Chapter IV: Leaves

We have learned in the previous chapters that the roots are the means by which a plant gathers out of the earth the various constituents which are needful to maintain its life.

The leaves have also to do their part in collecting from the air such gases as are required to effect the processes carried on within the substance of the leaf.

The leaf is really the digestive organ of the plant; it feeds, breathes, and gives off in the form of vapour any excess of water not required for its work. For these purposes sunlight and air are necessary.

A leaf consists of a stalk, called a petiole, and the flat green part, which we may call the blade.

If we hold a leaf up to the light we see a network of veins, and it is by their help the leaf becomes a broad expansion of tissue, so exposed that it gets the fullest possible benefit from the sunlight and air. This fibrous network gives strength to the leaf, and answers to the bones in animal structure.

The fibro-vascular bundles, which we see in the stem, go up through the petiole, and branch out in a beautiful and regular manner. We may observe this arrangement very clearly in a skeleton leaf, the midrib forming a backbone to the whole structure, while the smaller veins tend off to the edge of the leaf, and then overlap so as to form a system of girders supporting the edge, and preventing the wind from tearing the delicate tissues into shreds.

The arrangement of leaf network is called venation, and by a glance at it we can at once see to which of the great divisions in botany a plant belongs. If the fibres are straight and run parallel to each other without being netted, then we know the leaf is that of a plant which begins its life with only one seed-leaf; such are all the species of corn and grass, bulbs, palm-trees, bananas, and others.

The long name applied to this division of plants must be explained, as it is a term we cannot do without, and I must own it looks formidable until we understand its meaning.

The first leaf that comes out of a seed is called a cotyledon, from _kotúle_, a cavity, or cup. The Greek for one is _mónos_, so plants with one seed-leaf are called monocotyledons.

If we sow a date-stone or a few seeds of Indian corn in moist soil they will grow readily, and afford us nice little specimens of a one-seed leaf-plant.

If we see that a leaf has netted veins, then we know its seed produced two leaves at first,[8] so plants belonging to this great division are called dicotyledons.

[8] The Maranta and a few other plants are exceptions to this rule.

In order to watch the growth of two-leaved seedlings, we may select a broad bean, or some of the seeds out of tamarind jam; either will grow readily in a pot of earth, if it is placed in a sunny window, or near a stove, and kept moist.

Orange and lemon pips may sometimes be found sprouting within the fruit, and either of these seeds will germinate, and form charming little evergreen plants to brighten a town window-ledge.

Now we need not be afraid of those two long words which are used to describe one-leaved and two-leaved seedlings, since we know their meaning, and it will be interesting when we come across some new plant to see to which division it belongs, because knowing that will mean knowing a great deal besides.

All our English trees (with the exception of the firs, which have many seed-leaves) are dicotyledons; they increase their stems from the outside, and are therefore called exogens, and most of our plants belong to this division.

The monocotyledons increase from the centre, that is to say, the second leaf grows out of the first, and the third leaf and its stem grow out of the sheath of the second leaf, and so on; and this is the law of their growth, whether they be corn plants or palm-trees. These sheathing leaves and the straight veins will always enable us to recognise a one-seed leaf-plant at sight.

The development of the stem has a marked influence upon the arrangement of the leaves; these, in such plants as the cyclamen, sundew, or primrose, are said to be radical; that is, growing from the root. Close observation will reveal the cause to be the non-development of the internodes, the leaves being crowded upon a very short, suppressed stem, and thus we get the beautiful little rosettes we find in the daisy and plantain. When the stem is of greater length the leaves are ranged at definite intervals, the spaces between the leaves (the internodes) varying in length in proportion to the size of the leaf. Small leaves are thus much thicker upon the tree than larger ones. This will readily be seen if we compare a branch of sycamore with one of elm, the former having its large leaves much further apart than the latter.

Then, also, the arrangement of leaves upon the stem (_phyllotaxis_) varies much. If we take a spray of beech we shall find that its buds are placed alternately on either side of the stem, so that the third bud is exactly below the first, and the second bud is in a line with the fourth, and so on. This is also the plan of the elm, hazel, lime, hornbeam, and many other trees. In the alder and whitebeam the buds occur in three rows, and in some of the willows in series of eight.

