The Beauties of Nature, and the Wonders of the World We Live In Part 6

I shall never forget hearing Darwin's paper on the structure of the Cowslip and Primrose, after which even Sir Joseph Hooker compared himself to Peter Bell, to whom

A primrose by a river's brim A yellow primrose was to him, And it was nothing more.

We all, I think, shared the same feeling, and found that the explanation of the flower then given, and to which I shall refer again, invested it with fresh interest and even with new beauty.

A regular flower, such, for instance, as a Geranium or a Pink, consists of four or more whorls of leaves, more or less modified: the lowest whorl is the Calyx, and the separate leaves of which it is composed, which however are sometimes united into a tube, are called sepals; (2) a second whorl, the corolla, consisting of coloured leaves called petals, which, however, like those of the Calyx, are often united into a tube; (3) of one or more stamens, consisting of a stalk or filament, and a head or anther, in which the pollen is produced; and (4) a pistil, which is situated in the centre of the flower, and at the base of which is the Ovary, containing one or more seeds.

Almost all large flowers are brightly coloured, many produce honey, and many are sweet-scented.

What, then, is the use and purpose of this complex organisation?

It is, I think, well established that the main object of the colour, scent, and honey of flowers is to attract insects, which are of use to the plant in carrying the pollen from flower to flower.

In many species the pollen is, and no doubt it originally was in all, carried by the air. In these cases the chance against any given grain of pollen reaching the pistil of another flower of the same species is of course very great, and the quant.i.ty of pollen required is therefore immense.

In species where the pollen is wind-borne as in most of our trees--firs, oaks, beech, ash, elm, etc., and many herbaceous plants, the flowers are as a rule small and inconspicuous, greenish, and without either scent or honey. Moreover, they generally flower early, so that the pollen may not be intercepted by the leaves, but may have a better chance of reaching another flower. And they produce an immense quant.i.ty of pollen, as otherwise there would be little chance that any would reach the female flower. Every one must have noticed the clouds of pollen produced by the Scotch Fir. When, on the contrary, the pollen is carried by insects, the quant.i.ty necessary is greatly reduced. Still it has been calculated that a Peony flower produces between 3,000,000 and 4,000,000 pollen grains; in the Dandelion, which is more specialised, the number is reduced to about 250,000; while in such a flower as the Dead-nettle it is still smaller.

The honey attracts the insects; while the scent and colour help them to find the flowers, the scent being especially useful at night, which is perhaps the reason why evening flowers are so sweet.

It is to insects, then, that flowers owe their beauty, scent, and sweetness. Just as gardeners, by continual selection, have added so much to the beauty of our gardens, so to the unconscious action of insects is due the beauty, scent, and sweetness of the flowers of our woods and fields.

Let us now apply these views to a few common flowers. Take, for instance, the White Dead-nettle.

The corolla of this beautiful and familiar flower (Fig. 6) consists of a narrow tube, somewhat expanded at the upper end (Fig. 7), where the lower lobe forms a platform, on each side of which is a small projecting tooth (Fig. 8, _m_). The upper portion of the corolla is an arched hood (_co_), under which lie four anthers (_a a_), in pairs, while between them, and projecting somewhat downwards, is the pointed pistil (_st_); the tube at the lower part contains honey, and above the honey is a row of hairs running round the tube.

[Ill.u.s.tration: Fig. 6--White Dead-nettle.]

Now, why has the flower this peculiar form? What regulates the length of the tube? What is the use of the arch? What lesson do the little teeth teach us? What advantage is the honey to the flower? Of what use is the fringe of hairs? Why does the stigma project beyond the anthers? Why is the corolla white, while the rest of the plant is green?

[Ill.u.s.tration: Fig. 7.]

[Ill.u.s.tration: Fig. 8.]

