Flowers: The Supreme Achievement

Ever since the first plants with flowers began to grow on the earth, about one hundred million years ago when the higher insects were beginning to appear, they have endured and spread until they assumed their present position as the dominant plants in almost every possible environment. One of the predominant reasons for their success is the great efficiency, compared with other methods, of their reproductive system, which owes much to the activities of various members of the animal kingdom, most particularly the insects, for effecting pollination. The most conspicuous flowers can be thought of as grand ‘advertisement’ displays, employing sweet nectar or pollen, scent or colour, or, more frequently, a combination of these, as lures for pollinators.

However, some flowers have neither coloured petals, nectar nor scent; often they are small and inconspicuous as well, as in the grasses. These plants rely on other methods of pollination, the need for advertisement disappearing. The pigment chlorophyll, responsible for the green colouration throughout the plant kingdom, is the only colouring agent found in many wind-pollinated flowers—the wind has no eyes for beauty. Such plants have other floral modifications to ensure the effectiveness of their system.

Animal-pollinated flowers show a dazzling variety in the construction of their flowers; no single floral type can provide for all types of potential animal pollinator, so different plants have adapted themselves to pollination by specific groups by offering variations in their flower colour, scent, size, shape, arrangement and type of food available.

Generalized flower structure

Although flowers may show an enormous range in form and be used by taxonomists almost solely to establish distinctions between the smaller groups of the flowering plants, collectively known as the angiosperms, all flowers conform to a common plan. Within this plan the parts that are noticeable usually belong only to the sterile parts of the plant, and the reproductive organs that ensure the continuance of their life cycle are less prominent or even hidden. A look at the structure of a generalized flower will show how each part contributes functionally to the ultimate end—reproduction. Receptacle

The receptacle is the expanded end of a flower stalk upon which the floral parts are borne in whorls or spirals with very short spaces (inter-nodes) in between. The parts develop on the apex in a similar manner to the leaf development on a vegetative meristem, so that the receptacle corresponds to an apical meristem of a vegetative shoot. However, growth of the receptacle is limited and stops when all the flower parts have been formed. Buds do not form in the axils of the floral parts (at least only rarely) though they invariably do in the axils of foliage leaves. The receptacle varies widely in its shape and size with different species, and may undergo considerable changes with the development of the flower, but its basic role stays the same—that of a firm base and supporting structure for the more showy parts above. Sepals

The sepals constitute the outermost whorl of the floral parts, the calyx, and may be described as floral leaves. They may be separate, united into a single outer sheath, or in groups of united sepals. Usually green and leaflike in texture, they have a simple internal structure of loose parenchyma bounded by an epidermal layer on both faces, which are sometimes hairy. The green colour of the sepals comes from chloro-plasts in the parenchyma. The sepals protect the developing parts enclosed within while the flower is still a bud. As the flower blossoms, these sepals fold back so the petals can open, sometimes they may even wither and drop off. In some cases the sepals may change, becoming expanded and brightly coloured and appearing more like petals, whose purpose they often serve. This is typical of monocotyledons—for example the tulip—where sepals and petals are almost indistinguishable. Petals

The corolla is the term used to describe the petals of a flower. They are also floral leaves, but have very distinctive characters, being generally brightly coloured, scented, of expanded form and arranged in a showy fashion to attract animal pollinators. As with the calyx, the corolla may be a single unit.

Flower colours are due either to pigments in special living bodies inside the cells known as plastids, or dissolved in the cell-sap. In the former category belong the common carotenes and xanthophylls, collectively known as the : notenoid pigments, giving colours ranging from lemon yellow to tomato red. More than 60 varieties of carotenoid compounds have been isolated and they make up the second largest family of colour in the plant kingdom (green chlorophyll being the first). Anthocyanins belong in the latter category, pigments dissolved in the plants cell sap. They range in shades from palest pink, through vivid reds and blues, to flamboyant purple. Anthoxanthins also belong to this group and give colours from pale ivory to deep yellow. These are both frequently occurring plant pigments.

Sometimes chemical factors can affect the performance of the anthocyanins. For example there is a certain species of morning-glory which begins its daily cycle in the morning when the cell-sap is slightly acid and the flower colour displayed is a pale pink colour; by the evening, the cell-sap is mildly alkaline and the flower has turned blue. The hydrangea is similarly affected; slightly acid soil causes the flowers to be pinkish whilst under more acid or mildly alkaline soil conditions the flowers turn blue. However, colour variations within species are mainly controlled genetically, and even slight variations in shade may indicate a different genetic composition. The African violet contains an anthocyanin pigment called violanin, which is closely related to the pigment which colours delphiniums. As a result of the thorough research into the genetic behaviour of anthocyanins, African violet growers can now, by careful selection, produce specimens which range throughout the whole spectrum of anthocyanin colouring, from pink to blue, and even including white.

