Download Plant Cell Structure and Function: A Comprehensive Overview and more Study notes Advanced Education in PDF only on Docsity!
BBY 1214 PLANT STRUCTURE AND FUNCTION NOTES 1
Plant Cells Structure
The cell is the basic unit of life in all organisms. Like humans and animals, plants are also composed of several cells. The plant cell is surrounded by a cell wall which is involved in providing shape to the plant cell. Apart from the cell wall, there are other organelles that are associated with different cellular activities.
Plant Cell Definition
“Plant cells are eukaryotic cells with a true nucleus along with specialized structures called organelles that carry out certain specific functions.” One of the distinctive aspects of a plant cell is the presence of a cell wall outside the cell membrane.
Plant Cell Diagram
The plant cell is rectangular and comparatively larger than the animal cell. Even though plant and animal cells are eukaryotic and share a few cell organelles , plant cells are quite distinct when compared to animal cells as they perform different functions. Some of these differences can be clearly understood when the cells are examined under an electron microscope.
Plant Cell Structure
Plant cell structure includes various components known as cell organelles that perform di昀昀erent func琀椀ons to sustain itself. These organelles include:
Cell Wall
It is a rigid layer which is composed of cellulose, glycoproteins, lignin, pectin and hemicellulose. It is located outside the cell membrane. It comprises proteins, polysaccharides and cellulose.
The primary function of the cell wall is to protect and provide structural support to the cell. The plant cell wall is also involved in protecting the cell against mechanical stress and to provide form and structure to the cell. It also filters the molecules passing in and out of the cell.
The formation of the cell wall is guided by microtubules. It consists of three layers, namely, primary, secondary and the middle lamella. The primary cell wall is formed by cellulose laid down by enzymes.
Cell membrane
It is the semi-permeable membrane that is present within the cell wall. It is composed of a thin layer of protein and fat.
The cell membrane plays an important role in regulating the entry and exit of specific substances within the cell.
For instance, cell membrane keeps toxins from entering inside, while nutrients and essential minerals are transported across.
Nucleus
The nucleus is a membrane-bound structure that is present only in eukaryotic cells. The vital function of a nucleus is to store DNA or hereditary information required for cell division, metabolism and growth.
- Nucleolus : It manufactures cell’s protein-producing structures and ribosomes.
- Nucleopore : Nuclear membrane is perforated with holes called nucleopore that allows proteins and nucleic acids to pass through.
Plastids
They are membrane-bound organelles that have their own DNA. They are necessary to store starch, to carry out the process of photosynthesis. It is also used in the synthesis of many molecules, which form the building blocks of the cell. Some of the vital types of plastids and their functions are stated below:
Leucoplasts
They are found in non-photosynthetic tissues of plants. They are used for the storage of protein, lipid and starch.
Chloroplasts
It is an elongated organelle enclosed by phospholipid membrane. The chloroplast is shaped like a disc and the stroma is the fluid within the chloroplast that comprises a circular DNA. Each chloroplast contains a green coloured pigment called chlorophyll required for the process of photosynthesis. The chlorophyll absorbs light energy from the sun and uses it to transform carbon dioxide and water into glucose.
Chromoplasts
They are heterogeneous, coloured plastid which is responsible for pigment synthesis and for storage in photosynthetic eukaryotic organisms. Chromoplasts have red, orange and yellow coloured pigments which provide colour to all ripe fruits and flowers.
Sclerenchyma Cells
These cells are more rigid compared to collenchyma cells and this is because of the presence of a hardening agent. These cells are usually found in all plant roots and mainly involved in providing support to the plants.
Parenchyma Cells
Parenchyma cells play a significant role in all plants. They are the living cells of plants, which are involved in the production of leaves. They are also involved in the exchange of gases , production of food, storage of organic products and cell metabolism. These cells are typically more flexible than others because they are thinner.
Xylem Cells
Xylem cells are the transport cells in vascular plants. They help in the transport of water and minerals from the roots to the leaves and other parts of the plants.
Phloem Cells
Phloem cells are other transport cells in vascular plants. They transport food prepared by the leaves to different parts of the plants.
Plant Cell Functions
Plant cells are the building blocks of plants. Photosynthesis is the major function performed by plant cells.
