An Introduction to Nature-study

15. A TYPICAL FLOWER.

I. The wallflower.—After noticing the general habit of growth of a wallflower plant (Fig. 57), and especially the shape and venation of the leaves, make out the following parts in one of its flowers. On the top of the flower-stalk (called the receptacle) are:

(b) Four showy leaves arranged in the form of a Maltese cross, called petals. They are yellow, or red, or purplish in colour, are delicately scented, and have beautiful velvety surfaces. The four petals together constitute the corolla. Take off the petals one by one.

(c) Six stamens, each consisting of a greenish stalk or filament surmounted by a yellow, boat-shaped body, called the anther. The anther is a four-chambered box containing an enormous number of tiny yellow grains called pollen grains. Two of the stamens are shorter, and are fixed at a lower level on the receptacle than the remaining four. Take off the stamens one by one.

(d) A central pistil, shaped somewhat like a slender bottle. At the top, where the cork would come in a real bottle, is the notched stigma, slightly sticky. The neck is called the style, and the part corresponding to the body of the bottle is the ovary. Tear open the ovary with a needle to see the ovules, which in an undisturbed flower would have become seeds.

Watch bees visiting flowers. Does each bee confine itself to one kind (species) of flower at each journey, or does it visit several kinds indiscriminately? Try to discover what the bees are doing. Avoid alarming them.

The work of flowers.—The roots, stem, and leaves of a plant do a great deal of work, but, as it is performed for the benefit of the plant itself, it is all, in a sense, selfish work. Plants, however, like animals, grow old in time, and at last die. If they are not to become extinct it is evident that they must devote part of their energies to producing new individuals, and to sending these forth into the world as well equipped as possible for the battle of life. This unselfish and self-sacrificing part of a plant’s life-work is called reproduction; in the higher plants it is carried out by flowers.

The structure of a wallflower blossom.—The flowers of different groups of plants vary greatly in structure, but a good general idea of the arrangement of the parts of a flower can be obtained by examining the blossoms of a wallflower plant (Fig. 58). Other flowers may afterwards be compared and contrasted.

There are evidently at least eight leaves in the flower, but, unlike the green foliage leaves, these are not arranged spirally, but stand at nearly the same level on the end—called the receptacle—of the flower-stalk. The most external leaves are four in number, small, narrow, and purplish in colour. Each of these leaves is called a sepal, and the four sepals together constitute the calyx of the flower. Two opposite sepals are pouched at the base, forming pockets, in which a sugary fluid, called nectar, collects. Before the bud opens, the calyx is the only part of the flower which is visible. It is probably developed in the wallflower solely for the protection of the more delicate structures within. Next, inside the sepals, placed alternately with them, and standing a little higher on the receptacle, are four showy leaves arranged in the form of a Maltese cross. These leaves are called petals, and the four petals together form the corolla. The petals are delicately scented, and their surfaces have a beautiful velvety sheen.

When the sepals and petals are removed, there remain standing on the receptacle six stamens surrounding a centrally-placed pistil (Fig. 58, D). The stamens are the male part, and the pistil is the female part, of the flower. Each stamen consists of a greenish stalk or filament, surmounted by a yellow boat-shaped body called the anther. The anther is a box with four compartments (Fig. 59). When it is ripe, each compartment contains an enormous number of tiny, yellow grains called pollen grains; and when the anther bursts (as it does as soon as the flower opens) its inner face is covered by the yellow dust of the pollen.

The pistil bears a rough resemblance to a slender bottle, and consists of three distinct parts. The neck of the “bottle,” called the style, is short in the wallflower, and differs from an ordinary bottle-neck in being solid instead of tubular. At the top of the neck, where the cork would come in a real bottle, is a body called the stigma. The stigma of the wallflower pistil is hairy, notched, and slightly sticky, from the presence of a sugary solution which forms upon it. The part of the pistil, corresponding to the body of the bottle, is the ovary. It contains four rows of little white ovules, which are destined to become seeds capable of growing up and forming new wallflower plants.

The relations of the parts of the flower are well seen in Fig. 59.

