Chapter III: Tree Stems

A walk through a wood on a bright day in February will afford us many interesting intuitions about the growth of trees.

We are apt to think of winter as a dead season, and long for summer days once more, that we may pursue our botanical studies; but as soon as February begins there is already a secret work going on within the tree-stems, the sap is rising from the roots, and this ascent is easily to be traced if we look carefully at the trunks of those trees, such as the oak, elm, and others, which have rugged bark. The wood within is swelling; fresh layers of material will, a little later on, be added to the inner side of the bark as a result of this ascent of the sap.

As the bark is hard and inelastic, it cannot expand in proportion, and therefore has to crack and split in yielding to the internal pressure. If we look for these fresh cracks, we shall see the clean new bark within, which, before long, will harden and become of the same shade of grey as the rest of the stem.

It is at this season, too, that the plane-tree sheds off its fragments of bark in greatest quantity, as one may plainly see in the London squares, where this tree grows so remarkably well. Its stem is always peeling more or less throughout the year, and possibly that fact may be one of the reasons of its flourishing so well in the midst of smoke and fog.

Trees shed their bark in many different ways.

A reference to the illustrations will show the concentric rings of the horse-chestnut, the square pieces of the sycamore, which are due to the cleavage being both vertical and horizontal, the hexagonal shape of the divisions of the Scotch fir, the rugged bark of the Turkey oak, the sycamore and other species.

Where a woodpecker or a nuthatch has bored a hole into the living wood of a tree-stem, it is interesting to watch how the injury is repaired. New bark begins to form at the edges of the wound, and to this a layer is added each year, until at last the hole is filled up, and only a scar is left to show where it once existed.

I have been able to watch this repairing process going on for twelve years in the case of a Turkey oak, which was injured by lightning. I was watching the progress of the storm from one of our upper windows, and happened to be looking at this particular tree in the park, when out of a lurid cloud above it, a streak of forked lightning descended upon the tree, and rent off the bark of one side from the top to the bottom, carrying away portions of it to a distance of fifty feet or more, leaving a white gash which looked pitiful enough for many months. Year by year a wave of new bark rolls on, covering the bare place by slow degrees, but it is never destined to be quite healed in this case, for the inner wood was killed to some extent by the lightning, so it has become a home for the boring beetles, who are riddling it with holes wherein to lay their eggs.

Such a tree becomes a happy hunting ground for the woodpecker, who is attracted by the insect diet he finds there. The large holes he makes in getting at his prey will let in the rain, so that after a time the moist rotten wood forms a suitable place for various fungoid growths, and all these agencies work together for the destruction of the wood until the tree becomes a hollow stem, and the leafage above is solely produced by the sap carried upward by the bark.

Let us inquire a little more carefully into the formation of a tree-stem, and the different parts of which it consists. Some rather hard names are given to the four principal parts of a tree trunk but, by reference to the plate, and by knowing the meaning of the names, I hope they will soon be mastered, and then our future walks in the woods will be fuller of interest than ever, when once we understand something about the hidden work that is being carried on in those grand old trunks around us. A tree may be compared to a large manufactory. As we stand outside the building we see the brick walls and the roof, and smoke is coming out of the chimneys. We know that a great deal of work is being done inside, and carts are leaving its doors laden with the products of the machinery within, but how the work is done we cannot tell from the outside. We perhaps desire to obtain this knowledge, and under the guidance of the manager, we are taken from room to room and see the marvellous processes by which raw material is converted into exquisite fabrics, or it may be clay is turned into priceless china or porcelain. We leave the building full of wonder at the things we have seen, and those particular manufactures will ever afterwards be invested with a special interest for us, because we have seen with our own eyes how they are produced.

Just in the same way we shall look upon trees in a new light, if we are able in some measure to follow the processes nature is carrying on in them year by year so as to ensure the foliage, flower, and fruit, which minister so much to our pleasure and profit.

The four names we must learn about in order to understand the formation of wood are these. First the outer bark, called epidermis, from two Greek words _epi_ upon, and _derma_ the skin. _Cortex_, a Latin word meaning bark. Fibro-vascular bundles; this long phrase refers to certain threads or fibres which exist in stems and give them toughness and elasticity. From such fibres in the flax plant we obtain linen, and from the hemp fibres ropes are made. _Fibro_ comes from the Latin _fibra_, a thread or fibre; and _vasculum_ is Latin for a little vessel; we know the word better, perhaps, in another sense as _vasculum_, the tin box in which botanists place their plant collections.