Photosynthesis occurs in the chloroplasts of the plant cell. It is the process of preparing food by the plants, by utilizing sunlight, carbon dioxide and water. Energy is produced in the form of ATP in the process.
A few plant cells help in the transport of water and nutrients from the roots
and leaves to different parts of the plants.
Plant Tissues
Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Plant tissue systems fall into one of two general types: meristematic tissue and permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems , which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.
Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.
Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue. Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars.
Secondary tissues are either simple (composed of similar cell types) or complex (composed of different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes three different cell types: vessel elements and tracheids (both of which conduct water), and xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells (which conduct photosynthates), companion cells, phloem parenchyma, and phloem fibers. Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other (Figure 1). In stems, the xylem and the phloem form a structure called a vascular bundle ; in roots, this is termed the vascular stele or vascular cylinder.
Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.
Dermal Tissue
The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark, which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place (Figure 2). Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.
The main characteristics of parenchyma are:
They are living permanent 琀椀ssues that have the ability to divide at maturity and help in the regenera琀椀on and healing of wounds Parenchyma cells are the founda琀椀on of a plant as reproduc琀椀ve cells (spores, gametes) are parenchymatous in nature Single parenchyma cell of a zygote has an ability to develop into an en琀椀re plant. These cells are called “to琀椀potent” cells Parenchyma cells occur in the form of con琀椀nuous masses as homogeneous parenchyma 琀椀ssues e.g. in pith and cortex of stems and roots, mesophyll of leaves, the 昀氀esh of succulent fruits and in the endosperm of seeds Parenchyma cells may be associated with other types of cells to form heterogeneous complex 琀椀ssues such as parenchyma of xylem and phloem Parenchyma cells are essen琀椀al for ac琀椀vi琀椀es like photosynthesis, storage, secre琀椀on, assimila琀椀on, respira琀椀on, excre琀椀on and radial transport of water and solute
Structure of Parenchyma Cells
It is a living cell It has a prominent nucleus and protoplast The cells of parenchyma are isodiametric or polyhedral in shape. They may be polygonal, oval, round or elongated These cells are closely packed or may have small intercellular space They are made up of thin cell wall made up of cellulose, hemicellulose Plasmodesmata join the cells of the parenchyma 琀椀ssue They have several small vacuoles. In the older parenchyma, smaller vacuoles merge to become a large central vacuole, which may accumulate anthocyanin or tannins Water is abundant in the vacuoles of the parenchyma cells that act as a water reservoir Storage parenchyma cells may have thick xyloglucan walls e.g. in the endosperm of the date palm. The sugar is used during germina琀椀on and walls become thin The parenchyma cells of 昀氀owers and fruits contain chromoplasts Parenchyma cells may have a thick ligni昀椀ed wall that makes it di昀케cult to di昀昀eren琀椀ate it from sclerenchyma Hydraulic property of cells gives the parenchyma its mechanical strength Chloroplasts are present in the parenchyma cells that are speci昀椀ed to perform photosynthesis The parenchyma cells which perform a secretory func琀椀on, have dense protoplasm that is rich in ribosomes, Golgi bodies and a highly developed endoplasmic re琀椀culum
Types of Parenchyma Cells
Parenchyma cells can be categorised based on their structure, location and functions performed. The main parenchyma tissues are:
Chlorenchyma:
Cells which have chloroplast and perform photosynthesis
o The mesophyll cells in leaves which di昀昀eren琀椀ate into palisade and spongy cells In the other green parts of the plants like stems, sepal etc. o Transfer Cells: They play an important role in the transport of solutes over short distances. They have cell wall ingrowths, which greatly increase the surface area of the plasma membrane Sucrose is transported across the membrane through a proton/sucrose co- transport mechanism These are found in the areas of absorp琀椀on and secre琀椀on in plants like nectaries, salt glands and in carnivorous plants o Vascular Parenchyma: The parenchyma cells which are associated with vascular 琀椀ssues. These are of two types: o Phloem Parenchyma:
o It is made up of elongated, tapering and cylindrical cells having dense cytoplasm. Plasmodesmata connec琀椀ons occur between the cells through pits in the walls It stores food and other materials like resins, latex and mucilage Absent in monocotyledons
Xylem Parenchyma:
It is made up of thin-walled cells. The cell wall is made up of cellulose.