The leaves of the horse-chestnut are borne in pairs on alternate sides of the stem, and this plan is common to a number of plants, especially those of the type of the dead nettle and speedwell.

Quite a distinct arrangement is that to be found in the woodruff and bedstraws, where the leaves are placed in a ring (a whorl) at regular intervals on the stem.

The botanical student should carefully observe the differing methods of leaf arrangement, since, as branches are developed from buds, the varying order in their position must naturally modify the general aspect of a tree, and has also much physiological importance. We shall find that buds are so placed that each leaf shall receive its full share of sunlight and air, for it needs this position in order to enable it to carry out the wonderful work of assimilation which it has to perform.

The upper surface of a leaf is covered by a thin layer of cells, known as the epidermis (or skin); this does not prevent the light from falling through, and its outer surface is protected by a thickening, known as the cuticle. This is of great use in controlling the escape of moisture, otherwise the leaf would soon shrivel up in a hot sun. In a young seedling leaf the cuticle is not developed, and it can therefore breathe out moisture very rapidly; later on, when the cuticle is formed, it controls the escape of moisture, which can then only exude through the under surface of the leaf.

We can easily peel off a portion of the skin from the under surface of the leaf, and if we place it in a little water between two pieces of glass and look at it in a microscope we shall see that it consists of an extremely thin layer of cells, with numbers of little openings called stomata (from the Greek _stoma_, a mouth), answering somewhat to the lenticels to be found in young tree-stems, only those are solely for the admission of air, while these little mouths are to let in and out not only air, but water, vapour, and oxygen.

These stomata look like little crescent-shaped slits with a curved cell on either side, and as they curve more or less, the mouths are opened or shut as the plant may require. These little mouths play a very important part in the economy of the leaf, and they exist in immense quantities on its under surface.

It has been calculated that a million stomata exist on a single leaf of the lime tree. When the root has taken up more moisture than is required, then it is the office of these pores, or stomata, in the leaf to give out this extra water in the form of vapour, and we can thus see how the action of leaves must influence climate. If forests are recklessly cut down, the bare country, with no foliage to throw moisture into the air, may become an almost barren desert, and again in marshy places, where the air is too damp, a wise reduction in the number of trees may alter the climate to a healthy condition.

Remarkable results have been obtained by planting the Australian gum-tree, _Eucalyptus globulus_; it thrives well in malarious places, and at once produces a marked hygienic change in the air. A Monsieur Gimbert relates that “A farm some twenty miles from Algiers was noted for its pestilential air, and in the spring of 1867, 13,000 eucalyptus trees were planted there, since which time not a single case of fever has occurred.

“The gum-tree grows rapidly and absorbs as much as ten times its weight of water from the soil, and emits camphoraceous antiseptic vapour from its leaves. It is therefore often called the fever-destroying tree.”

Experiments have been made to try and find out how much moisture is really given out by leaves. It was found that a sunflower three and a half feet high, with a leaf expanse of over five thousand inches, exhaled one pint of liquid in the course of the day.

No wonder, therefore, that trees tend to make the air damp.

Each stomate leads into air spaces between the cells, and is thus connected with the interior of the leaf.

The tissue and cells of a leaf (bifacial)[9] can be understood by reference to the accompanying diagram. Between the upper and under surfaces of a leaf there is a layer, more or less thick, of soft green tissue known as _mesophyll_, and if we hold a leaf to the sunlight we shall see the veins traversing this tissue.

[9] That is, a leaf like the beech or sycamore, having an upper and under surface; vertical leaves, like the iris, have palisade tissue on both sides.

The upper part of the mesophyll consists of elongated cells arranged at right angles to the surface, and placed so evenly parallel to each other that they have been compared to the pales of a fence, and are called palisade tissue. These cells contain a quantity of the green substance called _chlorophyllon_ (from _chloros_, green, and _phyllon_, a leaf), so named because to this bright green substance we owe all the lovely verdure of our woods and gardens.

Below this palisade tissue is another of quite a different form, consisting of large spongy cells, and therefore known as spongy tissue.

In its intercellular spaces are stored those secretions which make certain herbs, such as thyme, marjoram, and others so fragrant when bruised.

The chemical changes which are ever going on in these various layers, require a constant supply of the outer air, and this is secured by the little openings, called stomata, on the under surface of the leaf, which have been already described; these constitute the breathing apparatus of the leaf, for they open and shut, and regulate the supply of air into little air chambers, from which it passes into the structure of the plant.