If a petal is examined microscopically, its epidermal cells will often show special peculiarities, especially in the ways in which they are fitted together. Often the cell walls have a wavy and ridged outline, giving the overall petal surface a beautiful mosaic appearance. The cuticle overlying the epidermis is often striated and very distinct patterns of lines may be formed to give the surface a roughened appearance. Grooves so arranged that they converge towards the centre of the flower, and which may also appear as darker lines, are known as ‘honey-guides’ since they are thought to direct insects towards the nectaries within the flower. Sometimes these lines may only become visible to our eyes if the flower is photographed using ultraviolet light. This fact leads to the intriguing question of colour perception differences between man’s and insect’s eyes.

The eye of a bee covers a different part of the colour spectrum from mans, so that bees see flowers in different colours: bees cannot see red, but they can see ultraviolet which man cannot. To a bee, a red poppy appears purplish; other flowers are black but the honey-guides appear as ultraviolet marks signposting the way to the nectaries. Again such flowers as rape, mustard and charlock which all look yellow to man, appear purple, yellow and crimson respectively to a bee, which perceives the little common daisy as a halo, since only the tips of its petals radiate ultraviolet.

In an experiment two notices were painted in white letters on a black background, the first being painted with process white which radiates ultraviolet light, the second with Chinese white, which does not radiate ultraviolet. The notices were ‘Bees may feed here’, and ‘No bees allowed here’. Because the second was invisible to the bees, they appeared to be able to read these notices!

Many flowers attract animal pollinators by means of scent as well as by colour. These scents, so familiar in certain flowers, derive from volatile oils produced by certain epidermal cells and their associated hairs, and their range is wide, through sweet, delicate, fiery, fresh, fetid and strong. Often they are a surer way of luring insects into visiting a flower than bright colours are, though most flowers combine the two just to make sure. Scent really comes into its own when the light fails and bright colours start to look drab and inconspicuous. Many of the most intoxicating scents are to be found amongst night-opening flowers, such as the evening primrose (Oenothera odorata), the stocks and the tobacco plant (Nicotiana). Frequently such flowers have expansive white or pale-coloured petals as well. These make the flower appear almost luminescent in the dim light and this, combined with the strong scent, advertise the plant to night-flying pollinators, mainly moths, to best advantage under these poor light conditions.

Not all flower scents are sweet, however. Where flies are the chief pollinators of a plant, smells are often fetid and repulsive to our noses. The arum is one example of this; another is the giant flower Rafflesia of Java, the largest flower in the world. This cabbagelike jungle flower, 6ocm (24m) in diameter, with its brown and purple colouration, looks and smells like rotting meat, causing hundreds of flies to be crawling over it at any one time. The African stapelias are called carrion flowers because of their odour.

Reproduction

The sepals and petals together constitute the perianth, which is the asexual part of the flower. The sexual part, which functions to produce, bear and protect the sex cells, consists of a male portion, androecium, formed by the stamens, and a female portion, gynoecium, represented by the carpels.

A stamen consists of two parts: a stalk or filament and a swollen part on top of it, the anther, which is usually a bright orange or yellow colour, though sap pigments can produce other colours, as in the tulip where anthers are often purple. An anther has two lobes, each lobe containing a pair of pollen sacs in which the pollen grains are contained. Pollen grains are formed from mother-cells which divide by mciosis (the process by which the number of chromosomes in a nucleus is halved forming a reduced ‘haploid’ nucleus). Cells packed with starch grains form a layer immediately surrounding the developing pollen, providing it with nutriment, whilst another, outer layer becomes fibrous with ligninthickened cells. By its uneven contraction upon drying this layer imposes strains upon the pollen sac walls, causing them to rupture. Splitting generally occurs along the grooves in between two pollen sacs where the epidermal cells are very small and easily broken apart. As the anther continues to dry out, so the edges of the now-separate pollen sacs curl away from the rupture line, finally disclosing their contents of tiny’ pollen grains to the outside.

When they are mature the pollen grains consist of little rounded cells, each covered in a thick protective wall which is composed of two layers, the exine and the intine. The exine has a thick cuticle and a very irregular surface; it may be covered in spines, pitted or sculptured in a manner characteristic of the species, but not all over, as little pits (germ-pores) are left through which the pollen tube breaks when the grain germinates. The intine, is a much more delicate structure, mainly composed of cellulose and pectic substances, and it is from this layer that the pollen tube itself develops. When it is shed, each little pollen grain contains within itself two nuclei: one is the pollen tube nucleus and the other is that from which two male gametes will arise.