o It stores food materials like starch, fats and other substances such as tannins and crystals o Radial conduc琀椀on of water takes place by ray parenchymatous cells o In the water-stress condi琀椀on, they help in preven琀椀ng damage to tracheids and vessels Storage Parenchyma: These store various substances like water, starch, proteins etc. They act as a food and water reservoir. o Stored protein is a good source of nitrogen for plants o In starch storing cells like in potato tubers, the endosperm of cereals and cotyledons, abundant starch-containing amyloplasts are present o Parenchyma cells may be specialised as a water storage 琀椀ssue in succulent plants such as Cactaceae, aloe, agave, etc. o In the underground storage like in potato tuber, it can ini琀椀ate the shoot growth and provide moisture for the ini琀椀al growth of growing parts Prosenchyma: These are 昀椀bre-like elongated cells, which are thick-walled and provide rigidity and strength to the plant
Cu琀椀cle present on epidermis helps in reducing transpira琀椀on in water stress condi琀椀on Thick-walled parenchyma cells provide mechanical strength to the plant Healing and regenera琀椀on: Parenchyma cells that retain their ability to divide even on maturity help in regenera琀椀on and wound healing. Tyloses present in the xylem parenchyma help in preven琀椀ng damage to vascular 琀椀ssues in the condi琀椀on of drought.
Collenchyma Tissue
Collenchyma tissue is a term given by a scientist named Schleiden in the year 1839. It is a kind of simple permanent supportive tissue that give mechanical strength to the plant. The collenchyma cells appear as elongated cells with a non-uniform thickened cell wall.
Collenchymatous cells originate by the modification of parenchyma tissues into the cells, which comprises a thickened cell wall due to the deposition of cellulose, hemicellulose and pectin like substances.
It exists under the epidermis layer of stem, leaves, petiole etc. and may or may not contain chloroplast. Collenchyma tissues are of many types, based on their location and cell arrangement. This post describes the definition, characteristics, types, structure and functions of the collenchyma tissue.
Definition of Collenchyma Tissue
Collenchyma tissue refers to the simple permanent tissue, which comprises axially elongated cells with a non-uniform and thickened cell wall (composed of pectin, cellulose and hemicellulose). Collenchymatous tissues are sometimes associated with vascular bundles and generally located in the hypodermis layer (underneath the epidermis ).
Collenchyma cells lack hydrophobic components. It only gives mechanical strength to the plants when the cells are in a turgid state. It possesses a prominent nucleus with developed cell organelles and compact cell arrangement.
Characteristics
Collenchymatous tissues show the following features:
- They are the living cells with living protoplasm and axially elongated cells.
- The cell wall is thickened towards the cell’s corner due to pec琀椀n, cellulose and hemicellulose deposi琀椀on. Some琀椀mes, tannin also deposits within the cell wall.
- Unlike sclerenchyma 琀椀ssue , a collenchyma cell comprises a non-ligni昀椀ed cell wall.
- Collenchyma cells are usually having a compact cell arrangement with li琀琀le or no inters琀椀琀椀al spaces.
- The collenchyma cells possess varying cell shapes and sizes.
- Generally, the collenchymatous 琀椀ssues are absent in monocots and the region of the root system.
- Collenchyma 琀椀ssues predominantly exist just below the epidermis of dicotyledonous stems, leaves etc.
- Their 琀椀ssues are elas琀椀c or extensible , which gives 昀氀exibility to the stems in bending without breakage.
Types
Depending upon the pattern of wall thickening and cell arrangement and their location, the collenchyma tissues are classified into the following types:
Angular Collenchyma
It seems polygonal in shape. The thickening pattern of the cell wall is towards the corner. The extra-wall material deposits on the vertical walls where the cells meet. It has a compact cell arrangement with no intercellular spaces. Example : Petioles of Cucurbita in the hypodermis layer.
Annular Collenchyma
It consists of rounded cells and possesses invariably thickened cell wall.