Fertilisation.—In order that an ovule may become a seed, its contents must mix with the contents of a pollen grain. The fusion of the two constitutes fertilisation. For fertilisation to take place, the pollen grain must first of all gain access to the stigma of the pistil. If this be prevented the flowers will wither without forming ripe seeds. (This may be proved easily by Expts. 18, 3 and 21, 2.) The sugary solution at the top of the stigma stimulates the pollen grains to growth, and each puts out a long tube which grows down the style. The living matter of the grains keeps near the tips of the tubes as these continue their journey down the style. At length the tubes enter the ovary and find the ovules. Each ovule has at one end a minute pore (the micropyle—p. 6), and a pollen tube finds this and enters it. The living matter of the pollen tube fuses with that of the ovule in the neighbourhood of the pore, and fertilisation is effected. It is now easy to understand that the comparatively insignificant stamens and pistil are the all-important parts of a flower.

How the wallflower advertises.—Botanists have proved that a flower produces more, and also better, seeds when it is fertilised by pollen from another flower of the same species. This is called cross fertilisation. The wallflower relies upon bees for the transference of the pollen from one flower to another; and it is solely to attract them that the petals are so delicately scented and brilliantly coloured, and that sweet nectar collects in the sepal-pouches. The gaily coloured petals are therefore advertisement placards which are hung out to attract the attention of bees. A bee comes to a wallflower for the sake of both nectar and pollen—the “bee-bread.” As the bee thrusts its proboscis down between stamens and pistil in search of the sweet liquid in the pouches, its head is pretty certain to come in contact with, and to brush off, some of the pollen dust hanging loose on the inner faces of the anthers. When the bee flies off to another wallflower and continues its search for nectar, it almost invariably leaves some of the pollen, from the first flower, on the hairy and sticky stigma of the second.

In almost all cases when a flower is brightly coloured it depends upon the help of insects for cross fertilisation.

16. THE WALLFLOWER FAMILY.

1. Shepherd’s purse.—Compare the shepherd’s purse (Fig. 60) with the wallflower. The flower is very much smaller, and white, but the parts have the same arrangement as in the wallflower, viz., four sepals, four petals arranged in the form of a cross, six stamens (two short and four long), and a central pistil, all arranged separately on the receptacle. Look down the plant, and notice that in the oldest (lowest) flowers, everything but the pistil has dropped off, and that this has become greatly enlarged to form a fruit. Cut some fruits open, both lengthwise and crosswise, and observe that each consists of two pocket-like chambers, separated by a thin partition on which the seeds are borne. Notice the manner in which the oldest fruits have opened naturally.

2. Other relatives of the wallflower.—Compare also the flowers and fruits of the stock and candytuft, and also cress, mustard, radish (Fig. 61), cabbage, and turnip, which have been allowed to “run to seed.” Examine the roots of the turnip and radish.

Make a note of the earliest dates on which you see the above plants in flower.

The wallflower family.—The plants of the family to which the wallflower belongs are of very great importance to mankind; for while not one of them is poisonous, many are extremely valuable as food-crops. They are all dicotyledons; that is, their seeds contain two cotyledons or “makeshift leaves,” as has already (Chapter I.) been seen in the case of the mustard. The net-like venation of the leaves of the full-grown plant also indicates this. There is a great family likeness between the flowers of this group, and they are easily recognised (Fig. 61 a, c) by the cross-shaped corolla and the six stamens (two short and four long). All the parts of the flower—sepals, petals, stamens, and pistil—are fixed separately on the top of the flower stalk or receptacle. The cross-arrangement of the petals has led to these plants being called Crucifers (cross-bearers). The shepherd’s purse (Fig. 60)—so-called from the shape of its fruit—is a common weed with small, white flowers. All stages of the flower may generally be found on the same plant. While at the top the buds may still be unopened, the flowers below have been fertilised (in this case generally self-fertilised); the sepals, petals, and stamens—having fulfilled their duties—have fallen off; the ovules have become seeds, and the ovule-box or ovary has become a seed-box or fruit, consisting of two bags separated by a partition which bears two rows of seeds on each side (Fig. 62).

