Chapter 3: Tissues — Plant and Animal Tissues
In Paris, around the year 1800, a young army surgeon named Xavier Bichat spent his nights taking bodies apart in the wards of the Hôtel-Dieu hospital. He was looking for something no one had quite named. When he opened the stomach, he found that its wall was not one substance but several layers, laid one on another like the cloth of a coat. When he opened a joint, he found the same kind of layering again — and, to his surprise, some of those layers looked like layers he had already seen elsewhere in the body.
So he tested them. He soaked them in acid, boiled them, dried them, left them to rot, and watched which layers behaved alike. Layers that survived the same treatments, he decided, were the same material wherever they occurred. By this slow and unglamorous method he sorted the human body into twenty-one such materials, and he called them by the French word for woven fabrics: tissus. From it we get the word tissue — a group of similar cells that work together at one job.
What makes his work remarkable is that Bichat distrusted the microscope and never used one. He never saw a single cell. He worked out that the body is built from a small number of repeating materials purely by watching how those materials behaved. He died in 1802, aged thirty.
Diagram
Fig. 3.0-1 — Layers In The Wall Of An Organ Simple line drawing of a cut-away block of the stomach wall shown as distinct stacked layers; label inner lining layer, connective layer, muscle layer, outer covering layer, and mark with a bracket ‘each layer is one tissue’.
3.1 Tissues — The Teams Of Workers
3.1.1 What A Tissue Is
Think of one cell working entirely on its own inside your body. It can take in food, use oxygen and stay alive, but that is about all. On its own it cannot cover a surface, or pull a bone, or carry water to the top of a tree.
The living body solves this by putting many similar cells side by side and giving them all the same job. A hundred cells doing the same thing together produce an effect that one cell could never produce alone. This is why bodies are built out of tissues rather than out of single cells.
Two things in that definition matter equally. The cells must be similar, because a team pulling in different directions achieves nothing. And they must share one function, because it is the sharing that makes their combined effect large.
Three Teams At Work
Your skin is covered by a tissue whose cells are flat and fitted tightly together, so that the sheet they form keeps germs and water out. Nothing about a single such cell is protective; it is the unbroken sheet that protects.
Underneath, the muscle tissue is made of cells that can shorten. When they all shorten at the same moment, the whole muscle pulls, and a limb moves.
A plant shows the same idea. Its conducting tissue is a set of cells joined end to end into long pipes. Water rises from the soil through these pipes to the leaves, and food made in the leaves travels down through them to the roots. Each of these tissues is examined closely later in this chapter.
3.1.2 From Tissue To Organism
A tissue is rarely enough on its own. Most jobs in a body need several different tissues arranged together in one place.
Your tongue is a good example, and Fig. 3.1-1 shows what it contains. Its surface is a covering tissue, its bulk is muscle tissue that moves it, and nerve tissue running through it carries the sensation of taste. Several tissues, one structure, one shared purpose. Such a structure is called an organ — a part of the body made of several tissues working together for a particular function.
Diagram
Fig. 3.1-1 — Several Tissues In One Organ Simple sectional line drawing of the human tongue with three call-out circles enlarging small areas; label covering (epithelial) tissue at the surface, muscle tissue in the body of the tongue, nerve tissue running through it, and blood supply.
Organs, in turn, rarely act alone. Your stomach cannot feed you by itself; it needs the mouth before it and the intestine after it. A group of organs that together carry out one whole life process is an organ system, such as the digestive system.
All the organ systems together make up the complete living individual, the organism. In a plant the same holds: the root system and the shoot system together make up the whole plant.
There is a definite order here, and it runs both ways. Cells build tissues, tissues build organs, organs build systems, systems build the organism. Read it downwards instead, and any organism can be taken apart into the cells it is made of. FC 3.1 sets out the ladder.
Flowchart
FC 3.1 — Levels Of Organisation In A Living Body Cell → tissue (similar cells, one function) → organ (several tissues, one structure) → organ system (several organs, one life process) → organism (all systems together)
3.2 Plant Tissues
3.2.1 The Two Basic Kinds Of Plant Tissue
A plant faces a problem an animal does not. It cannot walk to the light or to the water, so it must grow towards them, all its life. Growth means new cells, and new cells must come from somewhere.
Plants therefore keep two sorts of tissue at once. In a few small regions the cells go on dividing and making more cells. Everywhere else the cells have stopped dividing, have taken a fixed shape, and are busy at one settled job.
The dividing kind is called meristematic tissue, from the Greek meristos, meaning divided. The settled kind is called permanent tissue, because its cells keep the shape and the work they have taken up. FC 3.2 shows how the whole of a plant’s tissues fall under these two heads.
Flowchart
FC 3.2 — Classification Of Plant Tissues Plant tissues → branch 1: meristematic tissue → apical meristem, lateral (cambium) meristem; branch 2: permanent tissue → protective tissue (epidermis, cork), supporting tissue (parenchyma, collenchyma, sclerenchyma), conducting tissue (xylem, phloem); note: permanent tissue is formed from meristematic tissue, so branch 1 feeds into branch 2
3.2.2 Meristematic Tissue
Meristematic tissue is found at every growing point of a plant — the tip of each root, the tip of each stem, and the tip of each branch. It also lies in a thin layer between the bark and the wood of a tree, and it is this layer that makes a trunk thicker year after year. Fig. 3.2-1 shows where these regions sit in a whole plant.
Diagram
Fig. 3.2-1 — Where Meristems Lie In A Plant Whole-plant line drawing of a young dicot with four regions shaded and labelled: root tip, shoot tip, tip of an axillary bud, and a ring lying between bark and wood in the stem; add an arrow at the root and shoot tips marked ‘growth in length’ and arrows at the ring marked ‘growth in thickness’.
3.2.2.1 Characteristics Of Meristematic Cells
A cell whose whole business is dividing looks quite unlike a cell that has settled down, and Fig. 3.2-2 lets you compare the two. Its features are these:
- The cells are small.