Lamellar Collenchyma
It refers to the plate or tangential collenchyma , which possesses longitudinally elongated cells. The thickening pattern of the cell wall restricts the tangential walls. The intact rows of cells are arranged tangentially, leaving no intercellular spaces. Example : Stem of Sambucus in the hypodermis layer.
Lacunar Collenchyma
It refers to the tubular collenchyma , which possesses spherical or oval-shaped cells. The thickening pattern of the cell wall is towards the direct contact of intercellular spaces. Example : Petioles of Salvia, Malvia etc., in the hypodermis layer.
Peripheral Collenchyma
In this type, the collenchyma cells are located below the outermost epidermis layer and concentrated by one or more layers of parenchyma cells. It further subdivides into two kinds:
- Con琀椀nuous : Here, the collenchyma cells appear as a con琀椀nuous layer.
- Strand : Here, the cells appear as the discrete axial strands concentrated from one another by the parenchyma cells.
Conclusion
Therefore, a collenchyma cell is a supporting tissue whose cell wall material is irregularly distributed, due to which it has uneven cell wall thickenings. It is characteristically found in stems, leaves etc., of dicotyledonous plants.
Thus, we can conclude that the collenchyma tissue has three primary features like the presence of a living protoplast, thickened cell wall and axially elongated cells.
sclerenchyma
sclerenchyma , in plants, support tissue composed of any of various kinds of hard woody cells. Mature sclerenchyma cells are usually dead cells that have heavily thickened secondary walls containing lignin. The cells are rigid and nonstretchable and are usually found in nongrowing regions of plant bodies, such as the bark or mature stems. Sclerenchyma is one of the three types of ground, or fundamental, tissue in plants; the other two types are parenchyma (living thin- walled tissue) and collenchyma (living support tissue with irregular walls). Sclerenchyma cells occur in many different shapes and sizes, but two main types occur: fibres and sclereids.
Fibres are greatly elongated cells whose long, tapering ends interlock, thus providing maximum support to a plant. They often occur in bundles or strands and can be found almost anywhere in the plant body, including the stem, the roots, and the vascular bundles in leaves. Many of these fibres, including seed hairs, leaf fibres, and bast fibres, are important sources of raw material for textiles and other woven goods ( see also list of plant fibres).
Sclereids are extremely variable in shape and are present in various tissues of the plant, such as the periderm, cortex, pith, xylem, and phloem. They also occur in leaves and fruits and constitute the hard shell of nuts and the outer hard coat of many seeds. Sometimes known as stone cells, sclereids are also responsible for the gritty texture of pears and guavas.
nut
The hard shells of many nuts contain sclereids, which are a type of sclerenchyma cell.
tendril
Later, strong mechanical 琀椀ssue ( sclerenchyma ) develops in the tendrils, thus rendering them strong
enough to support the weight of the plant. In addi琀椀on to their twining character, some tendrils produce terminal enlargements that, on contact with a 昀椀rm surface, 昀氀a琀琀en and secrete an adhesive, 昀椀rmly
cemen琀椀ng the tendril to the support.
Meristematic Tissue
Meristematic tissues contain living cells with varied shapes. They possess a large nucleus, very small few vacuoles,and no intercellular spaces .The zone where these cells exist is known as meristem.
The cells of the meristematic tissue divide actively to form specialized structures such as buds of leaves and flowers, tips of roots and shoots, etc. These cells help to increase the length and girth of the plant.
Characteristics of Meristematic Tissue
The characteristics of meristematic tissue are as follows:
- The cells of these tissues are commonly called meristems.
- The meristematic tissue has the quality of self-renewal. Every time the cell divides, one cell remains identical to the parent cell, and the others form specialized structures.
- They have very small and few vacuoles.
- The meristematic tissue is living and thin-walled.
- The protoplasm of the cells is very dense.
- The meristematic tissues heal the wounds of an injured plant.
- The cells of the meristematic tissue are young and immature.
- They do not store food.
- They exhibit a very high metabolic activity.
- They possess a single, large and prominent nucleus.
Types of Meristematic Tissue
Meristematic Tissue On the basis of Origin
Promeristem
The earliest and youngest meristematic tissue. It originates from the embryo. The primary meristem arises from the promeristem. It is found in the root and the shoot tips.