Before going any further I must try and explain a little about the wonderful substance called protoplasm.[10]

[10] Greek: _proto_, “first”; _plasma_, “anything moulded.”

If we have ever watched a potter at work, we know he takes a lump of clay and moulds it according to his purpose, into a rough pot, or a lovely vase; now protoplasm seems to be just such a foundation material from which the Divine Creator causes animal and vegetable forms to proceed. _First material_ seems to me to be a term that actually expresses the meaning of the word protoplasm.

It lines the cell walls of leaves, it is capable of forming fresh cells, it can absorb moisture and other matters, it contracts and expands, it has power of movement, as one may readily see when a portion of a leaf is placed in a microscope, so as to show the grains of bright green chlorophyll circulating in the lining of each little cell.

Learned volumes would be needed to explain the nature of protoplasm, so I must be content with these simple facts about its nature, and proceed to the chemical action going on in leaves.

In ordinary atmosphere there is a very small quantity of a gas called carbon-dioxide.[11] The leaves absorb this gas from the air, and because there is so little of it, each tree needs to spread out an immense amount of foliage, that it may drink in, by its means, all the carbon-dioxide that can possibly be obtained.

[11] Carbon meaning charcoal, and dioxide meaning two parts oxygen. Sometimes called carbonic-acid gas.

When this gas comes in contact with the chlorophyll in a leaf, one part of the oxygen is set free, and returns to the air in a pure condition, thus making it more healthy for us to breathe; then the carbon and the remaining oxygen combine with water in the leaf cells, and form starch, the leaves retain the carbon, to build up their own structure; it enters indeed so largely into the composition of vegetable substance, that in some cases if we could burn one hundred parts of it, fifty parts of the ashes would prove to be carbon or charcoal.

In a rough sort of way we may see for ourselves how much carbon there is in woody fibre, by lighting an ordinary wooden match and letting it burn itself out; the black portion that remains will be a piece of charcoal not very much smaller than the original match.

Of course, in the process of burning, the match has lost the resin, and other organic substances which were stored up in the cells of the wood; all these have passed into the air, and only the carbon remains. If, however, instead of this slow manner of combustion, we had set light to a whole box of matches so that it burnt fiercely, the flame would have been strong enough to consume the charcoal, and nothing would then be left but mineral ashes.

When charcoal burners are at work in a forest, we may see them making a stack of wood, which they cover with a thick layer of clay so that the wood may burn away very slowly; in this case the charcoal will be left in the same way as when we burnt the single match.

As long as the upper side of leaves are soaking in the sunlight, starch is being formed, as I have described; but during the night the starch thus formed is dissolved, and passing through the leaf fibres finds its way into every part of the plant, either to be used in forming new tissue, or else to be stored up for future use.

The net-work of veins act as a service of tiny pipes, to convey the liquids up and down the petiole (leaf-stalk).

Generally the water given off from the stomata is in the form of vapour; but in some plants drops of water exude from the apex or point of the leaf through the water pores. In _Saxifraga crustata_, there are pores round the edges of the leaves, through which water, highly charged with lime and other salts, passes out, and as it evaporates a white deposit of lime remains which is quite visible in the form of a frosted edging to the leaves.

There is an American plant called the jewel-weed, which shows to perfection this power of distilling drops of water. I will quote a short description of its appearance at night-fall.

“Upon the approach of twilight, each leaf droops as if wilted, and from the notches along the edge, the crystal beads begin to grow until its border is hung full with its gems. It is Aladdin’s lantern that you see among a bed of these succulent pale green plants, for the spectacle is like dreamland.”[12]

[12] “Sharp Eyes,” by Wm. Hamilton Gibson.

A very similar effect may be observed if we visit a plant of Lady’s-mantle (_Alchemilla vulgaris_) at early morning after a warm dewless night; each leaf will be found beautifully decked with dewdrops at equal distances round the edge of the leaves where the pores have exuded the moisture with which they are charged.

Nasturtium and fuchsia may also be examined for this purpose and will show exudation from their leaf pores.

If a small quantity of wheat is grown in some cocoa fibre, it will illustrate this power of giving off water, for when the little blades are a few inches high, they will be found each morning tipped with a large dewdrop, the result of exudation during the night.