- They are usually cubical.
- The cell wall is thin.
- The nuclei are large.
- Vacuoles are almost absent.
- The cells are tightly packed, with almost no intercellular spaces between them.
- The cells actively divide, adding new cells to the plant.
- The new cells so produced are then transformed into mature permanent tissues.
Every one of these features follows from the single job of dividing. A large nucleus is needed because the nucleus controls division. A thin wall lets the cell split easily and lets a new wall be laid down between the two daughter cells. Vacuoles are almost absent because a big vacuole full of water would take up room the dividing cell needs for its own contents.
Diagram
Fig. 3.2-2 — Meristematic Cells Two-part figure. A: longitudinal section of a stem tip showing the dome of meristem, the young differentiating leaves around it and two axillary buds; label meristem, young leaf, axillary bud. B: high-magnification view of meristematic cells from an onion root tip during division; label cell wall, nucleus, chromosomes visible during cell division.
Point 8 deserves attention, because it links the two kinds of tissue. A meristematic cell divides into two; one daughter usually stays meristematic, while the other grows, changes shape and becomes a permanent cell. Every permanent tissue in a plant was once meristematic.
The meristematic tissue is classified into two categories according to where it lies: apical meristem and lateral meristem.
3.2.2.2 Apical Meristem
The apical meristem, also called terminal meristem, is located near the tips of roots and stems. It is present too in the growing young leaves near the tips of stems, and on the tips of axillary buds — the small buds sitting in the angle between a leaf and the stem.
Because it sits at the tips, this meristem lengthens the plant. A root pushing deeper into the soil and a shoot reaching higher into the light are both the work of apical meristem.
3.2.2.3 Lateral Or Cambium Meristem
The lateral meristem, or cambium, is situated below the bark. It is arranged not as a dome at a tip but as a cylinder running the length of the stem, and it adds cells on either side of itself.
The result is an increase in the girth, that is the diameter, of the stem. A tree that grew only at its tips would end up tall and impossibly thin; the lateral meristem is what allows a trunk to thicken enough to hold the weight above it.
3.2.3 Permanent Tissue
Permanent tissues are made up of cells that have lost their ability to divide. They take a permanent shape in order to perform some permanent function.
They may be living, in which case the nucleus is present, or dead, in which case the nucleus is absent. A dead cell may sound useless, but you will see shortly that some of a plant’s most important tissues are dead — which is possible only because their walls, not their contents, do the work.
According to the function performed, the permanent tissues are of three types: protective, supporting and conducting.
3.2.4 Protective Tissue
Protective tissue is found on the surface of roots, stems and leaves — that is, wherever the plant meets the outside world. Its cells have thickened walls, and it is the outer wall, the one facing outwards, that is thickened most.
The epidermis of a leaf is the clearest example. Its cells secrete a waxy waterproof material over their outer surface, so that water cannot escape freely from the leaf and germs cannot easily get in.
Cork cells in the bark of older stems contain another strong waterproof material, a fatty substance called suberin. Cork is dead tissue, yet it seals a trunk against water loss and injury for years.
3.2.5 Supporting Tissue
A plant has no skeleton of bone. What holds a stem upright, and stops a leaf from flopping over, is supporting tissue.
Supporting tissue is of several types. The three most important are parenchyma, collenchyma and sclerenchyma. They differ chiefly in how thick their walls are and where the thickening lies, and this one difference decides how much support each can give.
3.2.5.1 Parenchyma
Parenchyma is made of large, thin-walled cells, oval, circular or polygonal in shape. Usually a single large vacuole is present, filling most of the cell, as Fig. 3.2-3 shows.
It is found in the soft parts of plants, such as in the cortex, the outer region of a stem or root, and in the pith, the central region.
Diagram
Fig. 3.2-3 — Parenchyma Cells Two-part line drawing. A: parenchyma from the cortex of a root; label thin cell wall, single large vacuole, nucleus, intercellular space. B: parenchyma from a potato tuber; label small vacuole and starch granules stored in the cell.
Parenchyma has two further uses. It may store food, as it does in the potato, where the cells are packed with granules of starch. And it provides temporary support to the plant: when the vacuoles are full of water the cells press against one another and the soft parts stay firm, which is exactly why a plant short of water wilts.
Some parenchymatous cells, particularly in the leaf, contain chloroplasts and are termed chlorenchyma, from the Greek chloros, meaning green. Chlorenchyma helps the leaf to produce food by photosynthesis.
3.2.5.2 Collenchyma
Collenchyma takes its name from the Greek collen, meaning glue. It is made up of cells which are elongated, and whose cell wall is thickened at the corners, as Fig. 3.2-4 shows.
Thickening only at the corners is a compromise. It gives the tissue real strength, yet leaves the rest of the wall thin enough for the cell to stay alive and to stretch a little as the plant grows.
Diagram
Fig. 3.2-4 — Collenchyma Two-part line drawing of collenchyma. A: longitudinal view showing elongated cells. B: cross section showing several cells with the wall thickening clearly heaped at the corners where three cells meet; label cell wall thickened at corners, nucleus, cytoplasm.
Collenchyma is found in the leaf stalks and below the epidermis of stems. The tissue helps to support the parts of a plant, and it is the tissue that keeps a leaf stalk stiff enough to hold a leaf out flat in the light.
3.2.5.3 Sclerenchyma
Sclerenchyma, from the Greek scleros, meaning hard, is composed of long, narrow cells which have become dead. These cells develop very thick walls due to the deposition of lignin, a hard substance that stiffens the wall and makes it waterproof. Fig. 3.2-5 shows them in both views.
Once the wall is heavily lignified, nothing can pass in or out, and the living contents die. What is left is a thick-walled empty tube — of no use for storage or growth, but very strong. This tissue provides strength to plant parts, and it is found in stems and in the veins of leaves.
