RELATIONSHIPS in Flowering Plants and their inclusion in Natural Orders, or Families, as they are now more generally named, depend entirely upon the structure of the.
It may happen that all the plants of one family agree in their general morphological characteristics – all the plants in Crucifer, the Wallflower Family, are, for example,.
But this agreement is by no means the rule. There is very great diversity in the growth of the Clover of the meadows and the Laburnum of. The gardens – yet both these plants belong to Leguminosx. The Cinquefoil, creeping as an insidious weed along the garden beds, the erect Apple tree, the scrambling Wild, are all members of Rosacew – but structurally they are widely different.
In Flowering Plants it is on the flower, in the big majority of cases certainly, that the permanence of a species depends. Thethat are developed give rise to new individuals so that the race persists. Certainly bulbs and corms, instead of , are planted to flower in the garden beds. But in the course of time the appear sparsely and the dwindle in size – the Darwin Tulips lose the varied colour of their blooms, which become smaller and more insignificant each year. On the bulb farms of Holland especial care is taken to get good seeds from the various flowers. These are sown, and in three years’ time they have formed new bulbs as a result of basal swelling of the . In this way weakening of the strain is prevented.
In Chlamydomonas and in Hydra, in the plant as well as in the animal, sexual reproduction depends upon two nuclei, from different sources, meeting and fusing. A similar fusion of two nuclei of diverse origin is the basal fact in the sexual reproduction of. The male gamete is developed within a pollen grain – the female gamete is developed in an ovule within the ovary.
Speaking generally, the fertilisation of an ovule is dependent upon pollen from another flower. As the flowers are separated in space some agent must carry the detached pollen to the fixed egg-apparatus.
In Britain wind and insects are responsible for such transference of pollen.
The wind bloweth where it listeth, and it is in a more or less haphazard way that, through its intervention, pollen reaches the goal. Indeed, as far as the transference of pollen from one flower to another is concerned, there is no purposeful action even on the part of insects. Their visits have an aim, it is true, but the aim is in relation to their own immediate needs, and is in no way con- cerned with any good that may ultimately accrue to the plant.
Hunger is the most potent factor in determining the visits of insects to flowers. The food they find is nectar and pollen. In some quite exceptional cases theyon the juice of succulent floral – bees pierce the blue perianth leaves of the Wild and suck the abundant juice.
Nectar is produced by special glands in the flower for the sole purpose of attracting insects. Strictly speaking, it is only honey after it has been operated upon by a bee. The change from nectar to honey takes place in the bee’s mouth, where salivary secretion brings about the change. The nectar of the flower is a dilute solution, consisting, approximately, of 75 per cent. Water, and 25 per cent. Cane-sugar (C121122011) and grape-sugar (C61-11206) in equal proportions. In honey there is only about 20 per cent, of water, and of the 75 per cent. Of sugar present, only about per cent, is cane-sugar. There are also, in honey, certain organic acids. Speaking freely, however, it is very usual to talk of honey and honey glands in the flower.
Theof nectaries is different in different flowers, but is always such that an insect, inserting its proboscis to draw up the honey, plays, all unwittingly, an important part in , which precedes fertilisation.
Pollen develops in floral leaves called stamens. It is essential for the maintenance of the species – its use as insect food is incidental. In such insect-pollinated flowers as have no honey-glands there are usually many stamens, producing an abundant supply of pollen, above and beyond the plant’s own need. This is the case in Poppies and.
A delicate thermometer shows that the temperature in the centre of a flower is slightly higher than that of the air outside.
For this reason insects that have no home of their own seek shelter in flowers during a temporary cold or damp spell. On late summer or early autumn mornings earwigs are found between the pink or white sepals of the Japanese, in the spur of the Garden Nasturtium, and among the many flowers of the head. They have been sheltering from cold and damp all through the night. The flower has been their casual ward.
Again, but this is not so common, some insects visit flowers in order to lay their eggs within the ovaries. It is a wonderful instinct that leads to the deposition of the animal eggs among the ovules of the plant. When the insect larvx hatch out they find themselves surrounded by an abundant supply of food, for the ovules have now become seeds. Here again the provision is ample for the needs of both organisms. The number of seeds is such that after the larvx have eaten all they need, many still remain to fall to the ground and germinate.
This particular relation between insect and plant is beautifully illustrated in the interdependent life-histories of the Nottingham Catchfly and some of the Owlet Moths.
The flower is one of the wild, and its distribution is very local. There are certain districts in Cambridgeshire and Norfolk in which it may be found, and it grows near Dover also.
Its flowers, like those of the Tobacco plant, Evening, and Night Scented Stock, appear colourless and faded during the day. An hour or so before sunset they not only open out but they are, at this time, very sweetly scented.
The time of their expansion coincides with the nocturnal flights of moths. The Owlet Moth is attracted to the Nottingham Catchfly by the starry whiteness of the petals,the abundant honey at their base, and the sweet smell. Male and female moths alike visit the flowers for food, their long probosces enabling them to reach the honey. Their visits probably result in some transference of pollen from one flower to another, but the process is haphazard as they are inconstant visitors.
