The Buttercup Flower
This is a simple ’open ’type of flower. It is regular and hypogynous. The calyx consists of five sepals alternating with five yellow petals. At the base of each petal is a small flap on the upper side of the petal beneath which nectar is secreted. Both the sepals and the petals are borne in whorls but the numerous stamens and the carpels are spirally borne. The carpels are not united to form a single ovary and each contains a single ovule and is surmounted by a small hooked stigma. This flower is not particularly adopted for cross-though the anthers shed their pollen outwardly and away from the carpels which may help to prevent self-pollination.
Generally two or threeon short stalks are borne on each peduncle. The flower consists of the usual four whorls of floral . The calyx is made up of five sepals which have pointed tips and are joined together at the base, I.e. are gamosepalous. Five petals constitute the corolla, but two anterior petals have become joined together so that there are only 17a four free parts. These are a broad upper or posterior petal termed the standard, a pair of smaller lateral petals, the wings, and a lower or anterior boat-shaped structure, the keel.
Within the keel lies the andrcecium, consisting of ten stamens. The lower parts of the filaments of nine of the stamens are joined together for two-thirds of their length so as to form a tube which has a narrow slit along the top. The slit is filled by the single free stamen. The free ends of the stamens are curved upwards at right angles to the tube, within which lies the gynsccium. This consists of a single carpel with marginal placenation. The ovary is a very small pea pod and bears a style which is bent in a similar way to the stamens, and has a terminal stigma with a brush-like hairy base.
The Pollination of the Sweet Pea
The nectar of theis found in the trough formed by the filaments of the nine united stamens. This trough is hidden inside the keel of the corolla and can only be reached by a long-tongued insect such as a bee, which is heavy enough to depress the wings and so cause the keel to open. The pollen is shed from the anthers into the tube formed by the end of the keel. A bee alighting on the wings of the corolla causes the keel to be opened and the stamens and style to be pushed out at the tip. The style emerges first and then the pollen is swept out by the hairs on the style just below the stigma. As the bee lifts the free stamen and probes within the filament trough for the nectar, the stigma touches the lower side of its body and thus receives any pollen which has been left there after a previous visit to another flower. At the same time some of the pollen clinging to the hairs of the style is deposited on the bee ’s body.
When the beethe flower the keel rises again so as to enclose the stamens and style. In spite of this arrangement for cross-pollination the sweet pea flower is usually self-pollinated before the flower is fully open. In some cultivated forms the petals are too strong to be separated even by bees.
This is another example of a closed flower which can only be opened by bees. The four stamens lie under die upper lip with the stigma projecting slightly from between the anthers. When the bee opens the flower, its back becomes dusted with pollen, which it conveys to the stigma of the next flower it visits.
A similar arrangement of stamens and stigma is found in the openof the foxglove and the dead-nettle. Here the nectar can only be reached easily by bees owing to the exclusion of small insects by hairs growing inside the corolla tube. Pollination takes place in the same manner as in the snapdragon, by the back of the bee first touching the protruding lobes of the stigma before coming into contact with the anthers. Cross-pollination is more likely to occur in the foxglove than in the dead-nettle since the foxglove flower is protandrous. In the dead-nettle flower the stamens and stigmas ripen at the same time so that self-pollination is likely to occur should cross-pollination fail.
At first sight the sunflower plant appears to bear one large flower at the end of the, but closer inspection shows that this so-called flower is really an inflorescence, consisting of a great number of small sessile flowers crowded together on the flattened apex of the stem. Such an inflorescence is known as a capitulum, and the small flowers are termed florets. The capitulum is surrounded by a ring of overlapping green bracts which form the involucre.
The florets which occupy the inner region of the capitulum have tube-shaped corollas and are known as tubular or disc florets. Surrounding these is a single ring of ray florets, each of which has a corolla consisting of a short tube from which is given off a long strap-shaped part. In both types of floret the corolla is made up of five petals which are fused together, and which are indicated in the disc floret by five small points at the apex of the corolla. In the ray floret the four lines running down the strap-shaped part of the corolla represent the edges of the fused petals. The corolla of the disc floret is attached to the top of the syncarpous gynascium, which consists of two fused carpels containing a single ovule. The style is a long rod-like structure ending in a fork which forms the two stigmas and which projects just above the edge of the corolla. The stigmatic surfaces are on the upper sides of the fork. The andrcecium consists of five stamens which are inserted on the corolla and which are joined together by their anthers so as to form a tube round the style. The gynaxium of the ray florets is similar to that of the disc florets, but the ray floret has no stamens, and consequently is unisexual. The calyx is reduced in both types of floret.
The capitulum is characteristic of the order oftermed the Composita?, to which such species as the daisy, dandelion, groundsel and thistle belong.