These thread-like vessels are well called bundles, because they exist in little masses in the substance of the stem.

Most young people know what is called King Charles’s Oak in the stem of the brake fern, so plainly seen when it is cut across with a penknife. The dark markings are the ends of the fibro-vascular bundles which happen to resemble an oak tree in form, though some think them more like an eagle with outstretched wings, so the fern is named _Pteris aquilina_, from _aquila_, an eagle.

The fourth word is pith, the white substance in the centre of the stem, which can readily be seen by dividing a piece of elder branch, when the middle will be found full of white pith.

When we have these four parts of the stem clearly in our minds it will be possible to go on with our study and learn about the spaces between, which are filled with different kinds of cells.

The honey-comb formed by bees consists of small cells, little hollow spaces in which they store the honey or bee-food. Woody structure consists largely of cells of various shapes to contain sap and other substances. A beautiful specimen of cell net-work may be obtained by placing a thin slice of either white or yellow water-lily stem on a piece of glass and, holding it up to the light, a fine sort of lace-work will be seen. These are the cells which convey air and water through the stem up to the leaves and flowers. Or if we examine a flower petal with a magnifying glass we shall find it to be entirely composed of minute cells.

In these little spaces are stored very many and very different materials, all necessary to the growth of a tree; we shall try and learn about them by degrees; at present we must endeavour to obtain a clear idea of their structure.

A tree-stem increases in size yearly by the growth of fresh cells within the outer bark, and this active increase of tissue is due mainly to what is called the cambium layer, which is developed only in the spring and summer and does not exist in winter; it forms bast, or phloëm, on the outer side next the bark, and on the inner side next the pith it creates woody tissue.

Our English lime tree has a layer of fibre beneath the bark which is worth examination; it is the same in character, but not so wide or strong, as the bast which we import from Russia in mats to protect vegetation from frosts. Squirrels are very fond of this soft material; they strip it cleverly off the branches of our lime trees to form a warm lining for their nests.

It is easily found by cutting the outer bark off any small branch of lime within reach, when we can peel off the inner layer of bast, or phloëm, as botanists call it.

The phloëm from the lace-bark tree of the West Indies is like the finest possible net-work, and is used for many ornamental purposes. _Liber_ (Latin for the inner rind of a tree) is another term applied to this cell formation.

The study of different forms of woody fibre will be found most interesting.

I obtained one of my best specimens of it by placing a very old Swedish turnip in water for some months until the soft parts had melted away and only the round ball of fibre remained. If any one wishes to follow my example I would suggest placing the turnip and its pan of water in some outhouse where its perfume will not incommode any one. A maid came to me one day with a sad account of a fearful smell which had been noticed for some time in a lumber-room at the top of the house, and very naturally she thought that the plumber should be called in to remedy the evil. I had almost forgotten my interesting skeleton, but in due time I traced the odour to its right cause, and the turnip was banished to a distant spot, where many washings and some soaking in chloride of lime changed it into a really beautiful specimen of woody fibre. I possess now only a quarter of it, for botanists have so earnestly begged for pieces of it that I have been persuaded to share it with them.

I have sometimes picked up on the seashore old cabbage-stems bleached to a delicate ivory white, forming really beautiful instances of woody fibre. These we can prepare for ourselves, if desired, by soaking the stems in water until they can be brushed perfectly clean, and then bleached by mixing a little chloride of lime in water and letting them soak in it till they are white and free from odour.

In a manufactory there must of necessity be a series of windows on the different floors, not only to let in light but for purposes of ventilation. Now, the processes of tree-growth are carried on without light in the stem, but air is necessary, and it is supplied by means of small apertures called lenticels. These are not open holes, but are more like gratings which admit a small amount of air through loosely-packed cells.

These lenticels are the small brown specks which may be traced in great numbers on the young branches of almost any tree. They remain open through the spring and summer, admitting the needful air to the interior of the bark, but when the tree-growth is over for the season, and air is no longer needed, a layer of cork forms within the lenticel which entirely shuts it up and keeps out the wintry cold. Thus it remains sealed up till, by the growth of the cambium layer in the following spring, the corky barrier is split open and air is again admitted.

These lenticels are nature’s ventilators, opening and shutting in this curious way in order that the manufacture which is going on beneath the bark may receive from the outer air the various gases essential to the work which is being carried on within.