Diagram
Fig. 3.2-5 — Sclerenchyma Two-part line drawing of sclerenchyma. A: longitudinal section showing long narrow cells with tapering ends and very thick walls. B: cross section showing thick lignified walls and a narrow empty cavity in each cell; label lignified cell wall, narrow cavity, tapering end.
Table 3.1 sets the three supporting tissues side by side.
3.2.5.4 Simple Tissues
The above three categories — parenchyma, collenchyma and sclerenchyma — are grouped together as simple tissues, each being made up of only one type of cell.
That is the whole meaning of the word simple here. It says nothing about how important the tissue is; it says only that if you looked at a patch of parenchyma under a microscope, every cell in it would be a parenchyma cell.
3.2.6 Conducting Tissue
Water enters a plant at the root, hundreds of times further from the leaves than any cell could pass it by hand. Food is made in the leaves, and every living cell in the root needs a share of it. Something must carry both, in opposite directions, over long distances.
Conducting tissue, also called vascular tissue, provides that passage for water and dissolved materials to move up and down the plant. The two types of conducting tissue are xylem and phloem. Fig. 3.2-6 shows where they lie in a stem.
Diagram
Fig. 3.2-6 — Plant Tissues In A Stem Magnified block of a portion of a herbaceous stem, cut so that a transverse face and a longitudinal face are both visible; label epidermis, cortex parenchyma, sclerenchyma, cambium, phloem and xylem, with the transverse and longitudinal views marked.
3.2.6.1 Xylem
Xylem cells are elongated and thick-walled, and are somewhat in the form of tubular passages. They provide for the upward movement of water and dissolved materials absorbed from the soil by the roots to other parts of the plant.
The name comes from the Greek xylo, meaning wood, and that is what old xylem becomes. Older xylem tissue forms the wood of a tree and does not participate in transport any longer.
Each year a fresh ring of xylem is added, so you can determine the age of a tree by counting its annual rings, which are actually the xylem rings. Fig. 3.2-7 shows three such rings in a stem three years old.
Diagram
Fig. 3.2-7 — Annual Rings In A Woody Stem Diagrammatic sketch of a cross-section of a three-year-old woody stem with a wedge cut out; label cork, cork cambium, phloem, cambium, ray, and the three concentric rings of xylem, with a note that the three rings indicate three years of growth.
The xylem tissue consists of tracheids and vessels — collectively called tracheary elements — together with xylem parenchyma and xylem fibres. Their shapes are shown in Fig. 3.2-8.
- Tracheids are made up of elongated dead cells with large cavities without any contents. The cells are placed end to end forming a long tube. They have highly lignified cell walls, and they provide mechanical support by developing various types of thickenings in their walls.
- Xylem vessels, also called tracheae, are long tube-like structures meant for transporting water and dissolved minerals. They are made up of elongated cells placed end to end. Their cell walls are quite hard, thick and lignified, and their transverse walls get dissolved so that the row of cells makes one continuous water pipe.
- Xylem parenchyma consists of living parenchyma cells associated with the xylem. These cells serve for the storage of food, as sugars and starch, and also help in the conduction of water and minerals.
- Xylem fibres are made up of dead sclerenchyma cells. They are thick-walled, long, narrow cells with tapering ends, and their main function is to provide mechanical support to the plant.
Diagram
Fig. 3.2-8 — Shapes Of Xylem Elements Row of separate line drawings showing a narrow tracheid with pits, a vessel element with a simple perforation plate, a vessel element with a multiple perforation plate, and two vessels with ring-like and spiral wall thickenings; label pits, simple perforation plate, multiple perforation plate, wall thickenings.
3.2.6.2 Phloem
Phloem cells, named from the Greek phloos, meaning bark, provide a passage for the downward movement of food manufactured in the leaves to various parts of the plant. They also provide for the upward movement of the prepared food towards the growing new leaves.
Notice that this traffic runs both ways, while xylem traffic runs only upwards. Food is needed at the roots below and at the young leaves above, so the phloem must be able to deliver in either direction.
The phloem tissue consists of sieve tubes, companion cells, phloem parenchyma cells and phloem fibres, all of which appear in Fig. 3.2-9.
Diagram
Fig. 3.2-9 — Phloem Two-part line drawing of phloem. A: longitudinal section showing sieve cells joined end to end; label sieve cell, sieve plate, companion cell, phloem parenchyma cell. B: cross section; label sieve cell, sieve plate with sieve pits, companion cell, phloem parenchyma.
- Sieve tubes are made up of elongated cells placed end to end forming a long tube. The transverse walls, called sieve plates, are perforated with clusters of pores. They help in the transport of food from the leaves to storage organs and to other parts of the plant.
- Companion cells are living parenchyma cells closely associated with the sieve tube cells. They help in the functioning of the sieve tube cells.
- Phloem parenchyma is a parenchymatous tissue found associated with phloem. It is mainly concerned with the storage of starch, fat and other organic food material.
- Phloem fibres are sclerenchymatous cells found associated with phloem. They mainly provide support.
The sieve plate is worth a second look. Its walls are not dissolved away as in a vessel, only perforated, so food solution passes from cell to cell through the pores while the tube stays divided into cells. That matters, because unlike the dead xylem tube, a sieve tube is alive.
Table 3.2 sets the two conducting tissues against each other.
3.2.6.3 Vascular Bundles And Complex Tissues
The xylem and phloem collectively form the vascular bundles. The veins of leaves are examples of vascular bundles, and they are in continuity with the conducting tissues of the stem and the root.
This continuity is the point. A vein in the highest leaf is joined, without a break, to a vessel in the deepest root. Water taken up at one end can reach the other because the pipework is never interrupted.
The above two tissues, xylem and phloem, are also termed complex tissues, being made of more than one type of cell working together as a unit.
3.3 Animal Tissues
An animal moves, feels, feeds and heals, and it does all of this with a body that has no rigid box of cell walls to hold it in shape. The tissues it is built from are therefore quite unlike a plant’s.