There comes a time, however, when the relation between the female moth and the flower is much more intimate. Her under surface is covered with pollen from younger flowers that she has visited for honey. She now rests upon an older flower, where the stamens are withered, for a considerable period. She pierces the ovary wall with her ovipositor and lays her eggs within the ovary. The under surface of her body rubs against the stigma of the flower in such a way that much of the pollen she carries is left upon it. Her life-work done, she flies away, and shortly afterwards she dies.
The pollen deposited by the moth upon the stigma of the flower brings about the fertilisation of the ovules in the ovary, and they become seeds.
The eggs of the Owlet Moth hatch out and the larvx feed upon some of the seeds that they find in their temporary home.
The instinct of the mother moth has led her to provide for her offspring by putting her eggs in the most favourable situation possible. All unwittingly, she has, at the same time, benefited both the plant and her offspring by bringing about, and so ensuring the formation of seeds.
In August the fruit opens and the seeds are scattered. A little before this the larvx have bored their way out of the fruit and have spun down to the ground, where they change into the chrysalis stage just below the level of the soil.and chrysalids pass through a long rest period, for it is not until the following year that the seeds germinate and produce new plants, and the chrysalis coat bursts and the Owlet Moth emerges. In June the new Nottingham Catchfly is in flower and the new Owlet Moth is poising on its scented petals.
The stamen in which the pollen is produced is athat has become, in the course of evolution, especially modified for this particular work of pollen bearing. • The pollen develops in tubular cavities that run the length of the very obvious head of the stamen. This head is called the anther, and is generally supported on a 1 slender stalk, which is the filament. Between the lobes of the anther is the connective. In it is a vascular bundle which carries water and food to the developing pollen.
Pollen consists of hundreds of minute grains, each one of which is an independent cell. The cell-wall is two-layered.
The outer coat is thick, but in it there are numerous thin places. The inner wall is extremely thin.
The nucleus of the pollen grain divides, and two of the nuclei resulting from the divisions are the male gametes, destined to reach an ovule in the ovary. .
The ovary containing the ovules is the basal part of a carpel, another specially modified leaf. The upper parts of this leaf are designed to bring about the safe conduct of Male gametes from the pollen grain, to the female gamete that rests passively in each ovule. To this end a stigma and a style surmount the ovary. The stigma is projected upwards so that it may be in the best possible position for receiving pollen. The style is then necessary to act as a means of communica tion between stigma and ovary.
The stigma is usually somewhat sticky, or hairy, or uneven in surface. The ad- vantage of this is that, once pollen grains are carried to it by wind or insects, they are held in position. The outer coat of most pollen grains is somewhat sculptured, so that it, too, helps in setting up a frictional contact .
On the stigma the pollen grains absorb , moisture to such an extent that the in- ternal pressure is more than the outer wall can support, and it bursts at one of the thin places. But this is not all. As a result of the pressure the thin inner wall pushes through the gap as a short tube.
The whole proceeding is analogous to the collapse of a weak place in the outer tyre of bicycle or car. In this case, too, the thinner inner tube bulges through the gap.
The pollen grains now lie on the stigma, each with a little tubular projection, but the gametes they produce are still far from the waiting egg. To bring them into contact the short tube that emerges from each pollen grain, grows through the tissue that separates the stigma from the ovary.
The pollen tube, or pollen thread, secretes enzymes at its tip. These break down the cellulose walls of the tissue of the style and aid the pollen tube in actually absorbing the contents of the cells. Thus the pollen thread feeds as it bores onwards, and it is the food thus obtained that gives it the energy it needs for growth. As the pollen tube grows on, two male gametes are seen near its tip. Eventually the style is riddled by elongating threads from various pollen grains. They have made their way from the stigma at the apex to the ovary at the base.
In the coat of each ovule there is a minute gap called the micropyle (Greek micro, little – py/e, a gateway). The pollen threads are chemically attracted to these holes, and the tip of a pollen thread enters each micropyle. It thus carries the two male gametes right into the ovule. An opening at the tip of the tube now allows the gametes to escape and one of them fuses with the egg-cell of the ovule.
It sometimes happens in fruits, in a pea pod for instance, that one ovule escapes fertilisation. It has possibly grown a little during the development of the fruit, but it has not become a. It is small and shrunken-looking, and if it were put into the ground no new plant could grow from it.
The leaf, then, that bears the ovules has, speaking generally, three parts : the stigma, the style, and the ovary. Such a leaf is called a carpel. In the flower the carpels always occupy the central position, for they grow on the extreme apex of the flower stalk, which is, in this upper part, called the receptacle.
In some carpels the style is so short as to be almost nonexistent. This is the case in the Buttercup. The stigma here is somewhat hooked, and passes, almost immediately, into an expanded ovary, containing only one ovule.
There are many carpels clustered together at the apex of the receptacle of the Buttercup, and, all together, they are known as the pistil, or gyncecium . The latter name indicates the special function of this part of the flower, for it is derived from the – Greek gyne, a female – oikos, a house, and signifies the female part of the community.
