The Beauties of Nature, and the Wonders of the World We Live In Part 7

But such flowers are inconspicuous. Those which are coloured, say yellow or white, are of course much more visible and more likely to be visited by insects. I have elsewhere given my reasons for thinking that under these circ.u.mstances some flowers became yellow, that some of them became white, others subsequently red, and some finally blue. It will be observed that red and blue flowers are as a rule highly specialised, such as Aconites and Larkspurs as compared with b.u.t.tercups; blue Gentians as compared with yellow, etc. I have found by experiment that Bees are especially partial to blue and pink.

Tubular flowers almost always, if not always, contain honey, and are specially suited to b.u.t.terflies and Moths, Bees and Flies. Those which are fertilised by Moths generally come out in the evening, are often very sweetly scented, and are generally white or pale yellow, these colours being most visible in the twilight.

Aristotle long ago noticed the curious fact that in each journey Bees confine themselves to some particular flower. This is an economy of labour to the Bee, because she has not to vary her course of proceeding.

It is also an advantage to the plants, because the pollen is carried from each flower to another of the same species, and is therefore less likely to be wasted.

FRUITS AND SEEDS

After the flower comes the seed, often contained in a fruit, and which itself encloses the future plant. Fruits and seeds are adapted for dispersion, beautifully and in various ways: some by the wind, being either provided with a wing, as in the fruits of many trees--Sycamores, Ash, Elms, etc.; or with a hairy crown or covering, as with Thistles, Dandelions, Willows, Cotton plant, etc.

Some seeds are carried by animals; either as food--such as most edible fruits and seeds, acorns, nuts, apples, strawberries, raspberries, blackberries, plums, gra.s.ses, etc.--or involuntarily, the seeds having hooked hairs or processes, such as burrs, cleavers, etc.

Some seeds are scattered by the plants themselves, as, for instance, those of many Geraniums, Violets, Balsams, Shamrocks, etc. Our little Herb Robert throws its seeds some 25 feet.

Some seeds force themselves into the ground, as those of certain gra.s.ses, Cranes'-bills (Erodiums), etc.

Some are buried by the parent plants, as those of certain clovers, vetches, violets, etc.

Some attach themselves to the soil, as those of the Flax; or to trees, as in the case of the Mistletoe.

LEAVES

Again, as regards the leaves there can, I think, be no doubt that similar considerations of utility are applicable. Their forms are almost infinitely varied. To quote Ruskin's vivid words, they "take all kinds of strange shapes, as if to invite us to examine them.

Star-shaped, heart-shaped, spear-shaped, arrow-shaped, fretted, fringed, cleft, furrowed, serrated, sinuated, in whorls, in tufts, in spires, in wreaths, endlessly expressive, deceptive, fantastic, never the same from foot-stalk to blossom, they seem perpetually to tempt our watchfulness and take delight in outstepping our wonder."

But besides these differences of mere form, there are many others: of structure, texture, and surface; some are scented or have a strong taste, or acrid juice, some are smooth, others hairy; and the hairs again are of various kinds.

I have elsewhere[21] endeavoured to explain some of the causes which have determined these endless varieties. In the Beech, for instance (Fig. 15), the leaf has an area of about 3 square inches. The distance between the buds is about 1-1/4 inch, and the leaves lie in the general plane of the branch, which bends slightly at each internode. The basal half of the leaf fits the swell of the twig, while the upper half follows the edge of the leaf above; and the form of the inner edge being thus determined, decides that of the outer one also.

[Ill.u.s.tration: Fig. 15.--Beech.]

The weight, and consequently the size of the leaf, is limited by the strength of the twig; and, again, in a climate such as ours it is important to plants to have their leaves so arranged as to secure the maximum of light. Hence in leaves which lie parallel to the plane of the boughs, as in the Beech, the width depends partly on the distance between the buds; if the leaves were broader, they would overlap, if they were narrower, s.p.a.ce would be wasted. Consequently the width being determined by the distance between the buds, and the size depending on the weight which the twig can safely support, the length also is determined. This argument is well ill.u.s.trated by comparing the leaves of the Beech with those of the Spanish Chestnut. The arrangement is similar, and the distance between the buds being about the same, so is the width of the leaves. But the terminal branches of the Spanish Chestnut being much stronger, the leaves can safely be heavier; hence the width being fixed, they grow in length and a.s.sume the well-known and peculiar sword-blade shape.