The main kinds of animal tissues are four: epithelial tissue, connective tissue, muscle tissue and nervous tissue. Every organ in your body is some combination of these four, and FC 3.3 shows how each of them is divided.
Flowchart
FC 3.3 — Classification Of Animal Tissues Animal tissues → epithelial (squamous, cuboidal, columnar → ciliated columnar and glandular, stratified); connective (connective tissue proper → areolar, adipose, fibrous → tendons and ligaments; supportive → cartilage, bone; fluid → blood, lymph); muscular (striated or voluntary, unstriated or involuntary, cardiac); nervous (neurons, whose axons bundle together to form nerves)
3.3.1 Epithelial Tissue
Epithelial tissue is a thin, protective, continuous sheet of cells. The word continuous is the important one: the sheet has no holes in it.
Location. It covers the surface of the body and lines the various body cavities and internal organs, including the blood vessels. The outermost layer of the skin is epithelial tissue, and so are the lining surfaces of the mouth, the nose, the lungs and the stomach.
Shape. Epithelial cells may be flat, cuboidal or columnar in shape. In all cases the cells of the epithelial tissue fit tightly together, leaving no space between them.
Function. The epithelial tissue functions for protection, as on the skin; for absorption and secretion, as in the intestine; and for sensory perception. Its four structurally distinct categories are described below and drawn in Fig. 3.3-1.
Diagram
Fig. 3.3-1 — Kinds Of Epithelial Tissue In The Animal Body Five labelled line drawings in one plate. A: squamous epithelium, flat irregularly shaped cells with prominent nuclei. B: cuboidal epithelium, a row of cube-shaped cells. C: columnar epithelium, tall cylindrical cells. D: ciliated columnar epithelium, tall cells bearing cilia at their free ends. E: stratified epithelium, several layers with squamous cells above and cuboidal cells lowermost; label the nucleus in each type and the cilia in D.
3.3.1.1 Squamous Epithelium
Squamous epithelium is composed of cells which are thin and flat, with prominent nuclei, closely packed together. It is found lining the mouth and nasal cavities, and lining blood vessels and lymph vessels.
The squamous epithelium protects the underlying parts from mechanical injury, from germs, from harmful chemicals and from drying up. A flat cell covers a large area for very little material, which is exactly what a covering sheet needs.
3.3.1.2 Cuboidal Epithelium
Cuboidal epithelium is found in some parts of the kidney tubules, and in some glandular ducts such as those of the salivary glands and the pancreatic duct.
These are places where fluid is carried along a narrow tube and where substances pass through the wall of that tube. A cube-shaped cell is deep enough to do such work, and still shallow enough to keep the tube narrow.
3.3.1.3 Columnar Epithelium
Columnar epithelium contains vertically arranged cylindrical or brick-like cells. These cells are usually tall in size. Columnar epithelium is generally found in the inner lining of the stomach and the intestines.
Height is useful here. A tall cell holds far more machinery than a flat one, and the lining of the gut has to make digestive juices and absorb digested food, both of which need room inside the cell.
Cilia On A Lining
At some places in the body, such as in the lining of the trachea or wind pipe, the columnar epithelium is ciliated. The cells of ciliated epithelium have thread-like protoplasmic projections called cilia at their free ends.
The cilia constantly keep lashing, and by doing so they move the materials which enter these regions. Dust that you breathe in is swept back up the trachea instead of settling in the lungs.
Glands From An Epithelium
Glandular epithelium, also a kind of columnar epithelium, contains some large cells which secrete certain chemical substances. Such cells are common in the lining of the stomach and the intestine.
At certain places in the body the glandular epithelium is folded inward to form compact, hollow or tubular glands; the sweat glands, the tear glands and the liver are formed in this way. A gland, then, is not a new kind of tissue but an epithelium tucked into a pocket.
3.3.1.4 Stratified Epithelium
Stratified epithelium is found in the skin and in the cornea, the transparent front of the eye. It is composed of several layers, or strata, of the same or of different kinds of epithelial cells.
Wherever a surface is rubbed, scratched and worn away, one layer of cells is not enough. Stacking the layers means that the cells lost from the top can be replaced from below without the covering ever being broken.
3.3.2 Connective Tissue
Connective tissue binds one tissue with another, and also connects various organs, keeping them in their proper places. Without it a body would be a heap of separate parts.
It has three characteristics, and the first is unlike anything you met in an epithelium:
- An abundance of intercellular substance, called the matrix, filling the space between the cells.
- Fewer cellular elements.
- Fibres.
In an epithelium the cells touch and the sheet is the tissue. In a connective tissue the cells sit scattered in a matrix they themselves have made, and the properties of the tissue are largely the properties of that matrix — soft in one place, rubbery in another, stony in a third, liquid in a fourth. The connective tissue is classified as follows.
I. Connective tissue proper — serves for packing and binding the organs. II. Supportive connective tissue — gives support; the cartilages and bones. III. Fluid connective tissue — blood and lymph.
3.3.2.1 Connective Tissue Proper
Connective tissue proper has three sub-categories, and the first two of them are shown along with the supportive tissues in Fig. 3.3-2.
Areolar (packing) tissue is the most widely spread of all. It occurs beneath the epidermis of the skin, and it makes the skin elastic and helps it to withstand pulling strain. Wherever there is a gap between two organs, areolar tissue is likely to be filling it.
Adipose (fat) tissue has specialized cells which store fat. This tissue forms padding under the skin and around the kidneys and the eyeball. The padding under the skin acts as an insulation for retaining body heat, so the same tissue does two jobs at once — it cushions and it keeps warmth in.
Fibrous connective tissue is made up of elongated cells referred to as fibres. The fibres are strong and are bundled together by areolar tissue.
