BOTANY: THE WALLFLOWER

Flower

Parts of a Plant

A typical flowering plant such as the wall flower is composed of two parts, the root system and the shoot system. The root system of the wallflower consists of a large main root growing vertically down into the soil, and giving off many side branches in four vertical rows. Such a main root, which represents the first root of the seedling or young plant, and which is larger than any of the side roots, is called a tap root. The follow ing are the functions of the root:— To absorb water and salts from the soil.

To anchor the plant to the ground.

In some plants to act as a storage organ.

The Root and Root Hairs. —At the end of each root is a growing point which is protected by a conical cover ing of cells, termed the root cap. The outer cells of the root cap are slimy and become worn away as the root pushes its way through the soil. The sliminess helps the root to slide past the soil particles, and as the cells of the root cap are worn away, they are renewed by cell division from within. A short distance behind the tip is a zone about ½ to 1 in. long, where root hairs occur. The root hairs are delicate hair-like projections from the root, which are the absorptive organs of the plant and take in water and salts from the soil. Each root hair grows out from the root in between the soil particles to which it becomes attached.

The root hairs are very numerous, but they live for a few days only and then shrivel and die. As the root hairs die, new hairs arise towards the tip of the root, so that absorption from a fresh soil region takes place. The older parts of the root behind the root hair zone are brownish and are covered by a layer of cork cells which prevent the evaporation of water from the root.

Any root which develops from the primary root of the seedling, or from a branch of this root, is termed a true root, whereas a root which grows out from a leaf or stem is called an adventitious root.

The Shoot

The shoot system of the wallflower consists of a branched stem bearing leaves. At the tip of each stem is the growing point, which is protected by a number of young leaves which overlap so as to form a terminal bud. The leaves are arranged in a spiral round the stem, and in the angle between each leaf and stem, or axil, as it is termed, is a bud. These axillary buds are smaller than the terminal buds and generally remain dormant unless the terminal bud of the stem is injured, when they develop to produce shoots. The part of the stem where a leaf is fixed is called a node, and the stem region in between two leaves is the internode. The functions of the stem are:— To hold the leaves in a position where they may obtain plenty of light.

To conduct water and salts to the leaves.

To produce the flowers.

In some plants to act as storage organs.

The Leaf

A typical leaf consists of three parts :— The leaf blade or lamina, which is the flat expanded part of the leaf farthest from the stem.

The stalk or petiole.

The leaf base, which is the part nearest to the stem.

The lamina of the wallflower leaf has a smooth edge and is lanceolate in shape. The lamina is continuous with the leaf base, for there is no petiole. Such a stalkless leaf is said to be sessile. Running up the centre of the leaf is the main vein, from which branches are given off on either side. The side branches divide repeatedly, so that a close network of veins is formed. The functions of the leaf are :— To manufacture the food of the plant from the raw materials. To effect and control the exchange of gases between the plant and the air.

Stipules

The leaves of some plants bear a pair of flat outgrowths at the leaf base. Such outgrowths are called stipules, and their size and shape varies considerably in different plants. Stipules are absent in the wallflower, but the pansy leaf has large green stipules. The stipules of the pansy and rose remain on the leaf throughout its life, but those of the beech and many other trees drop off after the leaf has become fully grown.

Venation and Leaf Shape

The arrangement of the veins is termed the venation of the leaf. Leaves in which the veins form a close network, as in the wallflower, are said to have a reticulate venation. The leaves of grasses have several main veins running parallel, which are connected by short transverse branches. This is parallel venation. Leaves which have a single lamina are said to be simple leaves, while those in which the blade consists of several small leaflets are compound leaves. Many compound leaves have a main stalk along which leaflets are arranged in opposite pairs with a terminal leaflet at the end of the stalk, as in the ash and rose. Such leaves are termed pinnate. The leaflets of the horse chestnut leaf, however, radiate out from the end of the stalk in a similar fashion to the fingers of a hand, and are said to be palmate leaves.

Leaf Arrangement

The leaves of the wallflower occur singly at each node and are arranged spirally up the stem, but in other plants different arrangements are found. The leaves of the broad bean also occur singly at the nodes, but on opposite sides of the stem so as to form two rows of leaves. Consequently the leaves of this plant are said to be alternate.