The Pollination of Composite Flowers
The stamens of composite flowers ripen before the stigmas, and the anthers split along their inner sides so as to shed the pollen into the tube formed by the united anthers. The style is at first short, and lies below the pollen mass in the anther tube, with the stigmas folded together so that their surfaces cannot receive pollen. As the style lengthens, the tip pushes the pollen out of the anther tube, and consequently the pollen is exposed before the stigmas. The fork at the tip of the style then opens to expose the stigmatic surfaces. Owing to this order in the appearance of the pollen and stigmas it is unlikely that self-pollination will take place. Insects walking over the surface of the capitulum in search of the nectar will effect cross-pollination by the transference of pollen from one floret on to the stigmas of another. If a floret fails to be cross-pollinated, the stigmas may curl back so as to pick up any pollen which has remained sticking to the style.
Variation in the Shape of the Receptacle.
In the buttercup the receptacle is a conical structure on which the sepals, petals, stamens and carpels are arranged in order with the carpels at the top. In such a case the ovary is said to be superior and the flower hypogynous. Other examples of hypogynous flowers are the poppy, wallflower and primrose.
In many flowers, such as the cherry or plum, the receptacle is flattened or cup-shaped and bears the gynaxium in the centre, while the sepals, petals and stamens are attached to its edge. The rose flower has a deep cup-shaped receptacle which bears the carpels scattered over its inner surface. Such flowers are described as pcrigynous, and the ovaries are said to be superior.
When the gynaxium becomes sunk into the tissue of the cup-shaped receptacle and actually fuses with it, and the sepals, petals and stamens are borne on the top of the ovary, the flower is said to be epigynous and the ovary is inferior. Examples of such flowers are the daffodil, iris, apple, willow herb and all composite flowers.
Types of Inflorescences
Some plants, such as the
I tulip and daffodil, bear one flower only, but the majority have a special branch system which carries a number of flowers. Such a collection of flowers is called an inflorescence. The main stalk of an inflorescence is called the peduncle, and the flower stalk is the pedicel. The leaves in the axil of which the peduncle and pedicel arise are usually simpler than the vegetative leaves, being small and scale-like, and are termed bracts and bracteoles respectively. The branching of the inflorescence may be racemose or cymose. If the main stem of the inflorescence does not end in a flower, but continues growth, and the flowers are borne on side branches, the inflorescence is said to be racemose. The inflorescence is said to be cymose if the growth of the main axis of the inflorescence is terminated by the development of a flower, and growth is carried on by a lateral branch which overtops the main axis, develops a flower at its apex, and in its turn is outgrown by another lateral branch. There are several types of both kinds of inflorescence.
The flowers are borne on short pedicels on a long peduncle, e.g. wallflower.
The flowers are borne on a long peduncle, but are sessile, I.e. have no pedicels ; e.g. plantain.
This is similar to the spike except that the peduncle is thick and fleshy ; e.g. arum.
When the lower flowers of a raceme have longer pedicels than the upper ones, so that all the flowers lie at practically the same level, the inflorescence is termed a corymb ; e.g. candytuft.
The part of the peduncle which bears the flowers is short, so that the crowded pedicels radiate from the top of the peduncle and the flowers lie at the same level; e.g. ivy, cowslip.
The Compound Umbel
If a pedicel of an umbel instead of bearing a flower branches so as to form another umbel, the inflorescence is said to be a compound umbel ; e.g. hemlock.
If, in place of each pedicel of a raceme there is another small raceme, the inflorescence is termed a panicle ; e.g..
A compressed spike ; e.g. sunflower.
The One-sided Cyme
When the growth of the peduncle is terminated by a flower and a single side branch carries on growth, only to be overtopped in its turn by another side branch, the inflorescence is said to be a one-sided cyme ; e.g. forget-me-not.
The Two-sided Cyme
After the cessation of the growth of the peduncle two side branches develop opposite one another, and the inflorescence is a two-sided cyme ; e.g. campion.
Sometimes two types of branching are found in the same inflorescence. For example, the inflorescence of the garden geranium is a number of one-sided cymes arranged in an umbel. The flowers of the dead-nettle appear to be arranged in whorls with the younger whorls towards the tip of the plant. Actually each whorl consists of two clusters of flowers in the axils of bracts. Each cluster is a two-sided cyme in its first branching and a one-sided cyme in its later branching.
Hermaphrodite and Unisexual Flowers
Flowers which possess both stamens and carpels are said to be hermaphrodite. Many flowers, however, do not possess both, but have either stamens or carpels, and are termed diclinous or unisexual flowers. If the stamens only are present the flower is staminate, and if the flower possesses carpels only it is termed carpellary or pistillate. The hazel and the oak are examples of plants which bear both pistillate and staminate protandry, when the stamens ripen first, and protogyny, when the carpels ripen first. In a protandrous flower, for example the willow herb, the stamens shed their pollen before the stigma is mature and ready to receive it; and not until the stamens have withered is the stigma ripe. Other protandrous flowers are the snapdragon, columbine and delphinium, and the flowers belonging to the Compositaj, Labiatas and Umbellifera?. Protogyny is not as common as protandry, but the figwort and the plantain are examples of protogynous flowers. The lower flowers of a plantain spike are the oldest and have their stamens fully exposed. The ovaries of these lower flowers have been fertilized and the styles have disappeared, but the upper flowers of the spike are still unopened except for the protruding styles.