They form the tendons, which connect muscle to bone, and the ligaments, which connect bone to bone and hold them in position. A muscle can pull a bone only because something inextensible joins the two; that something is a tendon.
Diagram
Fig. 3.3-2 — Types Of Connective Tissue Three stacked line drawings. 1: fibrous connective tissue, with a non-living matrix containing scattered cells and wavy fibres; label matrix, fibres, nucleus, cell. 2: cartilage, showing rounded cells in pairs within a firm matrix; label matrix, cartilage cell, empty lacuna. 3: bone, showing concentric rings around a central canal; label Haversian canal, concentric lamellae, matrix, bone cell, with one bone cell enlarged alongside.
3.3.2.2 Supportive Connective Tissue
Cartilage is a non-porous tissue with a thickened intercellular substance, or matrix. It has no blood vessels and no nerves in it. Cartilage is semi-transparent and elastic.
Cartilages are found in the tip of the nose, in the external ears, in the trachea and the bronchial tubes, between the vertebrae, and at the ends of the long bones. In each of these places something firm is wanted that can also bend or squash a little and spring back.
Bone is a hard, porous tissue. Unlike cartilage it has a good supply of blood vessels and nerves running through it, which is why a broken bone can knit itself together again.
It consists of both living cells, called osteocytes, and a rigid mass of inorganic salts laid down around them. The salts give the hardness; the living cells maintain and repair the tissue. Table 3.3 sets the two supportive tissues side by side.
3.3.2.3 Fluid Connective Tissue
The third group is a surprise, because it flows.
Blood is composed of a liquid part and a cellular part. The liquid part is the plasma. The cellular part is the red blood cells, the white blood cells and the platelets, all of which are shown in Fig. 3.3-3.
Diagram
Fig. 3.3-3 — Three Kinds Of Blood Cells In Humans Line drawing plate of human blood cells: a group of biconcave red blood cells shown in surface and side view, two or three white blood cells with differently shaped nuclei, and a cluster of small irregular platelets; label red blood cells, white blood cells, platelets.
Blood counts as a connective tissue for the same reason cartilage does. Its cells are scattered, not packed; they lie in an abundant intercellular substance; and that substance, the plasma, is simply a matrix that happens to be liquid.
Lymph is the fluid surrounding the body cells. It is essentially the blood plasma that has oozed out of the blood vessels. It contains white blood cells and not the red blood cells.
Both blood and lymph are mainly concerned with transportation, and also with immunity, that is protection against diseases.
3.3.3 Muscle Tissue
Muscle tissue forms the muscles of the body. Muscles can contract, meaning shorten, and then relax, meaning return to their former length. Thus they help the body in its movements.
Notice what a muscle cannot do: it cannot push. It can only pull, by getting shorter. Everything a body does by muscle is done by pulling something towards something else.
Three kinds of muscles are shown in Fig. 3.3-4: striated, also called skeletal, striped or voluntary; unstriated, also called smooth, unstriped or involuntary; and heart or cardiac muscles.
Diagram
Fig. 3.3-4 — Kinds Of Muscles Three labelled line drawings. A: striated muscle, a bundle of long cylindrical fibres with dark and light bands across them; label sarcolemma, dark band, light band, nucleus. B: unstriated muscle, separate spindle-shaped tapering cells each with one central nucleus. C: cardiac muscle, short branched striated fibres joined end to end; label intercalated disc, oblique connection, nucleus, myofibrils.
3.3.3.1 Striated Muscle
Striated muscles are under the control of our will. They are made of long fibres, that is cells, which are nucleated and striated — light and dark bands run across the fibres, and it is these bands that give the tissue its name.
This muscle moves only when you want it to. These muscles constitute about 50 per cent of the body weight, and they are found in the arms, the legs, the face and the neck.
3.3.3.2 Unstriated Muscle
Unstriated muscles are not under the control of our will. They are made up of spindle-shaped, slender, tapering cells which are uninucleated, meaning each cell has a single nucleus.
They are found in the walls of the intestine, where they help in the passage of food; in the muscles of the iris of the eye; in the skin; in the lining of blood vessels; in the urinary bladder; and in the uterus.
These are all places where steady, slow work must go on whether you are thinking about it or not. Food would not move along your intestine at all if it waited for your permission.
3.3.3.3 Cardiac Muscle
Cardiac muscles are involuntary in function and are found only in the walls of the heart. The fibres of cardiac muscle are striated, uninucleated and branched, and they are relatively short.
Two properties set this muscle apart from the other two. It can contract without outside stimulation, so the heartbeat begins within the heart muscle itself. And it does not get tired soon, so it can go on contracting throughout life without rest.
Table 3.4 brings the three kinds together.
3.3.4 Nervous Tissue
Nervous tissue, also called neural tissue, constitutes the nervous system. This tissue is made up of specialised elongated cells called neurons, or nerve cells. A typical neuron is shown in Fig. 3.3-5.
Each nerve cell consists of a cell body called the perikaryon or cyton, which contains the nucleus. From the cell body arise one or more elongated hair-like extensions called dendrites or dendrons.
One of these extensions, the axon, may be very long — sometimes as long as one metre, or even three metres as in an elephant. Axons bundled together form a nerve.
Diagram
Fig. 3.3-5 — A Nerve Cell Line drawing of a single motor neuron drawn vertically: label dendrites, perikaryon or cyton (cell body), nucleus, Nissl granules, axon, neurolemma, myelin sheath, node of Ranvier, extra cells covering the axon, and axon endings at the far end.
The great length of the axon is the point of the whole design. A single cell can stretch from your spinal cord to a muscle in your foot, so a message travels the distance without ever being handed from cell to cell.
The nerve tissue is concerned with the perception and the responses of animals. Perception is receiving news of what is happening; response is doing something about it. A body with all four tissue types can sense a change, decide, and move — and that is the whole of what a tissue system is for.
3.4 Summary
- A tissue is a group of similar cells lying together and performing one specific function. Cells working as a team achieve what a single cell cannot.