When two leaves grow opposite each other at a node, they are said to be opposite. Generally, consecutive pairs of opposite leaves are arranged at right angles to one another, when they are said to be decussate. Such an arrangement is shown in the horse chestnut, sycamore and dead nettles.

When several leaves are given off at each node as in the cross-leaved heath, the leaves are said to be arranged in whorls.

The Inflorescence

A collection of flowers on a stem is termed an inflorescence, and there are many different types of inflorescences, differing in the arrangement of the flowers on the stem. The flowers of the wallflower inflorescence occur on a main stem or peduncle, which does not bear a flower at its apex but continues to grow and form fresh flower buds. The flowers are borne on short lateral branches or pedicels, which are separated by short intemodes. Such an inflorescence is termed a raceme. A leaf in the axil of which a peduncle arises is called a bract, and a leaf in the axil of which a pedicel arises is a bracteole. The wallflower is exceptional in having no bracteoles.

The Flower

A flower is a special part of the shoot modified for reproduction, and consisting of stem and special leaves called floral leaves. The stem, termed the receptacle, is very short, and the floral leaves are crowded together on it. There are four kinds of floral leaves, which are arranged in rings known as whorls and are termed sepals, petals, stamens, carpels. 1

The Calyx

The sepals form the outermost and lowest whorl of a flower and constitute the calyx. The sepals protect the inner whorls of the flower when it is in bud. The wallflower has four sepals, which are narrow scales, bluntly pointed at their tips and green or purplish in colour. They are placed in opposite pairs, and the lateral pair are placed slightly lower on the receptacle than the other sepals. Since the sepals are not united the calyx is said to be polysepalous. When the calyx of a flower is composed of sepals which are united it is termed gamosepalous. 1

The Corolla

Inside the sepals and attached slightly higher on the receptacle are the petals. The wallflower has four petals which together form the corolla. The corolla of a flower is usually coloured and scented, and serves to attract insects to the flower. The petals of the wallflower alternate with the sepals and are yellow or brown in colour. Each is oval in shape with the base produced into a long stalk, the claw, which is attached to the receptacle. As the petals are free and not joined together the corolla is polypetalous. A corolla in which the petals are united is said to be gamopetalous. 1

The Andrcecium

Six stamens, which constitute the andrcecium, occur inside the corolla. Each consists of a slender green stalk or filament, at the end of which is a yellow bilobed head termed an anther. A transverse section through an anther shows that each lobe of the anther has two chambers or pollen sacs containing yellow pollen grains. The stamens are arranged in two whorls, an outer whorl of two and an inner whorl of four stamens. The outer stamens are placed opposite the lateral sepals and are shorter than the other four, which are in pairs opposite the anterior and posterior petals. On the receptacle at the base of each short stamen is a small green pad with a moist surface. These pads are the nectaries which secrete a sugary solution for the attraction of insects which in their search for the nectar become dusted with pollen shed by the anthers when ripe. This they unconsciously brush on to the stigmas of the ovary, thus effecting pollination. 1

The Gynascium

Arising from the receptacle in the middle of the flower is a green stalk-like structure termed the gynascium or pistil. It consists of a hollow lower part, the ovary, which bears at its apex a short solid rod, the style. The style terminates in a bilobed head, the stigma, which is the receptive surface for the pollen grains.

The gynascium of a flower is formed of one or more floral leaves termed carpels. The gynascium of the wallflower represents two carpels which have become fused together. The ovary is divided into two cavities or loculi by a vertical plate named the septum. On each side where the septum joins the ovary wall there is a slight swelling termed the placenta. The placenta marks the line along which the floral leaves forming the gynascium have become joined at their edges. In both loculi of the ovary there are two rows of small oval bodies, the ovules, attached by short stalks to the placentas. The ovules are therefore fixed to the edges of the floral leaves. Under certain conditions the ovules develop into seeds.