This complete separation of the carpels of the pistil one from another occurs in the Buttercup Family (Ranunculacew), and in the Rose Family (Rosacew).
It is, however, much more usual to find the carpels united. In the Notting- ham Catchfly, as in thegenerally, the pistil is made up of three carpels. This is evident because there are three distinct stigmas and styles, but the basal parts of the carpels are most intimately united .
In the Canterbury Bell the styles also are joined, and only the three stigmas project freely .
In the Primrose there are five carpels, and the junction of these is complete, resulting in one round, knob-like stigma, one slender style, and an ovary which is one hollow chamber .
Thus there is every gradation from a pistil made up of free carpels to one that is made up of carpels so completely joined that it is difficult to tell their number by superficial examination.
Because of these variations the mor- phology of the pistil pKesents some little difficulty at first. It is, however; easy to remember that when the sepals, petals, and stamens have all been removed, that part of the flower which remains is the pistil, or gyncecium – this is made up of leaves called carpels – these carpels may be free from one another or may be joined in varying degree.
The floral leaves that form the whorl outside the pistil are the stamens. Together they constitute the andrcecium (Greek aner, andros, a male – oikos, a house).
Like the carpels, the stamens may be free from one another, standing up independently in the flower. This is much the commonest arrangement.
When they are joined, the union must be either between the filaments or the anthers.
In theand its relations (Leguminosw) there are ten stamens. These are united by their filaments in such a way that they form a sheath enclosing the ovary. In some members of the family all the stamens are joined in this way. In others one remains free .
In the Compositw, the Dandelion Family, the five stamens of each flower are united by their anthers, a condition which is known as syngenesious .
It is obvious that the stamens of the andrcecium and the carpels of the gyncecium are the essential leaves of a flower.
Upon them the persistence of the species depends.
They are the definite organs of sexual reproduction, and other parts that may be present are purely accessory.
The flowers of all grasses and of many trees are made up of these two parts only. For this reason they are inconspicuous, and it is a surprise to many people to learn that such plants bear flowers.
Often they are so simple that a flower consists of stamens only, or of carpels only. The male flower of the Birch is made up of two stamens, which fork and look like four. The female flower is merely one carpel. Because of their extreme simplicity they are massed together in long inflorescences, called catkins. As the male catkin sways in the wind its very light pollen is wafted to the female catkins, which are on the same tree. The two long stigmas of each carpel catch some of the pollen. To ensure pollination, not only is a vast amount of pollen produced, but the flowers are mature while the foliage leaves are still too small to hinder the free passage of pollen from the male to the female catkins.
Similar conditions obtain in the Oak. In the Hazel, Elm, and Poplar the leaf-buds do not burst until pollination is com- pleted . A further variation in the Poplar is, that the male catkins occur on one tree and the female on another.
In contradistinction to wind-pollinated flowers, those that are insect-pollinated have all, or some, of the following characteristics : they contain honey – they are brightly coloured – they are sweetly scented. That is, they are just as definitely conspicuous as wind-pollinated flowers are inconspicuous.
It is the two whorls surrounding the andrcecium and gyncecium that co-operate to give the insect-pollinated flower its most marked characteristics. These two sets of leaves form the perianth.
In most of the insect-pollinated flowers of plants known as Monocotyledons there are three very similar perianth leaves in each whorl. These alternate, so that the outer three leaves fill up the gaps which occur between cotyledons.
The long trumpet, or corona, of thegrows from six perianth leaves that are joined . In the Tulip the perianth leaves are free and fall off separately when the flower is dying. In the the outer leaves perhaps curve outwards just a little more at the tip than do those of the inner whorl. In the there is a good deal of difference in the two whorls – not only are they differently marked, but the three inner perianth leaves are shorter and thicker than the outer ones, and they are grooved on their inner surfaces, the grooves being nectaries .
I. most Dicotyledons the two whorls of the perianth are very distinct. The outer whorl, or calyx, is generally green, and its sepals may form a cup.
The colour, the scent, and often the honey also, depend upon the petals that make up the corolla.
Between the petals, and so prevents insects from forcing an entrance at these areas of least resistance. Such an entry would be prejudicial to the flower, because the visiting insect would get the honey provided without conferring any benefit in return. When it enters according to the set design, pollination is effected.
The sepals, too, help to hold the rest of the flower together. When a Poppy bud opens the petals withdraw from the sepals, which are sloughed off and fall to the ground. This accounts for the fragility of Poppy flowers – their petals have no permanence.
In some cases, again, the sepals take on the work of petals and attract insects by their colour and their size. The purple leaves of the, the yellow leaves of the Kingcup, the many-coloured leaves of the , are all sepals . Petals are entirely absent in these three flowers.
The petals of the corolla do not only play a part in pollination because their colour, size, and smell attract insects.
Quite frequently their shape is such that they .1 • accommodate, in a marked manner, the body, or part of the body, of one particular insect, it may be to the complete exclusion of others.
Frequently, too, the petals are so arranged that an insect, entering the flower, can only reach the honey by following a course that inevitably brings a definite part of its body into contact with the anthers and stigmas.
