An Introduction to Nature-study

The dandelion “flower” (Fig. 85) is also really a head consisting of a great many separate flowers. There are often between two and three hundred of these little flowers present in one head. Below the head are several green bracts, and these protect the flowers both in the bud and at night (Fig. 86). The flowers of the dandelion are all of one type. The corolla is strap-shaped (Fig. 86, 2), and of a beautiful yellow colour to attract insects. On the end of the strap may be seen five small teeth which indicate that it really consists of five united petals. At the base of the corolla is a tuft of fine hairs, which is the top of the calyx-tube, and at the bottom of the flower is a little white knob—the ovary. The five stamens are fixed on the inside of the corolla-tube. Their anthers are united to form a tube through which the upper part of the style, and the forked stigma protrude.

The thistle (Fig. 87) is another common member of the family. Its bracts are prickly, and are a protection from the attacks of animals. The flowers (Fig. 88) are all tubular. The common thistle distributes its fruit by a plume of radiating fine hairs—the calyx. The fruit is commonly known as “thistle down.”

The Compositae, as plants of this family are called, are found in all parts of the world. The family is the largest in the vegetable kingdom, and many of the plants included in it are of considerable importance.

23. THE FOXGLOVE FAMILY.

1. The foxglove.—Examine a flowering plant of foxglove (Fig. 89). Notice the general habit of growth. In the flower make out the five-lobed calyx, the irregular corolla with five petals joined to form a tube, the four stamens (two long and two short) fixed on the corolla tube (Fig. 90), and the form and attachment of the pistil. Watch bees visiting the flower. 2. The speedwell.—Compare the speedwell (Fig. 91), and notice that the corolla is more nearly regular than is the case with the foxglove, that it consists of four combined petals, and that two stamens are fixed upon it.

3. The musk.—Compare the musk. Dissect a flower and notice the forms and positions of the parts. Especially examine the pistil with its two-lobed stigma. With a hair, carefully touch one of the lobes of the stigma of a growing flower and watch how the lobes close. Do the lobes open again? Put a little pollen on, and watch to see if this time the lobes open again after closing.

Watch insects visiting the flowers and try to make out how the pollination of the stigmas is brought about.

The foxglove family.—In the primrose and the disc-flowers of the daisy are seen examples of regular, dicotyledonous flowers with five petals fused to form a corolla tube, and with five stamens inserted on the corolla. In the strap-shaped flowers of the dandelion the corolla is irregular, but still consists of five fused petals and bears the five stamens.

The foxglove (Fig. 89) and its relatives have also irregular corollas of joined petals on which the stamens are fixed; but the stamens are usually only four in number, two being long and two short as in Fig. 90, b. In the foxglove the stamens ripen and shed their pollen before the pistil of the same flower is mature. This prevents self-fertilisation, but bumble bees in passing from one flower to another convey the ripe pollen of the younger flowers to the stigmas of flowers which are ready for fertilisation.

The pretty blue speedwell (Fig. 91) is closely related to the foxglove, but its corolla has only four lobes instead of five, and some times these seem of almost equal size at the first glance. Generally, however, the corolla is very plainly irregular. The speedwell has only two stamens, while mullein has five.

The flowers of the musk are especially interesting, because they show so well what is called irritability,—the power which all living things possess of acting in a definite manner in response to a definite irritation or stimulus. We have already seen good examples of plant-irritability in the way a climbing stem winds itself round a support. The stigma of the musk flower has two flaps. If these are touched with a hair or bristle they quickly close together, but presently open again as if they had found out that they had been tricked. When, however, a little pollen is put on the flaps they close finally, for their whole object is accomplished.

Some of the plants in this family are poisonous, the foxglove being especially so in all its parts.

24. THE LABIATES.

1. The deadnettle.—Examine a deadnettle plant (Fig. 92). Notice the habit of growth, and write down a description of the shape and appearance of the stem and leaves. What is the shape of the flower? How many sepals, petals, and stamens has it? Do the stamens ripen first, or does the pistil? What insects do you find visiting the flower? Try to find out how they pollinate the stigma.

The labiates.—The deadnettle is a type of an easily recognisable family of plants. The stem is square in section, and the leaves are arranged upon it in opposite pairs at right angles to each other. The plants are hairy and have distinctive odours. The aroma of thyme, mint, marjoram, sage, etc., has led to the plants being used for flavouring food. None of the labiates is poisonous.

The shape of the flower is very characteristic, and is specially adapted to the visits of bees. The flowers are so modified that the lowest part of the corolla forms a platform on which the bee may conveniently alight, while the upper petals unite into an arched roof which protects the pistil and stamens.

The mechanism of cross-pollination is particularly well shown in the case of the sage (Fig. 93). The flower contains four stamens, but two of these have lost their use, and the others are modified in a strange manner. The whole stamen has somewhat the shape of a capital T, and at each end of the cross-piece is a pollen box. Usually the cross-piece (c, Fig. 93, 3) is not at right angles to the filament, but is swung up—the junction acts as a hinge—until it is nearly vertical (Fig. 93, 4). The pollen box s, which is at the lower end of the cross-piece c when this is vertical, contains hardly any pollen. The entrance to the honey tube is thus guarded by two pillars, the filaments (f) of the stamens; and the lower pollen box (s) of the cross-piece of each stamen is directly in front of the bee’s head as it stands on the lower lip of the flower. When it pushes forward its head to reach the nectar it comes in contact with the lower pollen boxes, and the cross pieces swing round on their hinges, bringing the upper pollen boxes down with a smack on the bee’s back (Fig. 93, 1), and sprinkling it liberally with pollen dust. Having shed their pollen the stamens shrivel up, and the pistil comes to maturity. As the pistil ripens, the stigma arches over (Fig. 93, 2) so as to scrape along the back of any bee visiting the flower for the nectar, and thus to wipe off the pollen which has been brought from a younger flower.