When the gynascium consists of two or more carpels which have become joined together it is said to be syncarpous. An example of a syncarpous gynascium is seen in the wallflower. 1

Placentation

The arrangement of the ovules within the ovary is known as the placentation. When the ovary is formed from a single carpel which is joined along its edges, as in the pea, the placentation is said to be marginal. If the carpels of a syncarpous ovary are joined by their edges so as to form a single chamber or loculus, and the placentas occur on the walls of the ovary, the placentation is parietal. Sometimes, however, the carpels of a syncarpous ovary do not meet at the edges, but are pushed in so that the walls meet in the middle of the ovary and the ovules are borne on the central axis so formed. Such an ovary has two or more loculi, according to the number of carpels, and the placentation is termed axile. The syncarpous ovary of some flowers contains a single large loculus, into which a knob bearing the ovules projects from the base of the ovary. Such placentation is known as free-central, and is well shown in the primrose.

Floral Symmetry

Most flowers can be divided into two halves in such a way that the two parts are similar. The primrose and wallflower can be divided symmetrically in several planes, and are said to be regular or actinomorphic. The sweet pea and violet, however, can be divided into similar halves in one plane only. Such flowers are termed irregular or zygomorphic.

Pollination

When the pollen is ripe the pollen sacs of the anther burst by longitudinal slits and so expose the pollen. The ovules within the carpels cannot develop into seeds unless the flower is pollinated. Pollination is the transference of pollen by wind or insects from a stamen to the stigma of the carpel. If the pollen from one flower is carried to the stigma of the same flower, the flower is said to be self-pollinated. If the pollen of a flower is carried to the stigma of another flower, cross-pollination takes place.

Fertilization

Each ovule is enclosed in a coat made of one or two layers. These layers are called the integuments. At the apex of the ovule is a small hole in the coat known as the ‘ micropyle. Within the ovule is a large vacuolated cell ’ containing several nuclei, one of which is the egg cell or ovum. When a pollen grain is placed on a stigma the sticky sugary surface induces a pollen tube to grow out from the grain. This pollen tube passes down the style until it reaches the ovary, where it enters an ovule by the micropyle. Within the f pollen tube are two male nuclei, and these pass into the ovule, where one of the male nuclei unites with that of the egg cell and thus fertilizes it. Fertilization leads to the formation of the embryo within the ovule, which then develops into the seed. Meanwhile the wall of the ovary alters and becomes the pericarp. The ripe ovary containing the seeds is called the fruit.

Experiment 1—To show the Germination of Pollen Tubes A 10 per cent, solution of cane sugar is prepared. A drop of this is placed on a microscope slide and ripe bluebell Called the embryo sac. pollen is scattered on to it. A style with its stigma is detached from the ovary of the same flower from which the pollen is obtained and placed with the stigma in the centre of the drop. A watch glass lined with damp filter paper is inverted over the drop to prevent its drying up. Normally after three to four hours, on examination under the low power of the microscope, the pollen tubes will be seen to have germinated, putting out tubes which grow towards the stigma, showing that an attraction produced by some chemical substance exuding from the stigma exists. The presence in each pollen tube of nuclei, one of which may later fuse with the nucleus of the egg cell in the ovule, can be shown as follows :—

A stain known as acetic-lacmoid is prepared by dissolving a few grains of dry lacmoid in a little 45 per cent, acetic acid solution and warming until the stain has dissolved. The liquid is filtered and one drop is added to the liquid containing the germinating pollen grains. After a couple of minutes a coverslip is placed on the drop and pressure applied through several pieces of folded blotting or filter paper. The slide is then gently warmed and examined under the high power. Near the end of each pollen tube will be seen one or two nuclei stained red.

The egg cell in the ovule and its nucleus can only be seen by examining thin transverse sections of ovules especially prepared and stained. 1

The Fruit

The fruit of the wallflower is of the type known as a siliqua. It is a narrow cylindrical structure about 2 in. long, with a cavity divided into two by the flat partition or septum which runs from top to bottom. When ripe the sides of the fruit become dry and split away from the placenta?, the split starting from below and working upwards. The placenta? which are joined by the septum remain after the sides of the joint have fallen off. The seeds are attached to the placenta? and eventually drop off or are blown away by the wind.

THE INTERNAL STRUCTURE OF THE WALLFLOWER STEM

If a transverse section of a wallflower stem is examined three regions are recognizable :—

An outer region, termed the cortex.