The female organ of the flower is the carpel, essentially a closed hollow container formed by the folding and fusion of a leaflike structure. Its basic function is to house the ovule which after fertilization becomes a seed. The part surrounding the ovule is the ovary, whilst typically the distal end is drawn out into a prolongation called the style, bearing a sticky pollen-receptive surface at its tip, the stigma. The ovules developing within the ovary are thus completely enclosed by sterile tissue, and it is this feature which essentially distinguishes the angiosperms (the flowering plants) from the gymnospcrms (the cycads and conifers).

Ovule development starts with a little outgrowth from a part of the carpel wall called the placenta. The small lump of tissue is called the nucellus and it gradually enlarges and becomes egg-shaped. As it does so, two sets of tissue grow up and around it forming two protective sheaths or integuments. It is from these integuments that the scedcoat is later formed. They do not quite completely enclose the nuccllus for a little passage is left at the apex; this opening is called the micropyle. Next the tiny ovule is raised clear of the placenta on a short stalk or funicle which arises from the ovule’s base.

Meanwhile, changes have been going on inside the nuccllar tissue. A mother-cell formed at the apex of the nucellus divides by meiosis into four, only one developing further, nourished by the nucellus. This megaspore cell develops into the embryo sac, a large vacuolate sac containing eight haploid nuclei. These eight nuclei are arranged in a particular fashion: three positioned near the micropyle end of the sac, the central one being the actual female nucleus or ovum; three more lying at the opposite end of the sac—the antipodal cells; and half-way along the sac are the two remaining nuclei the polar nuclei. The embryo sac, thus complete, brings the ovule into readiness for the fertilization process to take place.

Variations from the generalized flower

There are many variations amongst the flowers to the preceding general description, these variations being useful in the classification of the flowering plants, though they are interesting from other view-points as well. The variations cover arrangement, number, size and form of all parts.

The various parts of a flower are arranged on the receptacle in either a cyclic or a spiral fashion, or sometimes a mixture of both. The majority show the cyclic condition, with separate whorls of floral parts. Cactus flowers have their parts spirally arranged, while the buttercup, for example, has its calyx and corolla in two separate cycles and its stamens and carpels spirally inserted. The spiral condition is thought to be the more primitive one. Floral parts usually alternate so that petals do not come opposite sepals but between them, and stamens come between petals and so on.

There is also a difference in the relative positions of parts on the receptacle. This is chiefly related to the different ways in which the receptacle develops. If the final form of the receptacle is a dome, the flower parts are arranged in the order: sepals, petals, stamens, carpels, rising up the dome. The female part of the flower, the gynoecium, is thus in a superior position relative to the other parts, and this condition is known as ‘hypogynous’. The Ranunculaceae (the buttercup family) and Scrophulariaceae (foxglove family) have examples of hypogyny. The earliest flower forms were probably similar to those of the magnolias and the tulip-tree Liriodcndroii which have spirally-inserted parts on a long domelike receptacle.

Where the receptacle grows into a dish or saucer-shaped structure, the flower parts are as described above only flattened so that all parts are on the same level, with the gynoecium in the middle. This is the perigynous condition and different degrees of it are to be found in the rose family.

The other way in which the receptacle can develop is to become indented into a flasklike shape and in such a case the gynoccium occupies an inferior position relative to the other parts. This epigynous condition occurs in some mem- bers of the Rosaceac and also in the Compositae (the daisy family) where the receptacular flask is in fact closed by tissues arising from the carpels.

The nectaries, which may be specialized glandular regions or fully-developed glandular outgrowths, may develop as part of the receptacle or on one or more of the floral parts. In the cherry flower, the nectaries are to be found lining the perigynous receptacle, whilst in Viola, they are found in the stamen. Buttercup nectaries are tiny outgrowths of the petals.

Most flowers have their parts in definite constant numbers. If there are five sepals, then there are likely to be five petals and five or ten stamens. Five is a common number for the sepals though they do vary from the two-sepallcd poppy flower to the water-lily where the sepals are numerous. In the buttercup and rose families, however, the stamens and carpels occur in large numbers (’indefinite’) which vary from one plant to another and may even vary from flower to flower.

In some cases the flower is reduced by one or more of its floral whorls; sometimes the sepals or petals are missing, sometimes even both, and sometimes a flower becomes one-scxcd. Petals are entirely absent in the marsh marigold where the five large sepals, instead of being green, are brightly coloured and enlarged, taking on the function of the petals which are therefore no longer required. In the Christmas rose, though the sepals are white and petaloid, the petals remain but are very much reduced to small tubular nectaries. This kind of reduction of parts is an advanced evolutionary character. The other extreme is the multiplication of parts. This is commonly found in many of our cultivated flowers where there is a multiplication of petals and sepals. Although this makes the flower appear more attractive to man, it is a strange fact that many of them are actually sterile.