The honey of course serves to attract the Humble Bees by which the flower is fertilised, and to which it is especially adapted; the white colour makes the flower more conspicuous; the lower lip forms the stage on which the Bees may alight; the length of the tube is adapted to that of their proboscis; its narrowness and the fringe of fine hairs exclude small insects which might rob the flower of its honey without performing any service in return; the arched upper lip protects the stamens and pistil, and prevents rain-drops from choking up the tube and was.h.i.+ng away the honey; the little teeth are, I believe, of no use to the flower in its present condition, they are the last relics of lobes once much larger, and still remaining so in some allied species, but which in the Dead-nettle, being no longer of any use, are gradually disappearing; the height of the arch has reference to the size of the Bee, being just so much above the alighting stage that the Bee, while sucking the honey, rubs its back against the hood and thus comes in contact first with the stigma and then with the anthers, the pollen-grains from which adhere to the hairs on the Bee's back, and are thus carried off to the next flower which the Bee visits, when some of them are then licked off by the viscid tip of the stigma.[20]

[Ill.u.s.tration: Fig. 9.]

[Ill.u.s.tration: Fig. 10.]

[Ill.u.s.tration: Fig. 11.]

In the Salvias, the common blue Salvia of our gardens, for instance,--a plant allied to the Dead-nettle,--the flower (Fig. 9) is constructed on the same plan, but the arch is much larger, so that the back of the Bee does not nearly reach it. The stamens, however, have undergone a remarkable modification. Two of them have become small and functionless.

In the other two the anthers or cells producing the pollen, which in most flowers form together a round k.n.o.b or head at the top of the stamen, are separated by a long arm, which plays on the top of the stamen as on a hinge. Of these two arms one hangs down into the tube, closing the pa.s.sage, while the other lies under the arched upper lip.

When the Bee pushes its proboscis down the tube (Fig. 11) it presses the lower arm to one side, and the upper arm consequently descends, tapping the Bee on the back, and dusting it with pollen. When the flower is a little older the pistil (Fig. 9, _p_) has elongated so that the stigma (Fig. 10, _st_) touches the back of the Bee and carries off some of the pollen. This sounds a little complicated, but is clear enough if we take a twig or stalk of gra.s.s and push it down the tube, when one arm of each of the two larger stamens will at once make its appearance. It is one of the most beautiful pieces of plant mechanism which I know, and was first described by Sprengel, a poor German schoolmaster.

SNAPDRAGON

At first sight it may seem an objection to the view here advocated that the flowers in some species--as, for instance, the common Snapdragon (Antirrhinum), which, according to the above given tests, ought to be fertilised by insects--are entirely closed. A little consideration, however, will suggest the reply. The Snapdragon is especially adapted for fertilisation by Humble Bees. The stamens and pistil are so arranged that smaller species would not effect the object. It is therefore an advantage that they should be excluded, and in fact they are not strong enough to move the spring. The Antirrhinum is, so to speak, a closed box, of which the Humble Bees alone possess the key.

FURZE, BROOM, AND LABURNUM

Other flowers such as the Furze, Broom, Laburnum, etc., are also opened by Bees. The petals lock more or less into one another, and the flower remains at first closed. When, however, the insect alighting on it presses down the keel, the flower bursts open, and dusts it with pollen.

SWEET PEA

In the above cases the flower once opened does not close again. In others, such as the Sweet Pea and the Bird's-foot Lotus, Nature has been more careful. When the Bee alights it clasps the "wings" of the flower with its legs, thus pressing them down; they are, however, locked into the "keel," or lower petal, which accordingly is also forced down, thus exposing the pollen which rubs against, and part of which sticks to, the breast of the Bee. When she leaves the flower the keel and wings rise again, thus protecting the rest of the pollen and keeping it ready until another visitor comes. It is easy to carry out the same process with the fingers.

[Ill.u.s.tration: Fig. 12. Fig. 13.

Flower and Pollen of Primrose]

PRIMULA

In the Primrose and Cowslip, again, we find quite a different plan. It had long been known that if a number of Cowslips or Primroses are examined, about half would be found to have the stigma at the top of the tube and the stamens half way down, while in the other half the stamens are at the top and the stigma half way down. These two forms are about equally numerous, but never occur on the same stock. They have been long known to children and gardeners, who call them thrum-eyed and pin-eyed.