A middle region lying just within the cortex, and consisting of a ring of conducting strands termed the vascular bundles.

A central region, the pith.

The ring of vascular bundles together with the pith constitute the central cylinder.

The Cortex and Epidermis

The outermost layer of cells covering the stem form a protective skin called the epidermis. The epidermal cells are elongated in a direction parallel with the stem, and in transverse section are brick-shaped. They fit tightly together and have no chloroplasts. The outer wall of the cells is covered by a thin layer termed the cuticle, consisting of a substance called cutin, which is impermeable to water and gases. The epidermis bears single-celled. hairs, each in the form of a spindle lying parallel with the stem, and attached by a short stalk which fits in between the other epidermal cells.

The cortex consists of large cells with thin cellulose walls ; each cell contains a central vacuole and a lining layer of protoplasm with a nucleus. A collection of such cells forms a tissue known as parenchyma. The cells do not fit tightly together, for at the corners where two or three cells meet a little space is left. Such spaces are called intercellular spaces, and provide channels for the passage of water vapour and gases. The outermost layers of the cortex contain chloroplasts and are also characterized by having the cellulose walls thickened at the angles where several cells meet. Such modified parenchyma is termed collenchyma and has a mechanical function, for it strengthens and gives rigidity to the stem. The innermost layer of the cortex is called the endodermis, and consists of brick-shaped cells which have no intercellular spaces between them, and generally contain starch grains. 162.

The Central Cylinder.

The Pith, Medullary Rays and Pericycle

The centre of the wallflower stem is occupied by the pith, which is formed of large parenchymatous cells. In some plants the pith is absent and there is a large central air cavity. The regions between the vascular bundles are termed medullary rays, and are occupied by cells similar to those of the pith, but smaller. Just outside the ring of bundles is a single layer of cells lying next to the endodermis, and consisting of thin-walled cells. This layer is termed the pericycle.

The Vascular Bundles

The vascular bundles are the conducting channels of the plant along which food materials and water pass. In the transverse section of the stem are to be seen five large bundles together with ten or so smaller ones. A large bundle enters the stem at each node from the leaf stalk, and passes down to join the bundle entering the stem from the leaf vertically below the first leaf. Two smaller bundles also enter the stem from each leaf stalk. The bundles are joined by cross connections at the nodes.

The Structure of a Vascular Bundle

A vascular bundle consists of three parts :—

A hard inner part, the xylem or wood.

A softer outer part, the phloem or bast.

A narrow layer of cells, the cambium, which separates the xylem from the phloem. The Xylem

In the xylem the original cellulose of the walls of most of the cells has been altered by the deposition of a substance termed lignin, which gives a firm woody structure to these cells. Lignified cell-walls are impermeable to water and will not stretch so much as cellulose walls, but they are more elastic, I.e. after stretching they will return more readily to their original position. Lignificd cells have no protoplasmic contents, and their walls usually show irregular thickening.

The xylem cells are of three types :— Conducting cells. Wood fibres. Xylem parenchyma. The Conducting Cells

The conducting cells, which carry water to the leaves, are either vessels or tracheids. A vessel is not a single cell, but is formed from a row of cylindrical cells, the cavities of which have become continuous by the disappearance of the transverse walls. The remains of the cross walls are visible as rings which encircle the vessel at intervals. A tracheid is a single cell which is long and spindle-shaped.

The vessels are not lignified evenly, for some parts of their walls are thicker than others. In some vessels the thickening takes the form of a spiral which runs on the inside of the wall, while in others it is in the form of separate transverse rings. The larger vessels have a network of thickening, or have most of the surface of the wall thickened except for oval spaces termed pits. When the pits are enlongatcd transversely, scalariform thickening results.