Useful crucifers.—The turnip and the radish are largely cultivated for their roots (Figs. 63 and 64), and are then taken out of the ground at the end of their first season. As these plants naturally flower in their second year of growth and then die, they are called biennials. The production of flowers and fruit is a great strain on a plant, and it is to prepare for the effort that the turnip and radish store so much food in their roots during the first year as to give them a globular and spindle shape respectively. A carrot is not a crucifer, but it also adopts this device.

The cabbage is grown for its leaves. Varieties of the cabbage are Brussels sprouts, broccoli, and cauliflower; it is the very small flower-buds of the last two which are eaten. Cress and white mustard are eaten in the seedling stage. The seeds of the black mustard are ground and eaten as a condiment.

17. THE BUTTERCUP FAMILY.

1. The buttercup.—Notice the habit of growth, characters of the leaves, etc. Is the buttercup a dicotyledon? Make this observation with all flowering plants. (See, however, Chap. VIII., p. 163.)

In the flowers of a buttercup (Fig. 65) make out:

Watch bees and other insects visiting buttercups and notice how they stand on the flower to obtain the nectar from the nectaries. Does a bee on leaving go to another buttercup, or does it change to another kind of flower?

In a flower from which the sepals, petals, and stamens have fallen, notice the compound fruit (Fig. 66), consisting of ripened carpels. Open a carpel with a needle and pick out the single seed.

2. Other plants of the buttercup family.—Notice that in the anemone (Fig. 67) and marsh marigold (Fig. 68), the sepals appear to be absent (the three leaves under the anemone flower are not parts of the flower; they are called bracts). The apparent petals are really the sepals; it is the corolla which is absent. Observe the large number of stamens, and notice that the pistil consists of several separate carpels.

In what kind of ground have you seen these plants growing wild?

The buttercup family.—The buttercup (Fig. 65) and its relatives resemble the crucifers (1) in being dicotyledons (as is indicated (p. 40) by the venation of the leaves), and (2) in the fact that, of the parts which compose the flower, each group is arranged separately on the receptacle. For example, the stamens are not connected with either the calyx, the corolla or the pistil. Having noted these points of resemblance, however, we are met by some important differences. In the buttercup there are usually five sepals and five petals; there may be twenty or more stamens; and the pistil is not a single structure, but consists of a number of separate parts, each of which is called a carpel, and contains a single ovule. The pistil of a crucifer consists of only two carpels; these are welded together into a single structure, and until the fruit ripens a slight notch in the stigma is the only external indication that the pistil is in two parts.

After fertilisation the ovules become seeds; the sepals, petals, and stamens drop off; and the carpels swell up, forming a dry compound fruit (Fig. 66), consisting of several nutlets.

Two other common plants of this family are the anemone (Fig. 67) and marsh marigold (Fig. 68). In neither of these cases has the flower any petals, but the calyx has taken on the appearance of a corolla. In the marsh marigold it is large and yellow; in the anemone it is white or purple. The three green leaves immediately beneath the flower of the anemone are called bracts. They should not be mistaken for sepals.

1. The garden pea.—Notice again the habit of the plant: its compound, net-veined leaves with large stipules, and its method of climbing by tendrils which are modified leaflets. Examine the flower and make out (a) the calyx of five united sepals; (b) the curiously shaped corolla. The large upper petal is called the standard, the two at the side are the wings, and the lowest (really two locked together) is the keel; (c) the ten stamens. One (opposite the standard) is separate; the remaining nine have the lower parts of their filaments united to form a tube. Slit open the filament-tube and remove the stamens, noticing how they are attached to the other parts of the flower; (d) the pistil. Slit open the ovary and see the ovules in it. Watch the various stages of the formation of the fruit (pod).

2. Other plants of the pea family.—Examine also bean, vetch (Fig. 69), meadow vetchling (Fig. 70), clover (Fig. 71), laburnum (Fig. 73), and broom. Compare the habits of growth of the plants, and notice that they all have the same peculiar shape of flower. Dissect a flower of each. Notice that in laburnum and broom the ten stamens are all united. In clover the flowers are in heads. Notice how the leaflets of the clover plant close at sunset.

3. Fertilisation.—Dig up several red clover plants in early summer and pot them. Cover about half the plants with gauze, so fixed on wire frames that insects cannot get inside, and then put all the plants together where they will get plenty of sun. Water them regularly, and notice which plants ripen seed. How do you account for the differences?