- Several tissues together form an organ; several organs carrying out one life process form an organ system; all the organ systems together form the organism.
- Plant tissues are of two basic kinds: meristematic tissue, whose cells divide, and permanent tissue, whose cells have stopped dividing.
- Meristematic tissue lies at all growing points — root tips, stem tips, branch tips — and also between the bark and the wood, where it thickens the stem.
- Meristematic cells are small, usually cubical and tightly packed, with thin walls, large nuclei and almost no vacuoles. They divide actively, and the new cells mature into permanent tissues.
- Apical or terminal meristem lies at the tips of roots and stems, in young leaves near the stem tips and on the tips of axillary buds; it increases length.
- Lateral or cambium meristem lies below the bark; it increases the girth of the stem.
- Permanent tissues have taken a fixed shape and a fixed function. They may be living, with a nucleus, or dead, without one. By function they are protective, supporting or conducting.
- Protective tissue covers the surface of roots, stems and leaves. The epidermis of a leaf secretes a waxy waterproof material; cork cells in bark contain another waterproof material.
- Parenchyma is made of large thin-walled cells with a single large vacuole, found in the cortex and the pith. It stores food, as in the potato, and gives temporary support.
- Parenchyma cells containing chloroplasts, chiefly in the leaf, are called chlorenchyma and carry out photosynthesis.
- Collenchyma is made of elongated living cells whose walls are thickened at the corners. It is found in leaf stalks and below the epidermis of stems, and supports the parts of the plant.
- Sclerenchyma is made of long, narrow dead cells with very thick walls due to the deposition of lignin. It gives strength, and is found in stems and in the veins of leaves.
- Parenchyma, collenchyma and sclerenchyma are simple tissues, each made up of only one type of cell.
- Conducting or vascular tissue carries water and dissolved materials up and down the plant. Its two types are xylem and phloem.
- Xylem carries water and dissolved minerals upwards from the roots. Older xylem forms wood and no longer transports; its annual rings give the age of a tree.
- Xylem consists of tracheids and vessels, together called tracheary elements, along with xylem parenchyma and xylem fibres.
- Tracheids are elongated dead cells with large empty cavities and lignified walls, placed end to end. Vessels are formed of cells whose transverse walls dissolve, making one continuous water pipe.
- Xylem parenchyma is living and stores food while helping in conduction; xylem fibres are dead sclerenchyma cells that give mechanical support.
- Phloem carries food manufactured in the leaves downwards to the rest of the plant, and upwards to the growing new leaves.
- Phloem consists of sieve tubes with perforated sieve plates, companion cells that help the sieve tubes work, phloem parenchyma that stores food, and phloem fibres that give support.
- Xylem and phloem together form the vascular bundles, of which the veins of leaves are examples. Both are complex tissues, made of more than one type of cell working as a unit.
- The main animal tissues are four: epithelial, connective, muscle and nervous.
- Epithelial tissue is a thin continuous sheet of tightly fitted cells covering the body and lining its cavities, organs and blood vessels. It protects, absorbs, secretes and receives sensations.
- Its four categories are squamous, made of thin flat cells; cuboidal, in kidney tubules and glandular ducts; columnar, lining the stomach and intestines; and stratified, of several layers, in the skin and cornea.
- Ciliated columnar epithelium, as in the trachea, bears cilia that lash constantly and move materials along. Glandular epithelium secretes chemical substances and, folded inwards, forms glands such as the sweat and tear glands and the liver.
- Connective tissue binds tissues and organs and holds them in place. It has an abundant intercellular matrix, fewer cells and fibres.
- Connective tissue proper includes areolar tissue, which packs and makes skin elastic; adipose tissue, which stores fat and insulates; and fibrous tissue, which forms tendons joining muscle to bone and ligaments joining bone to bone.
- Supportive connective tissue is cartilage, which is non-porous, elastic and without blood vessels or nerves, and bone, which is hard, porous and well supplied with blood vessels and nerves.
- Fluid connective tissue is blood, made of plasma with red blood cells, white blood cells and platelets, and lymph, which is plasma that has oozed out of the vessels and contains white blood cells only. Both carry materials and provide immunity.
- Muscle tissue contracts and relaxes, producing movement. Striated muscle is voluntary and found in the limbs, face and neck; unstriated muscle is involuntary, with spindle-shaped cells, in the intestine, iris, blood vessels, bladder and uterus; cardiac muscle is involuntary, striated and branched, found only in the heart, contracting without outside stimulation and not tiring soon.
- Nervous tissue is made of neurons. A neuron has a cell body, the perikaryon or cyton, with hair-like dendrites; one extension, the axon, may be a metre or more long. Axons bundled together form a nerve, and the tissue governs perception and response.
Flowchart
FC 3.4 — The Chapter In One Chain Cells → tissue → organ → organ system → organism; branch A, plant tissues → meristematic (apical, lateral) and permanent → protective, supporting (parenchyma, collenchyma, sclerenchyma) and conducting (xylem, phloem); branch B, animal tissues → epithelial, connective (proper, supportive, fluid), muscular (striated, unstriated, cardiac) and nervous
3.5 Key Terms
Adipose tissue — connective tissue whose cells store fat, forming padding under the skin and around organs and keeping body heat in.
Apical meristem — dividing tissue at the tips of roots, stems and buds, which increases the length of a plant.
Areolar tissue — the most widespread packing tissue, lying beneath the skin’s epidermis and making the skin elastic.
Axon — the single long extension of a neuron, which may be a metre or more in length; bundled axons form a nerve.
Blood — fluid connective tissue made of plasma with red blood cells, white blood cells and platelets.
Bone — hard, porous supportive tissue of living cells set in a rigid mass of inorganic salts, well supplied with blood vessels and nerves.
Cardiac muscle — involuntary, striated, branched muscle found only in the walls of the heart, which contracts without outside stimulation and does not tire soon.
