CHAPTER XVII.

THE TISSUES BUILDING UP THE PLANT BODY

In our study of plants up to the present, we have only looked at their structures from the outside. We have examined the form, uses, and life of the parts of their bodies without looking for the details which might answer the question--“How are they built up?” Just as a house as a whole has a definite form, with rooms, and doors, and windows, each with their definite form and use, and at the same time every one of these things is built up of small separate bricks, tiles, pipes, and pieces of wood: so we find that the whole plant is composed of a number of definite parts, which are themselves built up of tiny individual parts, which we may take to correspond with the bricks of a house. Of course, they do not do this completely, for a plant is a living thing, and is far more complicated than a house, and each of the tiny individual parts is also a living, growing thing. These little building structures are called ~cells~ both in plants and animals, and they are so very small that you cannot study them fully without a microscope, and that is a very complicated and expensive thing, so we will leave it alone and only study what we can see of the structure of plants without it. Try, however, just to see a few cells under the microscope, so as to know what they are like. A ~typical cell~ has a ~wall~, within which is the actual ~living substance~, a clear, jelly-like mass, which contains many granules of food and stored material. Within this living substance is a more solid mass of still more actively living substance, which is called the ~nucleus~. Cells like these, or cells which were like these when they were young, and which have become modified for special work, build up the whole plant body (~see~ fig. 91).

You can get a good hand magnifying-glass for several shillings, and with this and a very sharp knife, you can find out something of the structure of the insides of plants, even though most of the cells are so small as to be out of sight except when looked at through a microscope.

Let us first cut as thin a slice as possible across a water-lily stem, and put it on a small piece of glass and hold it up to the light. Examine it with the magnifying glass, and you will see that it is not a solid mass of tissue, but that it is built up of a fine network like lace, with quite large spaces between the threads (~see~ fig. 92). These spaces are air spaces, and the fine lace-work threads are meshes built up of single rows of cells, which you may be able to see if your glass is a good one. Cells may be packed loosely like this, or they may be in more compact form something like a honeycomb, as you may see in the pith of an elder twig and many other stems. You can crush these soft cells between your fingers, and we cannot imagine that they could build up the hard, firm branches of trees.

Now examine the stem of a seedling sunflower, by cutting a very thin slice across it; you will see in it a ring of strands where the cells are smaller than those of the soft tissue and also much more closely packed (~see~ fig. 93). Then cut a thin slice longways down the stem, and you will see that these more solid strands are the cut ends of long strings of such tissue which run through the stem. The cells which build up these strings are not quite ordinary cells, but are exceptionally long, like water-pipes, and they have thickened walls. These cells do the carrying of water and liquid food up and down the plant (~see~ fig. 94).

You can see that the water travels up these cells if you cut across a stem near the root and place it in a little red ink. After a few hours if you cut a section several inches from the bottom of the stem you will find that these strands are coloured red by the ink which has passed through them, while the rest of the stem is very little coloured, or quite colourless. This shows us that these strands are the special water-pipes of the plant.

Large numbers of such cells closely packed together, and with some other hard cells between them, make up the wood in woody stems. Cut across a small twig of lime or oak and examine it with your lens. Outside is the brown bark, then within that some green cells and a little soft tissue, while most of the stem is made up of a mass of hard wood cells, among which you can see some of the larger water vessels distinct from the rest. All this hard tissue really corresponds to the joined up separate strands which we saw in the sunflower stem (~see~ fig. 95). Trees like the lime and oak, which live for a long time, grow for a certain amount every year, and each year they add a ring of wood to their stems. In old stems you can see clearly the rings of wood which have been formed by each year’s growth. This is another way of telling the age of the stem, and you should compare your results from this method with those you got from counting up the bud scars (~see~ p. 75).

Leaves, as you know, require much water, which comes to them up the stem through the “water-pipes.” You saw already the course of the water-pipes in leaves, for they are the “veins” which we found sometimes make a complete network, and sometimes run parallel in the tissue of the leaf. If you put a leaf stalk in red ink, you will see that the veins are connected with the water-pipe strands in the stalk, for they will both get coloured by the ink as it passes along them.

Just as in animals the whole body is covered over with a ~skin~, so in plants we find a special outside sheet of cells, which protect the inner tissues and form a thin skin. You can get this off very well if you break across an iris leaf, and pull along the thin, colourless layer on the outside. If you examine it with your lens, you may perhaps see something of the mosaic-like pattern of the cells which build it up. You should certainly see that it is colourless, although the tissue of the leaf beneath it is quite green.

On the large branches of trees and the bigger plants, we do not find this delicate protecting layer, but instead there is a thick brown ~cork~. When the cork layer gets very thick it splits irregularly as the tree grows too big for it, and so forms a rugged bark. The cork layers have much the same duty as the fine skin, only they are thicker and stronger, and more suited to hold out through the winter. You know already from daily life the practical use of cork, for you put it into bottles to keep the liquid in the bottle and the damp and dust in the air from entering. Just what the cork does for the bottle, the sheets of cork wrapping round the branches do for the plant. They prevent it from being dried up by cold winds, and they keep out the heavy rains of winter which would injure it. Roots have a cork coating also when they get old. As you may remember, it is only the tip of the root which can absorb water for the plant, so that in the young part of the root a cork layer would be very much out of place, and you will never find it there. You will find instead the little delicate root-hairs, which absorb water and pass it on to the older parts; these old parts do no more absorbing, they are only the water carriers and food storers, and so have no hairs and are protected by a layer of cork.

As we found before, plants ~breathe in~ air like animals, and you may ask how they can do this when they are covered with their thick air-tight layers of cork. Examine a fairly old elder twig, and you will see all over its brown skin numbers of darker brown spots. If you look at these with your magnifying glass, you will see that they are quite spongy and soft. They are the special entrances for air, and are the breathing spots or ~lenticels~ (~see~ fig. 96). They are to be found in all corky stems, although they are not always so easy to see as in the elder.

On the leaves and stems of many plants you will find a large number of ~hairs~. In some cases there are so many as to make the whole plant quite woolly, like the mouse-ear leaves. These hairs are protective, and keep the leaf warm and dry, and in some cases may shelter it from the sun. Hairs may consist of one cell, or several in a row, or of cells which are branched in a complicated way. Certain hair-cells protect the plant by stinging, as you can see if you watch a nettle-leaf with your magnifying-glass, and then rub your finger along it, only touching the hairs. You will find that it is they which sting you, and not the leaf itself.

Now we have found several kinds of tissues in plants, the ~skin~ and ~cork~ covering all over and protecting the rest; the ~central vessels~ or ~water-pipes~, corresponding to the veins and arteries of animals, the soft ~white ground~ tissue, which in some stems may be very loosely packed, and the soft ~green tissue~ in the leaves and young stems, which we found was the food-manufacturing part of the plant. There are also strands of simple ~strengthening tissue~, both by the water-pipes and in separate bundles in the soft tissue; these we may take as representing the bones of animals.

We have noticed (Chapter VIII.) that plants are sensitive to light and bend towards it, that they feel heat and cold, and that the stem and root seem to know when they are growing in the right or wrong positions, and bend accordingly. We know that we ourselves and the animals recognize such things by the help of nerves which carry messages to the brain. But where is the brain in plants, and the nerves? No true nerves have been found in plants, and it seems as though different parts of the plant were specially sensitive without there being any “brains.” So that we cannot speak of a central nervous system in even the highest plants as we can in the animals. In this respect they are built on quite a different plan from animals.