The pea family.—Plants of the pea family are found in all quarters of the earth. They are of very diverse size and habit of growth; the laburnum, for example, is a tree; the gorse is a bush; the broad bean has a strong, erect, herbaceous stem; the pea is a weak-stemmed climbing plant; the clovers are small herbs with flowers forming “heads.” Most of the members of the family agree in having a “butterfly-shaped” corolla (Figs. 72 and 73), which consists of three well-marked parts, viz., a large standard, a pair of wings, and two closely-connected petals which form a boat-shaped keel. There are ten stamens, and the filaments of nine of these usually cohere to form a tube surrounding the ovary. In the laburnum, gorse, and a few others, all the ten stamens are united. When a bee visits the flower, in search of nectar, it alights on the “wings” of the flower, and its weight depresses these and pulls down the keel. The anthers of the stamens are so placed with respect to the keel that this results in a mass of pollen being scraped off the anthers and forced out at the beak of the keel, or in the stamens being suddenly liberated and scattering pollen on the bee. The pollen sticks to the bee’s body, and some of it is almost certainly transferred to the stigma of the next flower visited. The quaint shape of the corolla is thus definitely adapted to the visits of insects; for the nectar is so placed that to obtain it the insect must carry off some of the pollen.

The calyx, corolla, and stamens are not obviously—as in the wallflower and buttercup—inserted separately on the receptacle, but seem to spring from a common base.

The fruit is a pod (Fig. 3), which opens when ripe along both margins and liberates the seeds. Laburnum seeds are poisonous, but the seeds of many other plants of the family (peas, beans, lentils, etc.) are valuable foods.

19. THE ROSE FAMILY.

1. The wild rose (Fig. 75).—With a sharp knife cut vertically through the middle of a wild rose. Notice that the receptacle forms a deep cup, and that the carpels of the pistil are enclosed in the cup. From the edge of the cup spring the five sepals, five petals, and numerous stamens. What is the great difference between a rose and a buttercup?

Trace the formation of the succulent fruit or hip from the receptacle cup of the flower. Cut through a rose hip, and observe the ripened carpels in the interior.

2. The blackberry.—Similarly examine a blackberry flower (Fig. 76). This is still more like a buttercup, but—as in the rose and the pea—the calyx, corolla, and stamens seem to spring from a common base, and not to be inserted separately on the receptacle.

Trace the formation of the compound fruit, and notice that each part is like a little plum or cherry.

3. The cherry.—Similarly examine cherry blossom (Fig. 77). The pistil consists of one carpel, and is fixed at the bottom of the receptacle-cup, while the calyx, corolla, and stamens are fixed on the margin of the cup. Trace the origin of each part of the fruit.

Compare the plum and apricot.

4. The apple and pear.—Cut vertically through an apple blossom (Fig. 74) or pear blossom (Fig. 78), and notice that the ovary is embedded in the receptacle, and that calyx, corolla, and stamens are fixed on the top of this. Cut across the ovary and see the five divisions (carpels).

The eatable part of the fruit is the swollen receptacle. Compare the hawthorn.

The wild rose.—The plants of the rose family are, most commonly, woody trees or shrubs. The leaves are provided with stipules in nearly all cases. The wild rose (Fig. 75) may be taken as a type of the group. It bears a superficial resemblance to a buttercup, but on dissection considerable difference in the arrangement of the parts is seen. In the rose, the receptacle is urn-shaped; from the margin of the urn spring the five sepals, five petals, and numerous stamens; while inside the urn the separate carpels of the pistil are inserted. In the blackberry (Fig. 76), raspberry, and strawberry the receptacle is knob-shaped, and the carpels are arranged on the outside of the knob, somewhat as in the buttercup. Here again, however, the calyx, corolla, and stamens differ from those of the buttercup in seeming to arise from a common base.