Mr. Darwin was the first to explain the significance of this curious difference. It cost him several years of patient labour, but when once pointed out it is sufficiently obvious. An insect thrusting its proboscis down a primrose of the long-styled form (Fig. 12) would dust its proboscis at a part (_a_) which, when it visited a short-styled flower (Fig. 13), would come just opposite the head of the pistil (_st_), and could not fail to deposit some of the pollen on the stigma.

Conversely, an insect visiting a short-styled plant would dust its proboscis at a part farther from the tip; which, when the insect subsequently visited a long-styled flower, would again come just opposite to the head of the pistil. Hence we see that by this beautiful arrangement insects must carry the pollen of the long-styled form to the short-styled, and _vice versa_.

The economy of pollen is not the only advantage which plants derive from these visits of Insects. A second and scarcely less important is that they tend to secure "cross fertilisation"; that is to say, that the seed shall be fertilised by pollen from another plant. The fact that "cross fertilisation" is of advantage to the plant doubtless also explains the curious arrangement that in many plants the stamen and pistil do not mature at the same time--the former having shed their pollen before the pistil is mature; or, which happens less often, the pistil having withered before the pollen is ripe. In most Geraniums, Pinks, etc., for instance, and many allied species, the stamens ripen first, and are followed after an interval by the pistil.

THE NOTTINGHAM CATCHFLY

The Nottingham Catchfly (Silene nutans) is a very interesting case. The flower is adapted to be fertilised by Moths. Accordingly it opens towards evening, and as is generally the case with such flowers, is pale in colour, and sweet-scented. There are two sets of stamens, five in each set. The first evening that the flower opens one set of stamens ripen and expose their pollen. Towards morning these wither away, the flower shrivels up, ceases to emit scent, and looks as if it were faded.

So it remains all next day. Towards evening it reopens, the second set of stamens have their turn, and the flower again becomes fragrant. By morning, however, the second set of stamens have shrivelled, and the flower is again asleep. Finally on the third evening it reopens for the last time, the long spiral stigmas expand, and can hardly fail to be fertilised with the pollen brought by Moths from other flowers.

THE HEATH

In the hanging flowers of Heaths the stamens form a ring, and each one bears two horns. When the Bee inserts its proboscis into the flower to reach the honey, it is sure to press against one of these horns, the ring is dislocated, and the pollen falls on to the head of the insect.

In fact, any number of other interesting cases might be mentioned.

BEES AND FLIES

Bees are intelligent insects, and would soon cease to visit flowers which did not supply them with food. Flies, however, are more stupid, and are often deceived. Thus in our lovely little Parna.s.sia, five of the ten stamens have ceased to produce pollen, but are prolonged into fingers, each terminating in a s.h.i.+ning yellow k.n.o.b, which looks exactly like a drop of honey, and by which Flies are continually deceived.

Paris quadrifolia also takes them in with a deceptive promise of the same kind. Some foreign plants have livid yellow and reddish flowers, with a most offensive smell, and are constantly visited by Flies, which apparently take them for pieces of decaying meat.

[Ill.u.s.tration: Fig. 14.--Arum.]

The flower of the common Lords and Ladies (Arum) of our hedges is a very interesting case. The narrow neck bears a number of hairs pointing downwards. The stamens are situated above the stigma, which comes to maturity first. Small Flies enter the flower apparently for shelter, but the hairs prevent them from returning, and they are kept captive until the anthers have shed their pollen. Then, when the Flies have been well dusted, the hairs shrivel up, leaving a clear road, and the prisoners are permitted to escape. The tubular flowers of Aristolochia offer a very similar case.

PAST HISTORY OF FLOWERS

If the views here advocated are correct, it follows that the original flowers were small and green, as wind-fertilised flowers are even now.