fertilization events. This is called double fertilization. One sperm nucleus unites with the egg cells contained in the ovary, forming the diploid zygote. The second sperm nucleus fuses with two polar nuclei in the ovule forming a triploid cell.      

5. Plant Growth and Development

Types of growth

                Growth is the process of cell enlargement. Differentiation is the process of cell specialization. Plants continue to grow their entire life. The apical meristem is a plant tissue consisting of actively dividing cells responsible for the increase in height of the plant. All growth from apical meristem is called primary growth. Primary growth continues throughout the life of the plant. Secondary growth is the growth that occurs from lateral meristem and it results in the increase in girth. The lateral meristem is a plant tissue consisting of actively dividing cells that produce secondary growth.

Primary Growth   

                The length of a plant shoot or root is increased by the primary growth. It starts as the cells of the apical meristem divide by mitosis. When the cell division ends, each cell grows long. Then the elongated cells grow into different specialized cell types. The shoot apical meristem produces the tissues that form stems, leaves and the organs responsible for sexual reproduction like flowers. The cells in the outermost part of the shoot develop into epidermal cells and the inner cells become the vascular tissue. The root apical meristem produces the root cap cells. The zones of cell division, elongation and differentiation are more clearly shown in the root.

Secondary Growth    

                Secondary growth only happens in woody species after the plants first year. One of the products of secondary growth is wood. Lateral meristem is where the growth cell arises and all the tissues formed are called secondary tissues. The lateral meristem is situated at the vascular cambium. After the first year of growth primary and secondary growth happens simultaneously woody species continue the primary growth and increase their length. They also grow in diameter through secondary growth from two lateral meristems.

                Growth rings are produced every year on trees. These growth rings are of different thickness. It is because of the change in environment. So growth rings can give information about the past environmental conditions in a region.

Environmental Factors That Affects Plant Growth and Differentiation

                All plants respond to changes in environment in some way. The main factors that affect these changes are light, water, temperature and the availability of nutrients. The presence of ther organisms and humans can change these factors.

Light    

                Plant use light energy for photosynthesis. Sunlight is actually a spectrum of different wavelengths of light, each with a different energy level. In an environment the particular wavelength of light that reaches a plant vary.

Seasonal Changes in Light

                The farther place from equator, the change in the intensity of light happens during different seasons. As the day length changes the wavelength of light that reaches the earth also change. Plants are able to detect the changes in light through photoreceptor cells in their leaves.

Photoperiodism

                Photoperiodism is the plant response to change in day length. An example is the timing of flowering. Plants that flower only when days are short are called short-day plants. Other plants are long-day that flower only when there is 12 hour or more of daylight. Photoperiodism can ensure that a plant flowers only when other environmental condition are likely to be best of reproduction.

Nutrients

                There are 2 categories of plant nutrients, they are macronutrients and micronutrients. Macronutrients are nutrients that are needed in larger quantities. Nitrogen, phosphorus and potassium are the macronutrients. Micronutrients are nutrients that plants need in only very small amount. They are boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc.

Temperature

                The rate of all cellular process is affected by temperature. The opening and closing of stomata depends on the temperature. If temperature is above or below the optimum temperature of the plant the plants will grow slowly.

6. Control of Plant Growth and Development

Plant Growth Regulators

                The body shapes of plant changes as they grow. Since plants cannot change their location this is an adaptation that allows plants to respond to changes. Plant growth regulators are chemicals that plants uses to modify their growth. The 5 main plant growth regulators are auxins, gibberellins, cytokinins, ethylene and abscisic acid.

Tropism and Plant Growth Regulators

                Tropism is a change in the direction of the growth of a plant according to stimulus. Tropism is also controlled by plant growth regulators. The tropism towards light is called phototropism. Gravitropism is the change in growth pattern of a plant in response to gravity. Thigmotropism is the directional change in growth pattern in response to touch.

Auxins

                Auxins are a group of compounds that act in similar ways on plant growth and cell differentiation. Auxins are present mainly on shoot apical meristem. Auxins promote cell elongation. During phototropism the side closest to the light contains less auxin than the other one. So the cells on the dark side elongate and the stem bends maximum towards the light and maximize the intake of light energy.

Gibberellins

                Gibberellins are the family of compounds that share a similar chemical structure. Gibberellins promote cell division and cell elongation depending on the tissue they are affecting. The environmental factors influence the effect of gibberellins on different plant tissues. Gibberellins also play a role in flower and fruit production in many species. They also plays a role in response of plant temperature change.

Cytokinins

                Cytokinins promote cell division. They are found on actively dividing tissues like meristems, young leaves and growing seeds. Cytokinins help to stimulate cell division in lateral buds when an apical bud has been removed. The normal development of roots and shoots are done with the help of auxins and cytokinins. Cytokinins also slow cell aging on in certain plant organs by inhibiting protein breakdown and stimulating protein synthesis.

Ethylene

                Ethylene is also called the plant stress hormone because it induces changes that protect a plant against environmental stress. This chemical also regulates the growth of roots and shoots around obstacles. It is also released at the site of a wound on a plant. The ripening of the root, shoot and root growth and differentiation, flower opening, dropping of leaves and fruits and leaf senescence are also stimulated by ethylene.

Abscisic Acid

                The primary role of this acid is to inhibit growth. ABA levels rise in response to the changes in temperature and light. ABA maintains dormancy in seeds and leaf buds. ABA controls the closing of stomata when the environment is dry.  

Bibliography- My greade 12 BIOLOGY text book.