Cartilage — non-porous, semi-transparent and elastic supportive tissue with a thickened matrix and no blood vessels or nerves.
Chlorenchyma — parenchyma containing chloroplasts, chiefly in leaves, which carries out photosynthesis.
Cilia — thread-like protoplasmic projections at the free ends of certain epithelial cells, which lash and move materials along.
Ciliated epithelium — columnar epithelium bearing cilia, as in the lining of the trachea.
Collenchyma — living supporting tissue of elongated cells whose walls are thickened at the corners.
Columnar epithelium — epithelium of tall cylindrical cells, lining the stomach and intestines.
Companion cell — a living parenchyma cell closely associated with a sieve tube cell and helping it to work.
Complex tissue — a tissue made of more than one type of cell working as one unit, as in xylem and phloem.
Conducting tissue — plant tissue, also called vascular tissue, that carries water and food up and down the plant.
Connective tissue — animal tissue with abundant matrix that binds tissues and organs together and holds them in place.
Connective tissue proper — the group of connective tissues that pack and bind organs: areolar, adipose and fibrous.
Cortex — the outer region of a stem or root, made largely of parenchyma.
Cuboidal epithelium — epithelium of cube-shaped cells, found in kidney tubules and glandular ducts.
Dendrite — a hair-like extension arising from the cell body of a neuron; also called a dendron.
Epidermis — the protective tissue forming the outer surface of leaves, stems and roots.
Epithelial tissue — a thin, continuous protective sheet of tightly fitted cells covering the body and lining its cavities and organs.
Fibrous connective tissue — strong bundled fibres forming tendons and ligaments.
Fluid connective tissue — connective tissue with a liquid matrix: blood and lymph.
Glandular epithelium — columnar epithelium containing large cells that secrete chemical substances; folded inwards it forms glands.
Lateral meristem — dividing tissue lying below the bark, also called cambium, which increases the girth of a stem.
Ligament — a fibrous cord connecting one bone to another and holding them in position.
Lignin — the hard substance deposited in cell walls, making them very thick and rigid, as in sclerenchyma.
Lymph — fluid connective tissue surrounding the body cells; blood plasma that has oozed out of the vessels, containing white blood cells but no red blood cells.
Matrix — the intercellular substance in which the cells of a connective tissue lie.
Meristematic tissue — plant tissue whose cells actively divide to produce new cells, found at all growing points.
Muscle tissue — animal tissue whose cells contract and relax, bringing about movement.
Nerve — a bundle of axons running together.
Nervous tissue — tissue made of neurons, concerned with perception and response.
Neuron — a specialised elongated nerve cell with a cell body, dendrites and an axon.
Organ — a structure made of several tissues working together for a particular function.
Organ system — a group of organs together carrying out one whole life process.
Organism — the complete living individual, formed of all its organ systems.
Parenchyma — living tissue of large thin-walled cells with a single large vacuole, found in the soft parts of a plant.
Perikaryon — the cell body of a neuron, containing the nucleus; also called the cyton.
Permanent tissue — plant tissue whose cells have lost the ability to divide and have taken a fixed shape and function.
Phloem — complex conducting tissue that carries food made in the leaves to the rest of the plant.
Phloem fibre — sclerenchymatous cell associated with phloem, giving support.
Phloem parenchyma — living parenchyma associated with phloem, storing starch, fat and other organic food.
Pith — the central region of a stem or root, made largely of parenchyma.
Plasma — the liquid part of blood, in which the blood cells are carried.
Protective tissue — plant tissue with thickened, waterproofed outer walls covering the surface of roots, stems and leaves.
Sclerenchyma — dead supporting tissue of long narrow cells with very thick lignified walls, giving strength.
Sieve plate — the perforated transverse wall between two sieve tube cells.
Sieve tube — a long tube of living cells placed end to end, through which food is transported in phloem.
Simple tissue — a tissue made of only one type of cell, as in parenchyma, collenchyma and sclerenchyma.
Squamous epithelium — epithelium of thin flat closely packed cells, lining the mouth, nasal cavities and blood vessels.
Stratified epithelium — epithelium of several layers of cells, found in the skin and the cornea.
Striated muscle — voluntary muscle of long banded fibres, found in the limbs, face and neck.
Supportive connective tissue — connective tissue that gives support: cartilage and bone.
Tendon — a fibrous cord connecting a muscle to a bone.
Tissue — a group of similar cells lying together and performing one specific function.
Tracheid — an elongated dead xylem cell with a large empty cavity and lignified walls, placed end to end with others to form a tube.
Unstriated muscle — involuntary muscle of spindle-shaped uninucleated cells, found in the intestine, iris, blood vessels, bladder and uterus.
Vascular bundle — xylem and phloem together, as seen in the veins of leaves and continuous through stem and root.
Xylem — complex conducting tissue that carries water and dissolved minerals upwards from the roots.
Xylem fibre — dead sclerenchyma cell in xylem, giving mechanical support.
Xylem parenchyma — living parenchyma in xylem, storing food and helping in conduction.
Xylem vessel — a continuous water pipe formed of cells placed end to end whose transverse walls have dissolved; also called a trachea.
3.6 Points To Ponder
A plant keeps only a few small regions able to divide, while most of its cells can never divide again. An animal repairs cuts and grows all over its body. What does a plant gain by restricting growth to its meristems, and what does it give up?
Sclerenchyma, tracheids and vessels all do their work after the cells have died. Can you find a general rule about the kind of job that a dead cell can do as well as, or better than, a living one?
Collenchyma thickens its wall only at the corners, sclerenchyma thickens it all the way round. Suppose a young stem that must still lengthen were to build sclerenchyma instead of collenchyma. What would happen to it?
Cartilage has no blood vessels, and bone is richly supplied with them. Both are supportive tissues in the same skeleton. What could make a blood supply worth having in one and not in the other?