Primary Meristem
It arises from the promeristem. Cells divide actively. It is present below the promeristem and forms the permanent tissue.
Secondary Meristem
It originates from the primary meristem.
The meristematic tissue is usually found in the apices of the root systems and the shoots and is in a continuous state of division.
Apical meristem ,
A region of cells capable of division and growth in the root and shoot tips in plants. Apical meristems give rise to the primary plant body and are responsible for the extension of the roots and shoots. Unlike most animals, plants continue to grow throughout their entire life span because of the unlimited division of these and other meristems.
As in other meristematic regions, the cells of the apical meristems are typically small and nearly spherical. They have a dense cytoplasm and relatively few small vacuoles (watery saclike enclosures). Some of these cells, known as initials, maintain the meristem as a continuing source of new cells and may undergo mitosis (cell division) many times before differentiating into the specific cells required for root or shoot growth. The cells that emanate from the apical meristem are arranged in lineages of partially differentiated tissues known as primary meristems. There are three primary meristems:
i) the protoderm, which will become the epidermis; ii) the ground meristem, which will form the ground tissues comprising parenchyma, collenchyma, and sclerenchyma cells; iii) the procambium, which will become the vascular tissues (xylem and phloem).
Root apical meristem
The root apical meristem, or root apex, is a small region at the tip of a root in which all cells are capable of repeated division and from which all primary root tissues are derived. The root apical meristem is protected as it passes through the soil by an outer region of living parenchyma cells called the root cap. As the cells of the root cap are destroyed and sloughed off, new cells are added by a special internal layer of meristematic cells called the calyptrogen. Root hairs also begin to develop as simple extensions of cells near the root apical meristem. They greatly increase the surface area of the root and facilitate the absorption of water and minerals from the soil.
Beginning with the root cap and leading away from the root tip, there are three distinct zones in which certain specific growth patterns dominate: cell division, cell elongation, and differentiation and tissue maturation. There is a gradual transition between these regions. The region of cell division includes the apical meristem and the primary meristems—the protoderm, ground meristem, and procambium—derived from the apical meristem. As is generally true of nonmeristematic regions elsewhere in the plant body, root length in the second region is increased as a result of cell elongation rather than by cell division. The region of differentiation and tissue maturation that follows is where the cells differentiate (i.e., change in structure and physiology into cells of a specific type) and where the first primary phloem and xylem, as well as mature root hairs, are clearly seen. In plants with woody roots (i.e., those of perennial dicotyledons), secondary growth, including secondary xylem and phloem as well as the periderm, are developed and add girth to the plant.
Shoot apical meristem
All the branches and stems of higher vascular plants terminate in shoot apical meristems. These are centres of potentially indefinite growth and development, producing the leaves as well as a bud in the axis of most leaves that has the potential to grow out as a branch. These shoot apical growing centres form the primary plant body.
.
Vascular tissue
A complex permanent tissue involved in the conduction of materials (e.g. water and nutrients) throughout the plant This vascular system is comprised of complex permanent tissues called vascular tissues. There are two major types of vascular tissues: (1) xylem and (2) phloem.
Xylem
The xylem tracheary elements consist of cells known as tracheids and vessel members, both of which are typically narrow, hollow, and elongated. Tracheids are less specialized than the vessel members and are the only type of water-conducting cells in most gymnosperms and seedless vascular plants. Water moving from tracheid to tracheid must pass through a thin modified primary cell wall known as the pit membrane, which serves to prevent the passage of damaging air bubbles. Vessel members are the principal water-conducting cells in angiosperms (though most species also have tracheids) and are characterized by areas that lack both primary and secondary cell walls, known as perforations. Water flows relatively unimpeded from vessel to vessel through these perforations, though fractures and disruptions from air bubbles are also more likely. In addition to the tracheary elements, xylem tissue also features fibre cells for support and parenchyma (thin-walled, unspecialized cells) for the storage of various substances.