In many flowers the particular design that is good for securing the best possible advantage from insect visits is also remarkably efficacious in protecting pollen. If pollen gets thoroughly damp it is useless. In one way or another the petals so protect it that it is not wetted even in the sharpest shower.
The Snapdragon owes its name to its tightly closed petals. The two lips of the corolla meet so firmly that a bumble-bee is the only insect able to force an entrance .
Honey is secreted by a little disk at the base of the ovary. As the bee, exerting strength, pushes well into the flower, its back rubs on the domed upper lip of the corolla.
Four stamens, two of which have long, and two short, filaments, are inserted on the corolla and follow the curve of the upper lip. Their anthers open in such a way as to let the pollen fall to the centre of the flower. Such anthers are introrse.
In this flower, as in so many others, the anthers open to let the pollen escape before the stigma of the flower is ready to receive it. That is, the flower is protandrous. In this very simple way self-pollination is prevented.
The bee visiting a Snapdragon with mature stamens, receives a charge of pollen on its back. It may visit one protandrous flower after another, thus increasing its pollen-load. When the stamens have shed their pollen the anthers wither and the slightly two-lobed stigma, which was at first very much in the background, now occupies their place. When the bee enters a flower at this stage, its back comes in contact with the stigma, and it leaves upon it some of the pollen it is bearing. In this way cross-pollination of the Snapdragon is brought about by bumblebees only.
The Figwort flower is protogynous, that is, the stigma in each flower is in a receptive state before the anthers are mature. It is pollinated by wasps, attracted in part by the raw-meat colour of the petals. The shape of the flower is such that it exactly accommodates this insect’s head and thorax. In this case it is the ventral part of the head and thorax of the wasp that gets coated with pollen.
Because the stamens and stigma rest on the lower lip of the corolla .
In the Columbine brightly coloured sepals alter- nate with very characteristic petals .
Each petal is funnel-shaped and drawn out into a long spur which secretes honey at its thickened distal end.
The length of the spur makes the honey inaccessible to any but long-tongued insects. In a newly opened flower the stamens give a firm resting place for an insect, whose ventral surface rubs off much of the pollen. In an older flower, when the anthers are withered, a similar platform is provided by five free carpels, which stand firmly upright, so that when they are in contact with the insect, pollen is left upon the five stigmas.
Thethat figure so largely in the herbaceous perennial border of every garden owe their beauty to the colour and form of the sepals. The two posterior petals are most un-petal-like in form. They are projected downwards as two long spurs which are the nectaries. These fit into a pouch of the posterior sepal, where the honey collects .
Again the honey is so placed, not merely that it can only be reached by long-tongued insects, but that these, in their quest, must arch their bodies over the mass of stamens that in a young flower block the passage. In this way pollen collects upon the insect’s ventral surface. In an older flower the stamens are shrivelled and out of the way, but the passage is now blocked by the three free carpels that the stamens surrounded. Their stigmas come in contact with that part of the insect’s body that is carrying pollen.
Thus pollination in the Columbine and Larkspur is brought about successfully, because the whole design of the flower makes it impossible for an insect to reach the honey without, at the same time, acting as an agent in the transference of pollen. In Monkshood a similar device obtains for securing cross-pollination. The blue perianth leaves of this flower are all sepals. The posterior sepal is in the form of a hood – this encloses two petals curiously modified to form nectaries .
The successive maturing of stamens and carpels, characteristic of Columbine and Larkspur, is a common device for ensuring cross-pollination. Generally speaking, seeds are better, that is, they produce better plants, if they have been formed by the inter- action of the pollen of one flower and the ovule of another.
Even if the two flowers have been growing on the same, as in Foxglove or , cross-pollination gives better results than self-pollination.
When the flowers grow on different plants, the result is better still.
Often, too, the strain is markedly improved when the crossed flowers have grown under different conditions, that is, separated in space. Darwin found that he got much better seeds as a result of his experiments when he cross-pollinated flowers growing on opposite banks of the Cam, than when the cross was made between flowers growing on the same bank.
It is not surprising, therefore, to find that protandry, as in Columbine and Delphinium, is not the sole device to secure cross-pollination.
In many flowers the stamens reach a lower level than the stigma. This is so in the pin-eyed flower of the Primrose, and is an arrangement that, at least, makes self-pollination difficult .
When the flowers are unisexual, as in Willow, Poplar, and Hazel, self-pollination is obviously impossible .
In the thrum-eyed flowers of the Primrose, not only are the anthers at a higher level than the stigma but, as in the pin- eyed flower, anthers and stigma reach maturity at the same time. Self-pollination would seem to be inevitable in the case of the thrum-eyed flower . As a matter of fact, it is only extremely rarely that it occurs. Time and again it has been demonstrated, in this particular Primrose, that if insect visits are prevented by shielding the flower in some way, it is most unusual for fertilisation to take place.
The explanation of this is found in the large size of the pollen grains of thrum-eyed flowers. They are too big to find anchorage on the stigma below them, for this is only adapted to receive the smaller pollen grains from the pin-eyed flowers.