Blood is classed as a connective tissue although it connects nothing mechanically. Is the classification a good one, or is it only a convenience? Argue for one answer using the three characteristics of connective tissue.
Almost every organ in your body contains all four kinds of animal tissue. Choose one organ, work out what each of the four is doing in it, and decide which of them the organ could least afford to lose.
3.7 Objective Questions
Multiple Choice Questions
- The plant tissue whose living cells have their walls thickened only at the corners is: (1 mark)
- parenchyma
- collenchyma
- sclerenchyma
- epidermis
- Which of the following is made of dead cells? (1 mark)
- Sieve tube
- Companion cell
- Tracheid
- Xylem parenchyma
- Growth in the girth of a stem is brought about by: (1 mark)
- apical meristem
- lateral meristem
- chlorenchyma
- phloem parenchyma
- A connective tissue in which the matrix is a liquid is: (1 mark)
- cartilage
- bone
- adipose tissue
- blood
- Ciliated columnar epithelium is best looked for in the: (1 mark)
- cornea
- trachea
- kidney tubule
- wall of the heart
Assertion-Reason
- For each of the following, choose the correct option:
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): A xylem vessel offers very little obstruction to rising water. Reason (R): The transverse walls between the cells of a vessel dissolve away, leaving a continuous tube. (1 mark)
- Assertion (A): An injury to cartilage takes a long time to mend. Reason (R): Cartilage contains no blood vessels. (1 mark)
- Assertion (A): Meristematic cells contain very large vacuoles. Reason (R): Meristematic cells divide actively to add new cells to the plant. (1 mark)
Match The Following
- Match each tissue in Column A with the description in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Adipose tissue | i. Connects one bone to another |
| b. Ligament | ii. Perforated transverse walls |
| c. Sieve tube | iii. Stores fat and insulates the body |
| d. Chlorenchyma | iv. Bundled axons |
| e. Nerve | v. Contains chloroplasts |
Fill In The Blanks
Cells of sclerenchyma develop very thick walls due to the deposition of ______. (1 mark)
The intercellular substance of a connective tissue is called the ______. (1 mark)
The cell body of a neuron is known as the ______ or cyton. (1 mark)
Xylem and phloem together form the ______ of a plant. (1 mark)
True Or False
Companion cells are dead cells associated with sieve tubes. (1 mark)
Cardiac muscle can contract without any outside stimulation. (1 mark)
Lymph contains white blood cells but no red blood cells. (1 mark)
Older xylem forms the wood of a tree and no longer takes part in transport. (1 mark)
Name The Following
The tissue that stores food in a potato tuber. (1 mark)
The epithelium made of several layers of cells, found in the skin. (1 mark)
The cord that connects a muscle to a bone. (1 mark)
The meristem present at the tip of an axillary bud. (1 mark)
The muscle tissue found in the wall of the urinary bladder. (1 mark)
Odd One Out, With A Reason
Parenchyma, collenchyma, sclerenchyma, xylem. (2 marks)
Cartilage, bone, blood, epidermis. (2 marks)
Tracheid, vessel, xylem fibre, sieve tube. (2 marks)
3.8 Check Your Concepts
Define a tissue, and state the two conditions a group of cells must satisfy to be called one. (2 marks)
Give a reason for each of the following: (2 marks)
- Meristematic cells have very small vacuoles.
- Cork protects a tree trunk although it is made of dead cells.
Name the two categories of meristematic tissue and state where each is located. (2 marks)
What is meant by a complex tissue? Give two examples from a plant. (2 marks)
Arrange the following in order of increasing size and explain the basis of your order: organ, cell, organism, tissue, organ system. (3 marks)
Differentiate between simple tissue and complex tissue, stating the basis of difference. (3 marks)
Explain, with one example each, how epithelial tissue carries out protection and secretion. (3 marks)
Why is blood classed as a connective tissue? Support your answer with two of the characteristics of connective tissue. (3 marks)
Differentiate between xylem and phloem on the basis of: (i) the material transported, (ii) the direction of transport, and (iii) whether the conducting cells are living or dead. (4 marks)
Describe the structure of a sclerenchyma cell and explain how that structure suits it to the work it does. (4 marks)
What would happen in each of the following cases? Give a reason in each case. (4 marks)
- The lateral meristem of a young tree stops dividing.
- The cilia lining the trachea stop lashing.
Compare striated, unstriated and cardiac muscle under these heads: control by will, shape of the cells, location in the body, and fatigue. (5 marks)
Trace the steps by which a row of living cells becomes a continuous xylem vessel, and state why this arrangement carries water better than a row of living cells would. (5 marks)
3.9 Figure-Based Questions
- Study Fig. 3.2-3, showing parenchyma cells. (4 marks)
- Name the structure that occupies most of the cell in part A.
- Name the granules stored in the cell shown in part B, and name the plant part from which it is taken.
- State one function of this tissue other than storage.
- Why can this tissue give only temporary support?
- Refer to Fig. 3.2-9, showing phloem. (4 marks)
- Name the perforated wall lying between two sieve cells.
- Name the small cell lying alongside the sieve cell, and state its function.
- Name the material transported through this tissue.
- State one way in which this tissue differs from xylem in the condition of its conducting cells.
- Refer to Fig. 3.2-7, showing the cross-section of a woody stem. (3 marks)
- What do the concentric rings of xylem indicate?
- Name the layer lying between the xylem and the phloem, and state its function.
- Why does the innermost xylem no longer carry water?
- Study Fig. 3.3-4, showing three kinds of muscle. (4 marks)
- Identify the muscle shown in part B and state one place where it is found.
- Name the structure marked between two fibres in part C.
- Which of the three is under the control of your will?
- Give one reason why the muscle in part C does not tire soon.
- Redraw the neuron shown in Fig. 3.3-5 and label the following on straight, non-crossing lead lines: perikaryon, nucleus, dendrites, axon, myelin sheath, axon endings. (5 marks)