The differences between the rose, blackberry, raspberry, and strawberry are more marked when the pistil has become a fruit. The fleshy part of the rose hip is the urn-shaped receptacle which encloses the ripened carpels. In the case of the blackberry and raspberry the receptacle is dry, and is surrounded by the compound fruit (Fig. 76, 3) of the several bodies like little plums or cherries. The eatable part of the strawberry fruit (Fig. 144) is the swollen receptacle, on the outside of which are the little yellow nutlets derived from the carpels of the flower.

In the cherry (Fig. 77), plum, and apricot the pistil consists of only one carpel, which is enclosed in the urn-like receptacle. After fertilisation, the greater part of the wall of the ovary becomes fleshy, and one of the two ovules contained in it becomes a seed. The “stone” is formed from the innermost part of the ovary wall.

The apple (Fig. 74) and pear (Fig. 78) have their five carpels embedded in the receptacle, and the rest of the flower stands on this part. The eatable portion of the fruit is the swollen receptacle. The hawthorn has usually only two carpels, and in the fruit the part derived from the receptacle becomes hard and horny. In other respects it is very similar to the apple.

1. The poison hemlock.—Examine this plant (Fig. 79) very carefully, remembering that it is poisonous. Notice the general habit of growth; the characters of the sheathing, compound leaves; the hollow ribbed stem; and also the arrangement of the flowers, which is characteristic of the family. From the top of a main flower-stalk several smaller stalks come off together, like the ribs of an umbrella. From the top of each of these spring the stalklets which bear the small white flowers. Notice the bracts at the points of origin of the stalks. Examine the flowers, and watch insects visiting them. Which insects are most commonly found on the flowers?

Compare the cow parsnip, the water hemlock, carrot, parsley, parsnip, and celery, carefully noting the points of resemblance and difference.

The parsley family.—The plants of this family may be recognised easily by the arrangement of the flowers. Several stalks spring together from the top of the main flower stalk and each of these again gives rise at its tip to a number of smaller stalks, at the ends of which the small flowers are borne (Figs. 79 and 80). The flowers are fertilised by the aid of insects, and as the nectar is on the surface it is accessible to small insects such as flies, beetles, etc. The flowers are rendered more conspicuous by being placed close together. The stems are usually hollow, and the leaves are alternate, and generally compound, with sheathing bases. Many of the plants of this family are very poisonous, and such should be carefully distinguished and whenever possible exterminated.

The poison hemlock (Fig. 79) varies in height from two to seven feet. It has a hollow stem which is spotted with purple in the lower part, and when bruised the leaves give off a smell like that of mice. Cattle are often poisoned by eating the plant in hay, and children have been poisoned even by blowing whistles made from the stem. The water-hemlock is extremely poisonous. It grows along the sides of pools. The stem is hollow, and the leaflets of the compound leaves are finely toothed. The root is a cluster of fleshy swellings, and has unfortunately a rather pleasant taste. Other poisonous plants of the family are the water dropwort and the fool’s-parsley. Among the harmless and useful members of the group are celery (when cultivated), carrot, parsnip, and parsley.

21. THE PRIMROSE AND COWSLIP.

1. The primrose.—Examine the habit (Fig. 81) of the plant, its underground stem, its spoon-shaped leaves—arranged in a rosette—and the manner in which the flowers spring from the stem. In the flower make out (a) the calyx, 5-pointed and with united sepals; (b) the corolla, consisting of 5 petals united below into a tube. Tear down the corolla-tube to see (c) the 5 stamens inserted on the corolla-tube. In some (“thrum-eyed”) flowers the anthers are at the top of the tube; in others (“pin-eyed”), they are halfway down; (d) the pistil, consisting of stigma, style, and ovary. In thrum-eyed flowers the style is short and the stigma is halfway down the corolla-tube; while in pin-eyed flowers the style is long and the stigma is at the top of the tube. Do you find both pin-eyed and thrum-eyed flowers on the same plant, or does one plant bear only one kind?

2. Fertilisation.—Cover up a plant of each kind with gauze, to keep insects from the flowers, and notice whether the covered flowers ripen seeds like the others.

3. The cowslip.—Compare the cowslip (Fig. 83), and notice that the main stalk gives off from the same point several smaller stalks, each of which bears a flower. Observe that the cowslip also has both pin-eyed and thrum-eyed forms of flowers.