In countries where the sun is intensely hot, if the leaves of trees were to be exposed to its full power, they would probably wither, and vegetation would perish.

Against this danger some trees are enabled to make special provision, by changing the form of their leaves, and their mode of hanging on the branch. In Australia, for instance, where the sun is almost vertical, the acacias and eucalyptus trees, instead of holding their leaves flat or horizontally as trees do in England, so that they may catch every ray of sunlight, avoid the heat as much as possible, by holding them edgeways to the light.

While eucalyptus trees are young, and partially shaded by surrounding vegetation, their leaves are flat and oval, and English seedlings of this tree usually retain such leaves from five to ten years, our climate not being hot enough to require the mature form of leaf which hangs vertically, and is of an entirely different form.

Reference to the plates will show a young shoot of _Eucalyptus globulus_ and a branch of the older leaves, with their edges only exposed to sunlight.

The curved sickle-shaped leaves of the eucalyptus afford very little shade to the traveller in Australia for this reason, that only fine intercrossing lines of shadow are seen on the ground. To make this clear, let my readers take a sheet of notepaper out of doors on a sunny day and hold it perfectly flat, so as to expose it to all the sunlight it can receive upon its surface, as if it were a growing beech-leaf, and it will throw a large shadow on the ground. Then hold it edgeways to the sun, and it will form the kind of thin line of shadow that would be cast by a mature eucalyptus leaf.

Preparation for the fall of the leaf begins in spring, when a fine line or ridge may be traced just below the junction of the leaf with the stem. This dark line is in reality a thin layer of cork, which, during the summer months, continues to grow inwards to form in due time a covering for the bare place on the stem that will be left when the leaf falls off; this is called the leaf-scar.

It is interesting to watch this line, growing more and more visible as the year goes on.

Another curious fact is, that some of the starch which the leaves have been making during the summer, becomes stored up in autumn at the base of the leaf-stalk, so as to afford nourishment to the bud which will arise out of the axil of the leaf. When a weak solution of iodine is applied to it, this starch turns blue, and in this way its presence can be ascertained.

The fall of the leaf appears to take place mainly because the starch has the effect of softening the cells of the leaf-stalk; as it dries up it loses its hold of the twig, and either the wind or a slight frost will suffice to bring the leaves down to the ground in showers.

Another reason for their fall is, that their year’s work is done. Like good servants, they have been hard at work all through the summer and autumn months, taking in stores of nourishment for the benefit of the tree, and giving out volumes of oxygen, so helpful for the maintenance of human life. They have secured and laid up sufficient nutriment for the development of the next year’s buds, and having done this, their special office being at an end, they fall beneath the tree to become leaf-mould, which, in its turn when fully decayed, will yield nourishing elements to be carried by the winter and spring rains to the tree roots.

I might add many more useful objects which we owe to trees, and I commend it to my young readers as an instructive study to try and make out a complete list of the useful products of our English trees. I imagine we do not yet know all that might be obtained from them, new discoveries continue to reveal their value in the way of medicines; for instance, the crystals of the willow (called salicine) are now frequently prescribed as a remedy for rheumatism. Euonomine and many others might be included amongst the valuable gifts which nature has stored in the cells of tree-stems.

Specimens to be obtained:—Leaves with straight veins, such as grass or corn, hyacinth, narcissus, or any bulbous plants; leaves with netted veins, such as oak, ivy, vine, &c.; monocotyledon seedlings; dicotyledon seedlings. Leaf-skin to be examined through a microscope, in order to see stomata, chlorophyll, network, and cells. Examine waterpores in leaves when exuding moisture. Observe shadows thrown by leaves held flat and edgeways to the sun. Compare young and old eucalyptus leaves. Observe line of cork below leaf-scars.

Leaves can be made into beautiful skeletons by soaking a good many together in a pan of soft water until the upper and under surfaces of the leaves are sufficiently decayed to be easily removed by a soft brush; the fibre which is left can then be bleached with chloride of lime. When mounted with fine wire these skeleton leaves form an interesting group to place under a glass shade.

CHAPTER V

_BUDS_

“Oh! who can speak the joys of spring’s young morn,
When wood and pasture open on his view,
When tender green buds blush upon the thorn
And the first primrose dips its leaves in dew.”

+Clare.+