As might be expected, then, the uplifted stigma of a pin-eyed flower is provided with hairs and irregularities that accommodate the large pollen grains of the thrum-eyed flowers.
Thus all is well arranged, and pollination takes place through the agency of long-tongued insects. The part of the proboscis that touches the anthers, and so collects pollen in one form of flower, is the part that touches the receptive stigma when a flower of the other form is visited .
The shielding of flowers just suggested is frequently resorted to in pollination experiments.
In the particular case of the Primrose the flowers can be shielded by putting a bell-jar over a whole plant because all its flowers are of one kind. There is no mixing of pin-eyed and thrum-eyed flowers.
In other cases the shielding is effected by tying up one individual flower of a plant in very fine muslin. Tulips are good to choose for preliminary experiments because their flowers are so large.
The six stamens of a flower must be cut off with care as soon as the flower pushes up from the bulb. A piece of fine butter muslin is then tied lightly, but firmly, over the flower head. Self-pollination cannot take place because the stamens have been removed. Cross-pollination cannot take place because the entrance of insects is effectually barred.
The result is that no seeds are formed and the ovary is of small size as compared with other free Tulips that surround it. This is a simple way of demonstrating the need for pollination as a preliminary to seed production.
It may be argued that theoff of the stamens has given the plant a shock which would in any case prevent seed formation. It is well, therefore, to remove the stamens of some unshielded flowers. These are accessible to insect visitors and are found to produce seeds normally.
In some flowers self-pollination is prevented by the very curious condition of self-sterility. The pollen grains of a flower simply do not germinate on the stigma of the flower that produced them. This is true of the Bitter Buttercup.
In a few cases there is actual antipathy between the pollen and stigma of the same flower. The flowers of someare actually killed when the pollen from their own stamens is put upon their stigmas.
At the other extreme there are flowers in which the whole aim is to secure self-pollination.
The petals in the flower of the Garden Pea are so firmly interlocked that no insect forces an entrance. Self-pollination must take place in this r pods normally contain the full complement of case, io peas.
In Orchids the pollen occurs in club-shaped masses. In the Bee Orchis these masses fall forwards on to the stigma of the flower when it is in any way disturbed. In this case an insect may bring about self-pollination by alighting on the flower and causing a slight tremor.
In addition to the sweet-smelling flowers of the Violet, the plant bears others which appear later. They are green and inconspicuous and never open. They have the appearance of buds and must be looked for quite low down, underneath the foliage leaves. In these flowers, which are called cleistogamous (Greek kleistos, closed – gamos, married), not only does self-pollination take place, but the seeds which they produce are invariably of good quality.
Here, then, are the two extremes. On the one hand, impossibility of self-pollination. On the other, impossibility of cross-pollination. Between the two are many cases of flowers that have, as it were, two strings to their bow. If cross-pollination does not take place, then they resort to self-pollination.
If the Scarlet Pimpernel has not been cross-pollinated the flower closes up and remains closed for two or three days. When it re-opens pollination has taken place.
If in spite of colour, scent, and honey the Wallflower has not been visited by insects, the four long stamens grow a little and bend, so that their anthers arch over the stigmas. The pollen then falls on to the stigmas and the ovules are fertilised.
In the Dandelion, Daisy,, and members of the Compositx generally, pollination never fails, and probably this accounts for the great success of the Family. It includes more than io per cent. Of the of the world and is represented in all parts of the globe.
In some members of this Family the flat inflorescence consists of strap-shaped flowers only, as in the Dandelion.
In others there are only tubular flowers, as in „Common Groundsel. In others ligulate, or strap- shaped, flowers are on the outside of the inflorescence and the tubular flowers are centrally placed, forming a compact disk.
In the tubular, or disk, flowers of the Giant Sunflower the course of pollination may be readily traced .
On examination, young unopened flowers are probably seen in the centre of the Sunflower disk. Surrounding these are slightly older ones in which the anthers are seen above the petals. In still older ones the hairy stigma has pushed through the joined anthers, raising their pollen above the general level. In flowers still nearer the circumference the stigmatic arms have parted, so that the actual stigmatic surfaces face upwards .
Now pollination may take place. Any insect, creeping over the flower head, may carry the pollen that is held on the erect stigmas to those that are outstretched in the older flowers. A bee, or butterfly flying from quite another Sunflower, may perform the same office.
There seems to be boundless opportunity for cross-pollination. In some cases, however, it is missed. Then the outstretched stigmatic arms curl towards the style and continue curling until their sticky stigmatic surfaces are in actual contact with the hairs of their under surfaces . The result of the contact is the transference of many of the pollen grains from the hairy under surface to the upper stigmatic surface. Thus self-pollination takes place and fruits are then formed from the flowers even when insect visits have failed.
The flowers of all members of the Composite are so extremely small that they would stand but little chance of cross-pollination if they were not massed together in a dense cluster, or inflorescence.
Aggregations of flowers to form inflorescences are extremely common. It is only in comparatively few cases that one solitary flower grows on the flowering axis. This is, however, the arrangement in the, and the Anemone. In such cases the inflorescence is said to be solitary, and, as the flower is at the end of the axis, it is also called terminal .