In the Sycamores, Maples (Fig. 16), and Horse-Chestnuts the arrangement is altogether different. The shoots are stiff and upright with leaves placed at right angles to the branches instead of being parallel to them. The leaves are in pairs and decussate with one another; while the lower ones have long petioles which bring them almost to the level of the upper pairs, the whole thus forming a beautiful dome.

For leaves arranged as in the Beech the gentle swell at the base is admirably suited; but in a crown of leaves such as those of the Sycamore, s.p.a.ce would be wasted, and it is better that they should expand at once, so soon as their stalks have carried them free from the upper and inner leaves.

[Ill.u.s.tration: Fig. 16.--Acer platanoides.]

In the Black Poplar the arrangement of the leaves is again quite different. The leaf stalk is flattened, so that the leaves hang vertically. In connection with this it will be observed that while in most leaves the upper and under surfaces are quite unlike, in the Black Poplar on the contrary they are very similar. The stomata or breathing holes, moreover, which in the leaves of most trees are confined to the under surface, are in this species nearly equally numerous on both.

The "Compa.s.s" Plant of the American prairies, a plant not unlike a small sunflower, is another species with upright leaves, which growing in the wide open prairies tend to point north and south, thus exposing both surfaces equally to the light and heat. Such a position also affects the internal structure of the leaf, the two sides becoming similar in structure, while in other cases the upper and under surfaces are very different.

In the Yew the leaves are inserted close to one another, and are linear; while in the Box they are further apart and broader. In other cases the width of the leaves is determined by what botanists call the "Phyllotaxy." Some plants have the leaves opposite, each pair being at right angles with the pairs above and below.

In others they are alternate, and arranged round the stem in a spiral.

In one very common arrangement the sixth leaf stands directly over the first, the intermediate ones forming a spiral which has pa.s.sed twice round the stem. This, therefore, is known as the 2/5 arrangement. Common cases are 1/2, 1/3, 2/5, 3/8, and 5/13. In the first the leaves are generally broad, in the 3/8 arrangement they are elliptic, in the 5/13 and more complicated arrangements nearly linear. The Willows afford a very interesting series. Salix herbacea has the 1/3 arrangement and rounded leaves, Salix caprea elliptic leaves and 2/5, Salix pentandra lancet-shaped leaves and 3/8, and S. incana linear leaves and a 5/13 arrangement. The result is that whether the series consists of 2, 3, 5, 8, or 13 leaves, in every case, if we look perpendicularly at a twig the leaves occupy the whole circle.

In herbaceous plants upright leaves as a rule are narrow, which is obviously an advantage, while prostrate ones are broad.

[Ill.u.s.tration: AQUATIC VEGETATION, BRAZIL. _To face page 145._]

AQUATIC PLANTS

Many aquatic plants have two kinds of leaves; some more or less rounded, which float on the surface; and others cut up into narrow segments, which remain below. The latter thus present a greater extent of surface.

In air such leaves would be unable even to support their own weight, much less to resist the force of the wind. In still air, however, for the same reason, finely-divided leaves may be an advantage, while in exposed positions compact and entire leaves are more suitable. Hence herbaceous plants tend to have divided, bushes and trees entire, leaves.

There are many cases when even in the same family low and herb-like species have finely-cut leaves, while in shrubby or ligneous ones they more or less resemble those of the Laurel or Beech.

These considerations affect trees more than herbs, because trees stand more alone, while herbaceous plants are more affected by surrounding plants. Upright leaves tend to be narrow, as in the case of gra.s.ses; horizontal leaves, on the contrary, wider. Large leaves are more or less broken up into leaflets, as in the Ash, Mountain-Ash, Horse-Chestnut, etc.

The forms of leaves depend also much on the manner in which they are packed into the buds.

The leaves of our English trees, as I have already said, are so arranged as to secure the maximum of light; in very hot countries the reverse is the case. Hence, in Australia, for instance, the leaves are arranged not horizontally, but vertically, so as to present, not their surfaces, but their edges, to the sun. One English plant, a species of lettuce, has the same habit. This consideration has led also to other changes. In many species the leaves are arranged directly under, so as to shelter, one another. The Australian species of Acacia have lost their true leaves, and the parts which in them we generally call leaves are in reality vertically-flattened leaf stalks.