Another arrangement resulting in cross-pollination is known as heterostyly, because the flowers exist in two forms with styles of different length. This feature is well shown by the primrose. The primrose has a calyx consisting of five sepals joined to form a tube, and a corolla of five petals also joined at their bases to form a corolla tube. Five stamens with short filaments constitute the andrcecium, and the syncarpous gynaxium, consisting of five carpels fused to form a spherical ovary, occurs at the base of the corolla tube. The two types of flower differ in theof the stamens and the length of the style. The pin-eyed flower has a long style reaching to the top of the corolla tube, and the stamens are situated half-way down the tube. On the other hand, the thrum-eyed flower has the stamens at the top of the corolla tube, and the style reaches only half-way up the tube. An insect seeking the nectar secreted at the base of the corolla tube of a pin-eyed flower will receive pollen on its head from the stamens half-way down the tube. Then, on visiting a thrum-eyed flower, the insect will deposit this pollen on the stigma of the short style, and at the same time its body will be dusted with pollen from the stamens at the top of the tube. Now, if it again visits a pin-eyed flower, the pollen on its body will be received by the stigma of the long style.
Methods of Pollen Transference in Cross-pollinated Flowers
Cross-pollination is usually brought about by the agency of wind or insects, and many structural adaptations are shown by flowers in connection with one or other of these modes of pollen transference.
Wind-pollinated flowers are characterized by the following features:
They produce great quantities of pollen, since most of the pollen is likely to be wasted and only a very small proportion will reach the stigmas. The pollen is light and smooth, and is frequently produced in stamens with long filaments which swing easily in the wind ; e.g. grasses. In the oak, hazel and other trees the male flowers are arranged in a hanging inflorescence called a catkin, so that the pollen is easily shaken out.
In order that the pollen distribution may not be hindered by the leaves the flowers are either arranged in long-stalked inflorescences, which stand well above the leaves, or mature before the leaves are out. The elm, for example, flowers in February and March, long before its leaves are unfolded. The flowers of the beech and oak open while the first leaves are just beginning to unfold, and have withered before the leaves are fully out.
The flowers are not brightly coloured, and the perianth is usually small and inconspicuous. They have no scent or nectar.
The stigmas of wind-pollinated flowers are usually large and feathery, and the styles are long so that the projecting stigmas may catch any pollen that is floating in the air.
Insect-pollinated flowers, to which class most of our British flowers belong, generally show many of the following features :—
The flowers have large brightly coloured corollas, and usually possess a scent and secrete nectar.
The pollen grains have rough, spiny surfaces, and consequently easily become stuck to the body of an insect.
The surface of the stigma is sticky, so that once placed on the stigma the pollen is not easily brushed off.
The shape of the flower may be specially adapted for the visits of insects, and the nectar so placed that in order to reach it the insect becomes dusted with pollen. In certain cases the flower is adapted for the visits of a particular kind of insect. Thus flowers which are pollinated by moths have the nectar secreted at the bottom of a long corolla tube, so that it can only be reached by such long-tongued insects. Such flowers, e.g. honeysuckle, are light coloured and have a strong scent, and usually open in the evening. Butterfly flowers, such as the pink, also have deep-seated nectar, but open during the day. Bee flowers are usually blue and have a sweet scent; the nectar is hidden, and the shape of some flowers is such that only large-bodied insects like humble bees can obtain the nectar. Other bee flowers are white dead-nettle, larkspur and sage.
The Floral diagram
In order to illustrate theof parts of a flower, drawings representing longitudinal and transverse sections must be made. The transverse section is an imaginary ground plan passing through all parts, and is termed a floral diagram. The side of the flower towards the stem of the plant is called the posterior side of the flower, and the side towards the bract is the anterior side. The posterior side of the flower is shown in the floral diagram by a small dot or circle placed outside the sepals. When the members of the calyx or corolla are joined together this is indicated on the diagram by small brackets connecting the members concerned., A small line drawn from each stamen to the corresponding petal indicates that the stamens are inserted on the corolla.
The Floral Formula
The essential features in the structure of a flower can be represented by the floral formula. In this the different whorls are indicated by letters, behind which are placed figures showing the number of parts in the whorl. Thus K stands for calyx, C for corolla, A for andrcccium, and G for gynaxium. When a perianth is present this is shown by the letter P. The sign 00 is used when the parts in a whorl are numerous, and brackets denote that the parts enclosed are united. A horizontal line below G shows that the gynaxium is superior and one above that it is inferior.
Buttercup: K5, C5, Aoo, G 00.
Sweet pea: K, C5, A + 1, Gi.
Wallflower: Kz+Z, C4, A2+4, G*.
: P3+3,A3+3, G.