25. THE LILY AND SNOWDROP FAMILIES.

1. The hyacinth.—Take up a plant (Fig. 94) entire and notice the underground bulb with roots springing from its lower surface, and the long narrow leaves. Is the venation of the leaves parallel or net-like? Is the hyacinth a monocotyledon or a dicotyledon? See the bract at the base of each flower-stalklet.

Examine the flower. Its leaves cannot be distinguished into calyx and corolla, but are quite similar to each other in size, shape, and colour. They are therefore called the perianth. The perianth leaves are united to form a tube. Tear down the tube to see the six stamens fixed on it. Are they all on the same level? Examine the pistil and cut the ovary across to see the ovules in the three joined carpels. Which is fixed at the higher level, the perianth or the base of the pistil?

2. Other plants of the lily family.—Examine also the white lily, tulip, star of Bethlehem, and lily of the valley, and notice that in spite of small differences they are all monocotyledons (how do you know this?) and all have the perianth fixed below the ovary.

3. The snowdrop.—Compare and contrast the snowdrop (Fig. 95). Make out that it is a monocotyledon, but that its perianth is inserted above the ovary. This is the great point of difference from the lily family. Notice also how the stamens are fixed.

4. Other plants of the snowdrop family.—Examine the daffodil (Figs. 55 and 96) and narcissus. Arising from the short perianth tube of the daffodil is a longer one which is often mistaken for a corolla; it is called the corona. None of the flowers hitherto described contains anything corresponding to a corona. The corona in the narcissus is short. As in the snowdrop, the perianth is fixed above the ovary. Observe how the stamens are fixed, and notice the dry leaf beneath the flower.

The lily family.—Either the wild hyacinth (Fig. 94) or the cultivated single hyacinth may be taken as a good representative of this family. The first point which strikes the student on examining the general “habit” of the plant is the character of the long sheathing leaves. Their veins do not form an obvious network, such as is seen in the leaves of dicotyledons, but run lengthwise and roughly parallel to each other in the manner characteristic of grasses and other monocotyledons (p. 40). The leaves are narrow, and are not divided into blade and stalk; they and the flower stalk spring from an underground bulb (p. 84) which consists chiefly of the swollen leaf-bases of a previous season. A separate calyx and corolla are not to be distinguished in the flower; the six leaves being all alike in size, shape, and colour. These six leaves hence receive a special name, and are called the perianth. The perianth leaves are united into a tube, on the inside of which the six stamens are arranged in two series of three each. In the middle of the flower, and fixed above the insertion of the perianth, is the pistil, which consists of three united carpels.

It will be noticed that the parts of the flower are in threes. There are six united perianth leaves (three inner and three outer), six stamens (also in two series), and three united carpels. This is very common—though by no means universal—in monocotyledons.

All the plants of the lily family—including the tulips, the true lilies, lily of the valley, asparagus, onion, etc.—agree in being monocotyledons, and in their flowers having a conspicuous perianth (for attracting insects) and six stamens, and in the ovary being above the insertion of the perianth.

The snowdrop family.—Plants of this family are very similar to those of the lily family; in fact in only one respect can any sharp line of demarcation be drawn between the two groups; in the snowdrop and its relatives the other parts of the flower stand upon the ovary (Fig. 96). The flowers of some plants of the family, e.g. the daffodil, possess a tubular outgrowth of the perianth, which is called a corona. It is often mistaken for a corolla.

26. THE ARUM LILY.

1. The cuckoo-pint.—Examine the habit of the plant, and then cut open the “flower.” You will probably find a number of small flies inside. Examine the central rod and make out the stamens and pistils on it. Which ripen first?

The arum “lily,” with its humble relative the cuckoo-pint (Fig. 97), merits special mention; for, in the first place, it is not a lily at all, and secondly, it furnishes an extremely interesting example of pistils coming to maturity before anthers, which is rather rare.

What is generally called the “flower” of the cuckoo-pint, or “lords and ladies,” consists of a big curled leaf with a purple rod sticking up in the middle. Near the bottom of the rod, but hidden from sight by the lower part of the leaf, the true flowers arise. The chamber containing them is shut in by a series of stiffish hairs which point downwards. Below the hairs the rod supports a series of anthers and, near the bottom of the chamber, several pistils. On cutting open the chamber one nearly always finds a number of small flies, covered with pollen, which they have brought from another arum. The flies get in easily enough, but once in they are prisoners, for the down-pointing hairs prevent them from getting out again. The pistils near the bottom of the rod ripen, and are fertilised by the pollen the flies have brought. After a time the anthers above ripen and shed their welcome pollen on the hungry captives. Soon after this the hairs at the top of the chamber shrivel up, and the flies, once more covered with pollen, are at liberty to return to the outer world and, untaught by experience, to repeat the experiment on another “flower.”

EXERCISES ON CHAPTER VI.