In other cases the stem itself is green, and to some extent replaces the leaves. In our common Broom we see an approach to this, and the same feature is more marked in Cactus. Or the leaves become fleshy, thus offering, in proportion to their volume, a smaller surface for evaporation. Of this the Stonecrops, Mesembryanthemum, etc., are familiar instances. Other modes of checking transpiration and thus adapting plants to dry situations are by the development of hairs, by the formation of chalky excretions, by the sap becoming saline or viscid, by the leaf becoming more or less rolled up, or protected by a covering of varnish.

Our English trees are for the most part deciduous. Leaves would be comparatively useless in winter when growth is stopped by the cold; moreover, they would hold the snow, and thus cause the boughs to be broken down. Hence perhaps the glossiness of Evergreen leaves, as, for instance, of the Holly, from which the snow slips off. In warmer climates trees tend to retain their leaves, and some species which are deciduous in the north become evergreen, or nearly so, in the south of Europe. Evergreen leaves are as a rule tougher and thicker than those which drop off in autumn; they require more protection from the weather.

But some evergreen leaves are much longer lived than others; those of the Evergreen Oak do not survive a second year, those of the Scotch Pine live for three, of the Spruce Fir, Yew, etc., for eight or ten, of the Pinsapo even eighteen. As a general rule the Conifers with short leaves keep them on for several years, those with long ones for fewer, the length of the leaf being somewhat in the inverse ratio to the length of its life; but this is not an invariable criterion, as other circ.u.mstances also have to be taken into consideration.

Leaves with strong scent, aromatic taste, or acrid juice, are characteristic of dry regions, where they run especial danger of being eaten, and where they are thus more or less effectively protected.

ON HAIRS

The hairs of plants are useful in various ways. In some cases (1) they keep off superfluous moisture; in others (2) they prevent too rapid evaporation; in some (3) they serve as a protection against too glaring light; in some (4) they protect the plant from browsing quadrupeds; in others (5) from being eaten by insects; or, (6) serve as a quickset hedge to prevent access to the flowers.

In ill.u.s.tration of the first case I may refer to many alpine plants, the well-known Edelweiss, for instance, where the woolly covering of hairs prevents the "stomata," or minute pores leading into the interior of the leaf, from being clogged up by rain, dew, or fog, and thus enable them to fulfil their functions as soon as the sun comes out.

As regards the second case many desert and steppe-plants are covered with felty hairs, which serve to prevent too rapid evaporation and consequent loss of moisture.

The woolly hairy leaves of the Mulleins (Verbasc.u.m) doubtless tend to protect them from being eaten, as also do the spines of Thistles, and those of Hollies, which, be it remarked, gradually disappear on the upper leaves which browsing quadrupeds cannot reach.

I have already alluded to the various ways in which flowers are adapted to fertilisation by insects. But Ants and other small creeping insects cannot effectually secure this object. Hence it is important that they should be excluded, and not allowed to carry off the honey, for which they would perform no service in return. In many cases, therefore, the opening of the flower is either contracted to a narrow pa.s.sage, or is itself protected by a fringe of hairs. In others the peduncle, or the stalk of the plant, is protected by a hedge, or chevaux de frise, of hairs.

In this connection I might allude to the many plants which are more or less viscid. This also is in most cases a provision to preclude creeping insects from access to the flowers.

There are various other kinds of hairs to which I might refer--glandular hairs, secretive hairs, absorbing hairs, etc. It is marvellous how beautifully the form and structure of leaves is adapted to the habits and requirements of the plants, but I must not enlarge further on this interesting subject.

The time indeed will no doubt come when we shall be able to explain every difference of form and structure, almost infinite as these differences are.

INFLUENCE OF SOIL

The character of the vegetation is of course greatly influenced by that of the soil. In this respect granitic and calcareous regions offer perhaps the best marked contrast.

There are in Switzerland two kinds of Rhododendrons, very similar in their flowers, but contrasted in their leaves: Rhododendron hirsutum having them hairy at the edges as the name indicates; while in R.