A solitary inflorescence that is, at the same time, axillary is found in the Scarlet Pimpernel . Here each flower stalk, bearing one flower only, grows in the axil of a leaf. The leaf then receives the special name of bract. Any leaf in the axil of which a flower grows is a bract.
Little leaves are frequently borne on the stalks of individual flowers. These are bracteoles, or prop hylls. Two small prophylls occur on either side of the flower stalk in the Violet . In the Daffodil two prophylls have united to form the green, protecting wrapping of the flower bud – when the flower has opened, this remains on the stalk as a brown and shrivelled structure. A protective involucre of three leaves surrounds the flower of the Anemone and Love-in- a-Mist .
Specialised bracts are found in many flowers whose fruit is a nut.
The Hazel nut is enclosed in a lobed cupu/e, or husk, which is formed from the persistent bracts of the female flower .
The cup of the acorn is made up of numerous leaves which have become woody after fertilisation has taken place . The leafy nature of the cup is plainly seen in the fruits of the Turkey Oak.
Similarly the rough husk of Beech nuts and the very prickly cupule of the Spanish Chestnut are formed from modified leaves .
In vegetative branches a distinction has already been made k between monopodial and sympodial growth. A similar difference is met with in the growth of the flowering axis.
In the Foxglove the main flowering axis goes on growing almost indefinitely at the tip, new flowers being produced laterally. Counting the fruits already formed on the lower part of the axis, and the flowers, and the buds, it is not unusual to reach as high a number as 350, and this with the possibility of still more buds developing round the growing point.
Plainly this is monopodial growth.
In Stitchwort and Gypsophyllum the first-formed, that is the oldest, flower is produced at the tip of the flowering axis, and therefore effectually puts an end to any further growth in length. Lateral shoots grow to a higher level, but here again the growth is strictly limited, or definite, because each ends in a flower. The same thing occurs in the lateral branches to which they give rise.
Here, then, the growth is sympodial.
An inflorescence that exhibits such sympodial growth is cymose.
A racemose inflorescence is one in which monopodial growth obtains.
Differences in cymose inflorescences depend upon the position of the lateral branches. (r) It may be, as in the Stitchwort, that the branches are in pairs, each flower arising in the axil of a bract on either side of the terminal flower . These secondary branches also bear paired branches – so do the tertiary branches in their turn – and so on. Sometimes the branches grow equally. More corn- monly one member a of pair grows less strongly, or is even completely suppressed. For this reason diagrammatic exactitude is seldom attained.
Such a cymose inflorescence is a dichasium. (2) Another possibility is that the branches may come off singly, one being developed to the right, another to the left. This produces a zigzagging effect. The, Buttercup, and Forget-me-not have this type of monochasial cyme, which is called scorpioid .
The first basal part of the inflorescence axis and the branches become equally thickened. The result is that the zigzagging is not always obvious, for the axes of different orders (r , 2, 3, 4, 5) tend to straighten and look like one. (3) In some Monocotyledons the branches of the cymose inflorescence all arise on the same side. This form, which is known as helicoid, occurs in Rushes and the.
An inflorescence that is racemose may have a long or a short main axis. (1) The raceme itself often has a very long axis. The lateral branches are flowers whose flower stalks, or pedicels, are nearly equal in length. Each flower de, velops in the axil of a bract, as in the Foxglove, Antirrhinum, and Toadflax . (2) In a corymb the flowers grow upon the long axis at some distance one from another, just as they do in the raceme. But as the pedicels of the lower flowers are long, all the flowers reach the same level round the growing point . Candytuft is a typical corymb. Many other members of Cruciferx are corymbs in the upper part of the inflorescence, but bractless racemes in the lower, because the pedicels do not elongate. (3) The spike is a much less common r ac emo se inflorescence. The long axis in this case bears sessile flowers, that is, flowers which have no pedicels but grow directly on the peduncle, or main. It is found in Plantains . (4) In the catkin the raceme is often pendulous and the flowers are always unisexual . (5) The spadix is still less common. It differs from the spike in that its axis is thick and fleshy and the whole is surrounded by one very large bract or spathe. This is the inflorescence of the Arum Lily and of our Wild Arum – the Cuckoo Pint or Lords and Ladies .
Pollination in the Arum is most interesting. The very simple female flowers are at the bottom of the spadix. A short distance above these are the male flowers. Above these, again, are sterile flowers with downwardly directed, hair-like prolongations. These hairs occur just where there is the constriction in the spathe, so that the whole design is much like that tof a lobster pot.
Little insects, such as small flies and midges, probably covered with pollen from another Arum, find an easy entrance into the spathe, down and between the sloping hairs. They are fussy because they find no food – their escape is prevented by the sterile flowers. In their purposeless wanderings they leave much of the pollen they were carrying upon the receptive stigmas of the basal female flowers. After this the stigmas wither and exude drops of honey by way of compensating the insects for their imprisonment. The sterile flowers wither also, and a way of escape is clear, but it must be taken over the staminate flowers, which have now shed their pollen. Thus the insects become loaded with pollen once again, and not profiting by experience, they enter another spathe andanother set of female flowers.