The innermost region of the xylem, termed the protoxylem, is formed earlier than the outer region or metaxylem, and is characterized l by the small size of the vessels, which have annular or spiral thickening. The Wood Fibres

These are spindle-shaped lignified cells, which lose their protoplasmic contents and have thick walls provided with narrow pits. The wood fibres have a mechanical function only, and do not serve to conduct water. The Xylem Parenchyma

Occurring between the vessels and the fibres are fairly thick-walled rectangular cells which have protoplasmic contents, and frequently contain starch grains. These cells constitute the xylem parenchyma, which acts as a storage and packing tissue. The Phloem

Unlike the xylem cells, the cells of the phloem are not lignified but have walls of cellulose. There are three types of phloem cells :— Sieve tubes. Companion cells. Phloem parenchyma. The Sieve Tubes

The sieve tubes are more or less cylindrical cells arranged end to end in long rows. The cross walls are perforated by many fine pores and are termed sieve plates. Each sieve tube contains a thin lining of cytoplasm containing a nucleus and enclosing a mucilaginous cell sap. The contents of each sieve tube are continuous with that of the next by narrow strands which pass through the pores of the sieve plates. It is not always easy to see the sieve plates, but they are particularly large and easy to examine in the cucumber or in the vegetable marrow. The Companion Cells

Lying alongside each sieve tube is a narrow cell filled with dense cytoplasm containing a nucleus. This is the companion cell, and it is formed together with the accompanying sieve tube by the division of a single mother cell. The Phloem Parenchyma

This name is given to the large thin-walled parenchymatous cells that occur amongst the sieve tubes and act as a storage and packing tissue. The protophloem is the outermost region of the phloem, and is the part formed first. The Cambium

Between the xylem and phloem are two or three layers of thin-wallcd cells which constitute the cambium. Each cell is elongated and contains abundant cytoplasm and a nucleus. The central row of cells is continually dividing so that layers of cells are produced on the inside and outside. Those on the outside gradually alter so as to become the phloem cells, while those on the inside become xylem.

THE INTERNAL STRUCTURE OF THE ROOT OF THE WALLFLOWER

If a transverse section of a young root of a wallflower is examined two regions are clearly seen :—

An outer cortical region.

An inner central cylinder containing the bundle.

The Cortex

This consists of large colourless parenchymatous cells with well-developed intercellular spaces. For a short distance behind the root tip the outer layer of cells, termed the piliferous layer, which has not a protective cuticle, as in the stem, bears structures known as root hairs. These are long slender projections from many of the cells of the piliferous layer, which are able to absorb water and salts from the soil. Each root hair has a thin cellulose cell wall with a lining layer of cytoplasm containing a nucleus. The cytoplasm encloses a central vacuole full of cell sap.

The cells of the inner layer of the cortex, called the endodermis, fit tightly together without leaving intercellular spaces, and are characterized by patches of cutin laid down along the middle of the radial walls.

The Central Cylinder

The tissues occurring inside the endodermis constitute the central cylinder. A band of xylem lies across the middle of the cylinder so that there is no pith, and on either side of the xylem is a bundle of phloem..

The protoxylem occurs at the ends of the xylem band, so that development of the xylem takes place from without inwardly. In the young root there are two separate xylem strands, but these become joined as the xylem develops inwardly. The bundles of xylem and phloem are separated by parenchymatous tissue.

In the older regions of the root, the cells of the piliferous layer die while the cell walls of the adjacent layer of cortical cells thicken. This layer is then known as the exoderniis.

Differences between the Structure of a Root and of a Stem.

The root bears root hairs and has no cuticle, while the stem has no root hairs and has a cuticle.

The xylem and phloem strands of a root alternate, while those of a stem are situated on the same radii.

The pith area is absent or very small in the root, but clearly present in the stem.

The protoxylem is on the outside of the bundle in the root, but on the inside in the stem.

THE INTERNAL STRUCTURE OF THE WALLFLOWER LEAF

A transverse section through a leaf shows that the upper and lower surfaces of the leaf are protected by an epidermal layer similar in structure to that of the stem. The epidermal cells have the outer wall covered with a cuticle, and seen in surface view have an irregular shape. In transverse section they are brick-shaped. Hairs similar to those on the stem occur on both epidermal layers, but the lower epidermis differs from the upper in having a greater number of hairs and in possessing a number of minute pores termed stomata. Between the epidermal layers the internal tissue of the leaf, termed the mesophyll, is divided into two regions, an upper palisade region and a lower region termed the spongy mesophyll.