Xylem formation begins when the actively dividing cells of growing root and shoot tips (apical meristems) give rise to primary xylem. In woody plants, secondary xylem constitutes the major part of a mature stem or root and is formed as the plant expands in girth and builds a ring of new xylem around the original primary xylem tissues. When this happens, the primary xylem cells die and lose their conducting function, forming a hard skeleton that serves only to support the plant. Thus, in the trunk and older branches of a large tree, only the outer secondary xylem (sapwood) serves in water conduction, while the inner part (heartwood) is composed of dead but structurally strong primary xylem. In temperate or cold climates, the age of a tree may be determined by counting the number of annual xylem rings formed at the base of the trunk (cut in cross section).
are the openings that allow gases and water to pass through. Each pore is flanked by two bean- shaped cells called guard cells. Together, the pore and guard cells make up what is known as a stomata.
Since they are the only openings in the epidermis, stomata regulate what is able to pass through the dermal layer. Gas exchange occurs through these tiny openings, which makes it possible for the plant to make food and release waste.
Secretory Tissues
The cells directly concerned with secretions like resins, essential oils, mucilage, latex and similar substances together constitute secretory or special tissues.
These cells have neither common origin nor morphological continuity. They may occur as isolated patches in any part of the plant, or may form well-organized structures.
The cells belonging to the first type have rich cytoplasm and prominent nuclei, whereas those of the second type are large cells with well-developed cavity where the secretion remains stored up. The ducts containing essential oils and mucilage are the examples.
Glands:
Glands are well-organized secretory structures, composed of diverse types of cells. The secretory materials are produced and liberated by the protoplasts of the cells. The substances may be at once exuded from the seats of formation as in nectaries, or they may remain stored in some cavity inside.
The glands may be external or superficial, naturally formed on the epidermis; or they may be internal where the cavities correspond to the intercellular spaces formed either schizogenously or lysigenously.
Glandular hairs and trichomes, common in many plants are superficial in origin. The common glands are those which secrete digestive enzymes, known as digestive glands, nectar-secreting ones or nectaries, and, similarly, resin ducts, oil ducts, latex-secreting glands called laticiferous ducts, and water- secreting ones called hydathodes.
Digestive Glands:
It is an established fact that plants in general have intracellular digestion. Here the living cells secrete enzymes, no specialised structure being present for the purpose. The insectivorous plants possess special digestive glands which secrete proteolytic (protein-digesting) enzymes and thus part of the nitrogen requirement is obtained from the bodies of the insects they catch. So it is a case of extracellular digestion.
Nectaries:
These are special glands usually located on the floral parts. They secrete the sugary substance, nectar or honey, and thus attract the pollinating insects. These glands are superficial, usually consisting of epidermal cells.
In some cases the cells forming the glands are columnar or papillose ones (Fig. 547A), with dense protoplast, whereas in others the secretory cells may be more or less like normal epidermal cells, but without cuticle.
Nectar is exuded through the walls and exposed on the outer surface of the gland. Nectaries may also occur on purely vegetative parts, in that case they are called extra-floral nectaries.
Resin Ducts and Oil Ducts:
Substances like resins, oils, gums are copiously secreted in gymnosperms in general and also in many angiosperms. The materials are secreted and conducted through special glands, which are also called ducts.
The oil ducts of umbellifers, though localised and limited in extent, are also formed in the same manner. The characteristic oil glands present on the rinds of citrus fruits like lemons and oranges are formed lysigenously. The cavities remain filled tip with essential oils and other substances due to disorganization of tissues (Fig. 518B).
Laticiferous Ducts:
Long tube-like bodies containing the viscous fluid, latex, occur in a large number of angiospermic families. These are called laticiferous ducts or tubes. Latex is usually a milky, often yellowish or watery fluid, which is readily exuded when the plants containing it are injured.
From ontogenetic point of view laticiferous ducts are of two types, viz., non-articulate latex ducts or latex cells and articulate latex ducts or latex vessels. They are also called simple and compound laticifers respectively. There is no difference between the two, so far as the contents are concerned.
Non-articulate latex ducts or latex cells (Fig. 548A), are single cells. They arise as small meristematic cells at the very embryonic stage. With the growth of the plants these cells also elongate and make their way through other tissues.
The growth of the tip is mainly apical and often branches are given out. Thus as much elongate tube-like bodies growing continuously they penetrate into all the tissues of the plant, even invading the newly-formed organs like leaves, buds and lateral roots.