Primitive flowers have many parts, all separate and freely inserted on the receptacle. This type of condition is characteristic of the buttercup family, the Ranunculaccae. As they advance on the evolutionary scale, flowers generally simplify; sometimes parts go missing as mentioned above, sometimes parts fuse to- gether laterally, forming petal and stamen tubes as in the cardinal flower. The bird’s foot trefoil has nine of its stamens joined together, whilst the tenth is free. Where the number of carpels in a flower is small and definite, these are frequently found fused together as in the gooseberry which has two carpels fused along their longitudinal margins. Some parts, most particularly the stamens, show a tendency to become inserted on an adjacent floral whorl rather than on the receptacle. Flowers with their stamens inserted on the petals are to be found in such families as the Compositae, Scrophulariaceae and Labiatae (the mint family). In some orchids, the stamens are inserted on the carpels instead. Such reduction and fusion of parts makes for greater all-round simplicity in the flower; it is not in any way a degradation for it in fact leads to greater efficiency in the flower, both economically and in pollination processes.

Regular and irregular flowers

A flower may be regular or irregular. ‘A regular flower means that a vertical split through the centre could be made in any one of several planes and the parts would be identical in every case, for example, as in the buttercup. A regular flower is said to be actinomorphic, and is the result of having each part of a whorl identical with all the others and all evenly disposed in the whorl.

The pea flower has five petals. Looking straight towards the inside of the flower, one large and spreading petal stands up as a background to the rest; this is called the standard. Flanking the standard at either side are two winglike petals, which are therefore called the wings, whilst at the bottom of the flower two smaller petals lie facing each other and together looking something like a ship’s keel. They are therefore collectively called the keel. Examining this pea flower, it is clear that there is only one vertical plane through which it could be cut in order to get two halves of the flower symmetrical. The plane would pass vertically downwards through the standard and between the two wings, and bisecting the keel. So the flower is irregular. This irregular condition is also referred to as zygomorphy and is the end result of either fusion of floral parts, absence of one or more parts of a whorl, or differing sizes of parts within a whorl, most often of the petals and/or sepals as we saw in the pea.

Members of the Crucifcrae, the cabbage family, show yet another condition. They have flowers which can be divided equally in only two planes at right angles, and the half-flowers produced by cutting in one such plane are different from those produced by cutting in the other, though the flower is still considered to be actinomorphic. Flowers with spirally inserted parts, such as in the cacti, cannot be divided into two exactly equal halves however many cuts are tried, and are hence termed asymmetric.

Flowers with irregularly shaped petals are often the result of specialized adaptation to a particular pollination method; there is division of labour within the petal whorl. The upright standard and flanking wings of the pea for example serve together to advertize the flower whilst the keel functions as an attractive solid-looking landing base for a visiting insect, the weight of which upon alighting depresses the keel downwards, exposing the sexual parts which stick up onto the underside of the insect.

The flowers of the family Compositae, which includes daisies, dandelions, hawkweeds, deserve special mention. What at first appears to be a petal in the flower head is actually a complete flower, often called a floret. These florets are all tightly packed together to amass a broad expanse of colour to attract visiting insects. In some Compositae there is a sharp division of labour amongst the florets. For instance, in the black-eyed susan, the outer insectattracting yellow ray is composed of sterile flowers with conspicuous petals, whilst the inner circle is made up of hundreds of fertile ones, with/their tiny inconspicuous fused petals, ensuring efficient pollination.

The ways in which flowers are arranged on a plant is termed the inflorescence. Flowers may develop singly or in groups. They may be at the end of the main shoot or one of its branches, with or without a stalk. Amongst insect-pollinated flowers it is common to find small flowers grouped together to provide a visually more attractive expansive inflorescence, for example the much-stalked, flat-headed type of inflorescence characteristic of the carrot family, the Umbelliferae. Single flowers tend to be larger and more showy, such as the tulip or the wood anemone.

Most flowers are hermaphrodite, that is they possess both male and female parts, the stamens and the carpels, together in the one flower. However, some flowers possess either stamens or carpels, but not both, and are thus unisexual. Male and female flowers may occur together in the same inflorescence, or there may be unisexual flowers together with ‘normal’ hermaphrodite flowers in the same inflorescence; both s. these conditions are to be found in the Com-positae and the Gramineac (the grass family). Sometimes separate male and female flowers occur together on the same plant but in different inflorescences. This happens in the hazel and oak trees, and such a species is called monoecious. Dioecious species, which include willow and poplar, have male and female flowers on separate plants. Occasionally, as in the Com-positae, flowers develop which have neither stamens nor carpels. These flowers are thus neuters, which may seem a contradiction of terms, in view of the picture drawn of flowers as reproductive devices. However it must be remembered that such flowers are in fact highly specialized for their purpose in life; they direct all their energies into being attractive, thus luring insects to the fertile flowers, with which they are in close association.

Sorry, comments are closed for this post.