The palisade tissue consists of three or four layers of cylindrical cells, arranged with their long axes at right angles to the surface of the leaf and containing many chloroplasts. Small intercellular spaces occur between the cells. The spongy mcsophyll is built up of irregular cells loosely arranged, with large intercellular spaces between them. The cells also contain chloroplasts, but these are not as numerous as in the palisade cells.

The veins in the leaf are the vascular bundles, and these are continuous with those in the stem. The large vascular bundles of the leaf have a surrounding sheath of parenchymatous cells, which accompanies them throughout their branching. The bundles become smaller and smaller at each branching, and finally the phloem disappears so that each bundle consists of a few spiral tracheids only.

The Structure of a Stoma

The lower epidermis of the leaf is pierced by numerous small pores, each of which is bounded by two curved, sausage-shaped cells termed the guard cells. A single pore with its guard cells is known as a stoma. The guard cells, unlike the other epidermal cells, contain chloroplasts and also starch grains. A transverse section through the guard cells shows that the walls facing the pore are thickened. The size of the pore can be altered according to the curvature of the guard cells, which depends on the fluid pressure in them. When the pressure is low the guard cells are straight and the pore is closed, but when the pressure is high they become curved and the pore is opened.

The Growth of the Wallflower Plant

During the life of the wallflower its stems and roots gradually increase in length and thickness owing to the formation of fresh tissue by cell division. Tissues which are capable of cell division are termed meristems, and those responsible for increase in length occur at the tips of the stems and roots and are known as apical meristems. Increase in thickness is brought about by the activity of the cambium, which finally has the form of a hollow cylinder of cells lying between the xylem and phloem.

The Growing Points of the Shoot and Root

The apex of the stem is protected by overlapping scales, within which lie the small undeveloped foliage leaves. A longitudinal section through the tip of the stem shows that the apex is slightly dome-shaped, and consists of thin-walled cells which have dense protoplasmic contents and which are actively dividing. Slight ridges on the apex represent the rudiments of leaves, and farther back the leaves have the form of small projections curving over the apex. In the axils of the larger leaf rudiments there are slight lumps formed from the superficial tissues of the stem ; these represent the axillary buds which will develop into side shoots.

The apex of the root is covered and protected by the conical root cap, and consists of meristematic tissue similar to that of the stem apex. The outer layer of the meristem gives rise to the root hair layer and also to the tissue of the root cap. The branch roots do not arise as superficial projections as do the branch shoots, but develop deep in the internal tissue of the main root, and grow out to the exterior through the cortex. In most plants the side roots occur in vertical rows opposite the xylem groups. Sometimes this is the case in the wallflower, but generally in this plant there is a double row of side roots opposite each xylem group.

The Increase in Thickness of the Stem

The cambium, which lies between the xylem and phloem of the vascular bundles, is composed of thin-walled living cells which are brick-shaped in transverse section and are elongated in the direction of the length of the stem. In the wallflower there are two or three layers of such cells, the central one of which is divided by tangential walls so as to cut off cells towards both phloem and xylem. The cells formed on the side of the xylem divide once or twice and then gradually develop into conducting tissue, wood fibres or parenchyma. The cells cut off on the outer side of the cambium develop, after one or two divisions, into sieve tubes, companion cells or parenchyma. As a result of the formation of this new tissue the vascular bundles increase in size.

At this stage the cambium of the bundles forms an incomplete ring of dividing tissue, and the gaps in the ring are occupied by the medullary rays. Soon, however, a line of cells in each medullary ray opposite the cambium of the bundles regains its power of division, and thus the circle of cambium is completed. As the cells of the complete cambium continue to divide, a ring of tissue, the secondary xylem and phloem, is laid down in between the xylem and phloem of the original bundle. At the centre of each of the original medullary rays the cambium does not form conducting tissue but gives rise to a narrow band of parenchyma which maintains the continuity of the rays. At other places in the cambium parenchymatous cells which give rise to secondary medullary rays are formed. These rays are radial sheets of tissue, two or three cells thick, which generally penetrate only a short distance into the xylem and phloem, and do not extend very far in a vertical direction. The cells of the medullary rays frequently contain starch grains, and act as reserve food stores.

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