The primrose and cowslip.—In the primrose we have flowers of a type differing from all those previously considered in this chapter. Not only are the sepals joined together to form a five-toothed calyx-tube, but the five petals are also joined together to form a corolla-tube, and the stamens are fixed on the corolla-tube. There are two kinds of primroses, known to country children as pin-eyed and thrum-eyed flowers respectively (Fig. 82). The two forms grow on separate plants. In a pin-eyed primrose the style is long, and the stigma—looking somewhat like the head of a pin—is at the top of the corolla-tube; while the stamens are halfway down. The thrum-eyed primroses have their stamens at the top, while the stigma of the pistil is halfway down the tube, exactly opposite the place where, in the pin-eyed form, the stamens are inserted. This curious state of things was a great puzzle to botanists until Darwin cleared up the mystery. A bee, thrusting its proboscis down a pin-eyed primrose in search of the nectar at the bottom, dusts it with pollen about halfway down—just in the place which will come in contact with the stigma when the animal visits a thrum-eyed flower. And the pollen from the thrum-eyed form adheres to the part of the bee which will presently touch the stigma of a long-styled flower. This beautiful and simple arrangement makes it practically certain that each primrose shall be fertilised by pollen from the other form. In the thrum-eyed form, however, it is possible for pollen to fall upon the stigma and produce self-fertilisation.

It is obvious that the cowslip (Fig. 83) is closely related to the primrose. The difference lies chiefly in the character of the flower-stalk. In the cowslip this is long, and it bears at its top several stalklets, each of which ends in a flower. As in the primrose, cross-fertilisation is secured by some flowers being pin-eyed (long styled) and others thrum-eyed (short styled).

22. THE DAISY AND ITS RELATIVES.

1. The daisy.—Take up several daisy plants (Fig. 84) entire, and wash away the soil from the roots. Notice how the stems—some of which are underground—are connected together. Draw a leaf. What advantage is it to the plant to have leaves of the shape noticed? Cut vertically through the “head,” and notice that what is usually called the “flower” really consists of a large number of small flowers.

The central or disc flowers are tubular. Which disc flowers open first, those near the middle or those nearer the edge of the disc? Pick off a flower and notice the 5-toothed corolla. Tear the corolla down with a needle, and observe the tiny stamens (5) fixed on the corolla-tube. The anthers are joined together. Notice the divided stigma of the pistil. The white and pink ray flowers have strap-shaped corollas. They have no stamens, but each has a pistil like that of a disc flower. What do you think is the object of the ray flowers being so conspicuous? Why do they close over the disc at night?

Notice the large number of green bracts below the disc.

2. The dandelion.—Compare the dandelion (Fig. 85). Notice that all the flowers are strap-shaped, like the ray flowers of the daisy. Pull one out, and make out the strap-like corolla, the five stamens with joined anthers, and the double stigma (Fig. 86, 2). Notice the tuft of fine hairs below the corolla and above the knob-like ovary. The tuft of hairs is the top of the calyx-tube.

When the flowers have been fertilised, the yellow corollas wither, and each calyx-tube elongates until it is about an inch long, the tuft of fine hairs being still at the top (4). Blow a “clock,” and notice how easily the fruits are detached from the disc and how slowly they settle. What advantage is this to the plant?

3. The thistle.—Compare the thistle (Fig. 87). The bracts are very prickly. Is this an advantage? Are the flowers tubular or strap-shaped? Examine the fruits (“thistle down”) and compare with those of dandelion.

The daisy.—What is generally called the “flower” of the daisy (Fig. 84) really consists of a very large number of small separate flowers set close together on a flattened disc or receptacle. The group of flowers is called a head. On the lower surface of the head are several green leaves, or bracts, which protect the bud before it opens. The flowers of a daisy-head are of two kinds. The white or pink straps set round the edge of the head are the corollas of the ray flowers. They have no stamens, but a pistil with a divided stigma is present in each. The disc flowers are yellow and tubular. The corolla consists of five united petals. On it are fixed five stamens, the anthers of which are joined together. The pistil is like that of a ray flower. No calyx is present in either the ray or disc flowers of the daisy.

The daisy is fertilised by the aid of insects which are attracted by the strap-like corollas of the ray flowers.