The fruits are the bright red berries that are found in the woods and under the hedges in early autumn. They look abruptly placed on the thick stem because all the upper part of the spadix has died away. 6. An umbel has a main axis from which spring a number of pedicels of equal length, so that the whole inflorescence looks a little like an umbrella blown inside out. A simple umbel is less common than a compound one . In the latter the branches that arise on the main axis give rise to other branches instead of bearing flowers directly.
Umbels, both simple and compound, are the characteristic inflorescence of Umbelliferw, the family which includes the Carrot, Parsnip, Parsley, Hog-weed, and Hemlock.
The inflorescence of the Primrose is a simple umbel. In this case the short main axis is a flat rhizome. A similar rhizome, seen above the surface of the soil in the flower-pot, occurs in, which also belongs to the Primulacem.
In Cowslip and Oxlip it is obvious that the inflorescence is an umbel . 7. In the capitu/um a flattened axis expands to form a conical or disk-shaped receptacle. On this, numerous sessile flowers grow. The whole head is surrounded by a number of bracts which form an involucre .
Every member of the Compositx has this particular type of inflorescence.
In many plants the inflorescence is neither definitely one thing nor the other, being a mixture of cyme and raceme.
In the Deadnettle the growth of the main axis is indefinite. The growing point keeps its freedom and produces new lateral growths for some time, while the axis is elongating.
The white two-lipped flowers that grow in the axils of the paired bracts are, however, disposed in paired cymes . The oldest flower of each half of the square stem has a younger one on each side of it and each of these is flanked by a small scorpioid cyme.
The meeting of the paired cymes makes an apparent whorl of flowers, to which the name of verticillaster is given.
Just as there is sleep movement in foliage leaves, so is there the corre- sponding tropism in flowers. When Dande- lions are picked they hide their beauty by closing up. They perform a sleep movement at the wrong time of day.
In the Dandelion, as in all Composites, the sleep movement is not the response of one single flower, but of a whole inflorescence. The mechanism of the closing up of the inflorescence at night differs in different plants. Pernel is seen in many flowers of a similar open form. Poppies,
In all the Composites the pollen is pushed up by the growth of the style and therefore it is in a very exposed position. The sleep movement is designed to keep the pollen dry when rain or dew is falling., , Rock Rose, and Herb Robert,
In the Dandelion and Goatsbeard and all the plants with 1 igulate flowers, each corolla strap bends inwards, so as to protect either its own pollen or that of its neighbours. All bend downwards before rain comes on or before dew falls.
When tubular flowers occupy the centre of the inflorescence they are protected from the wet by the ligulate flowers and bracts, which change from a horizontal to a vertical position, then arch over towards the centre and make an excellent tent-like shield.
The devices adopted by flowers for keeping pollen dry are very varied. In addition to barring the entrance to all insects but the bee, the firmly closed corolla of the Snapdragon also keeps out rain . In this flower there is no possibility of the pollen getting wet. It is obvious that this could not be a universal arrangement. The very perfection of pollen protection would make pollen transference impossible. The principle of the tightly closed flower is not economically sound.
The flat, open arrangement of petals in the Scarlet Pimpernel is just at the other extreme. Five stamens stand rigidly erect at right angles to five horizontally placed petals. And yet so perfect is the protection here that the plant has gained the popular name of Poor Man’s Weatherglass. Normal.
When the atmosphere is becoming charged with moisture an inequality of basal growth, on the outer surface of the petals, produces extra cells at this point. By their intrusion the individual petal is pushed upwards and forwards. By the time the five petals have met and closed in the central part of the flower, the flower stalk itself has bent and brought the whole flower into an inverted position.
The subsequent opening of the flower in a dry atmosphere depends upon additional basal growth on the inner surface of each petal. When the flowers are fully expanded the short pedicels have turned through a considerable angle and face downwards. Rain, falling on the perianth, does not reach the stamens. If, when the flowering time was over, the Hyacinth fruits were in this position, the seeds would all fall down together in one spot. To ensure better distribution the pedicels change position once more, so that the fruits become erect – when they open and the wind blows, the seeds are sufficiently widely scattered .
The infinite variety that occurs in flower structure is further exemplified in the relative positions that the floral leaves occupy on the thalamus, or receptacle.
In the Buttercup the numerous free carpels of the pistil occupy the apex of the receptacle.
The numerous free stamens are at a slightly lower level. The five free petals are inserted below the s. stamens. The five free alternating sepals are below these again . The floral axis is ex- tremely short, and the nodes are very close together. The flower is, in fact, a dwarf shoot bearing variously modified leaves.
A dissection of a Whiteis a beautiful demonstra- tion of floral leaf-modification. When sepals, petals, and stamens are arranged in a long line the transition from one form to another is most striking . The firm greenr sepals, apart from size, bear some resemblance to the foliage leaves. They are followed by somewhat less sturdyi leaves which are greenish-white. Succeeding these are leaves which are whitish-green. Then come large, spreading, white petals. These are followed by narrower ones, on some of which there is a lateral pollen-sac near the tip. Others, still narrower, may bear two such sacs. There is still further narrowing of the leaf, and gradually the petals pass into the numerous stamens with slender filaments and elongated anthers.