I have said that many and various things are stored in the stem-cells of trees. It would occupy too much space to attempt to make anything like a complete list of the liquids and solids which are obtained from trees, but I will enumerate a few of those with which we are familiar from their usefulness in every-day life.

Turpentine is obtained from various kinds of firs—the Scotch fir, larch, and others. Burgundy pitch from the spruce fir. A kind of tar is also prepared from Scotch fir and larch. From various kinds of cinchona we obtain quinine, so valuable as a remedy for fever. Camphor is a product of a Chinese tree. Tannin, by which skins are converted into leather, is obtained from the bark of the oak-tree. A kind of sugar is made from the sap of the maple, which is largely used in America. Gum arabic and a great number of gums used in medicine are produced by foreign trees of various kinds. The interior pith of a West Indian palm tree produces the sago of commerce.

Stems, like every other part of a plant, are to be seen in endless variety when we come to examine them for ourselves. In common garden plants such as the calceolaria and petunia, the consistence is soft, and such stems are known as herbaceous; these generally die down in autumn. Roses and rhododendrons have stems of a harder and more rigid character, and seem to be intermediate between the soft herbaceous stems and tall tree trunks.

If in some country ramble we resolve to make the trunks and bark of trees our study, we shall find much that is interesting and well worthy of observation.[7]

[7] For instance, I have noticed some curious examples of trees growing together. A Turkey oak and Silver fir in my own grounds are closely united at the base. The fir-seed and the acorn must have germinated in such close proximity that the stems have almost grown into each other. The group of beeches shown in the plate gives another example of interlacing stems and roots.

The Lombardy poplar, with its tall bending stem, the graceful willow and the silver birch, contrast strongly with the thick and sturdy trunks of the elm and oak. Even these two differ, the wood of the elm being short and brittle, whilst that of the oak is hard and flexible. Again, we may note the slender drawn-up stems of trees growing thickly together in a wood, where light and air are in a measure shut out, and compare them with other specimens standing in a park in free air and light. There we see trees, growing as nature intended, with grand sturdy trunks and welldeveloped branches spreading out on all sides. Lastly, in this chapter, we may note the climbing stems; these are especially numerous and diversified in their manner of growth. Almost every part is modified and adapted to assist the stem to climb. The common ivy develops upon the surface of its stem numerous rootlets, and by their clasping nature the ivy is enabled to ascend the smoothest tree-trunk. The hop and the convolvulus climb by means of their habit of twining around some rigid stem or twig. Then the peas and vetches send out little clasping tendrils in the place of leaflets, whilst that lovely ornament of the hedges—traveller’s joy—climbs by occasionally using the leaf stalk for a clasping holdfast. Not less interesting are the plants that climb by means of their hooks; the common bramble is of this kind; it scrambles over the hedge in a very enterprising and aggressive manner, while its spines and hooks effectually prevent it from slipping back.

A very highly developed organ of climbing is that to be found upon the stems of the small Virginian creeper (_Ampelopsis Veitchii_). On the points of its small tendrils we shall discover little globular, crimson-coloured pads, which, when pressed against a tree or wall, secrete a kind of vegetable glue. This fixes the tendril and enables the weak slender stem to climb upwards. In these instances, as well as others mentioned earlier in the chapter, we have again evidences of how wonderfully plants are adapted to their wants and environment.

Things to observe and collect:—The various ways in which trees shed their bark; how trees repair holes in the stem; fibres in flax stem and in hemp; specimens easily obtained by sowing linseed and hemp-seed; section of brake-fern stem; section of elder stem; thin section of white or yellow water-lily stem; flower petal; piece of bast matting; West Indian lace-bark; turnip and cabbage stalk prepared as specimens of woody fibre; lenticels on various trees; suitable leaves for skeletonising—holly, magnolia, tulip-tree, pear, poplar, aspen, mahonia, plum and maple; suitable capsules—poppy, stramonium, henbane, winter-cherry, campanula, and the calyces of the yellow-rattle.

CHAPTER IV

_LEAVES_

“These naked shoots
Barren as lances, among which the wind
Makes wintry music, sighing as it goes,
Shall put their graceful foliage on again,
And more aspiring, and with ampler spread,
Shall boast new charms, and more than they have lost.
Then each, in its peculiar honours clad,
Shall publish even to the distant eye,
Its family and tribe.”

+Cowper.+