The position of the floral leaves is always referred to that of the pistil. Thus in the Buttercup the pistil is said to be superior, because it occupies the apex of the receptacle. The stamens, petals, and sepals, being at a lower level than the pistil, or gyncecium, are inferior or hypogynous (Greek hypo, below) .
In some flowers the actual apex of the re- ceptacle stops growing, while growth continues at the regio surrounding it in such a way that the whole receptacle becomes a disk. The pistil occupies the centre of the disk and the sepals are on its outer edge. All the floral leaves are growing at the same level. No one part is superior or inferior to another.
Again taking the pistil as the point of comparison, the stamens, petals, and sepals are said to be perigynous (Greek pen, around).
Such an arrangement obtains in theand other members of the Leguminosae .
In the Rose Family the perigynous arrangement in many of the flowers is extreme.
Spirwa affords the simplest example .
Here the circumference has continued its growth in such a way that the actual apex of the axis lies in a depression and the receptacle is in the form of a rather deep saucer.
In(Avens) localised growth at the apex of the receptacle produces a tapering projection on which the numerous free carpels grow . The rest of the perigynous floral leaves are actually at a lower level, because they are inserted on the very slightly concave, disk-like expansion.
This arrangement is also found in the, but the projection in this case is rounded, not pointed . After fertilisation the projection grows greatly. It becomes red and succulent and sweet and is much more obvious than the fruits . Thus in the Strawberry the part that is so delicious to eat is the receptacle – the actual floral axis. The fruits, formed from the numerous free carpels, are the small hard bodies, dotted all over its surface.
In the Blackberry, Raspberry, and Loganberry the same form of receptacle occurs, but, after fertilisation, it is the individual fruits that become juicy and sweet and increase in size . The receptacle, with the calyx still persisting, is the part pulled out when the fruits are being prepared for eating.
The receptacle in the Plum, Cherry, and Almond resembles that of Spirxa . The margin, however, grows still more, so that the actual apex lies in a much deeper depression.
Unlike most members of the Rosacem the pistil is made up of one carpel only.
After fertilisation the urn-shaped part of the receptacle drops off and the fruit is formed entirely from the single ovary. The stimulus of fertilisation induces marked changes in its wall, so that the fruit coat or pericarp develops excessively and is differentiated into three layers .
The outermost of these is thin, but resistant. Wax deposited in the cutin of its epidermal cells produces the beautiful bloom of the Plum. This layer of the fruit coat is called the epicarp. The next layer is sweet and juicy and is the mesocarp. The innermost layer is hard and shell-like and is called the endocarp. Within the endocarp lies the kernel or seed.
Such a fruit as this, with a three-layered pericarp, is called a drupe.
In the Rose the receptacle is similar to that of the Plum, but as it narrows at the top it is still more urn-shaped . Furthermore it is persistent. After fertilisation it increases in size, turns red, and is the familiar hip of the hedgerows.
When the stamens and petals have fallen off, the numerous stigmas are still seen projecting through the narrow opening of the urn. When a vertical section is cut, the styles, too, are plainly seen, passing upwards from the enclosed fruits.
In the Apple the condition found in the Rose goes one degree further.
The narrow opening of the urn is obliterated, for the receptacle completely encloses the ovaries of the carpels, which are five in number and slightly joined. The five styles and stigmas project upwards in the centre of the flower, surrounded by stamens, petals, and sepals .
It would now be absurd to speak of perigyny, for the ovary is very definitely below all the rest of the flower. The extreme development of the receptacle has, in fact, carried up the three outer whorls of the flower, so that the ovary now occupies an inferior position, while all the rest of the flower is superior, or eptgynous (Greek epi, upon).
It is clear that the part we eat in the Apple is the swollen receptacle. The actual fruit is the five-chambered core, formed from the basal parts. Of the five carpels .
Further fundamental differences in floral structure depend upon the arrangement of the ovules within the ovary. This arrangement is known as their placentation . To understand this it must be remembered that a carpel is a modified leaf and that it bears marginal ovules, more particularly on the broad basal part of the leaf.
When the leaf folds inwards so that the margins meet, the ovules are in a row on the line of junction. This arrangement is shown excellently in the free carpels of Columbine and Delphinium. In this case the placentation is marginal .
When two or more carpels similar to this join, so that all the margins meet, forming a syncarpous ovary, the ovules lie very near the centre, in the angle or axil formed by the infolding of each leaf. In this case the placentation is axile and is well seen in the large ovary of the Tulip.
It may happen, however, that the margins of the leaves are only very slightly infolded. Then, when two or more carpels join, the fused margins form ridges on which the ovules grow. This is the parietal placentation of the Violet. In the Poppy the margins project towards the centre, as may be seen in a transverse section of its ovary.
Lastly, the ovules may grow on a short column of tissue in the centre of the ovary, absolutely free from its walls. This is the free-central placentation of the Pinks, the Primrose, and the Scarlet Pimpernel.