CHAPTER VII
THE MICROSCOPE AND PLANT LIFE
The science of botany consists of many branches and, in most of them, the microscope is the scientist’s constant aid. The study of bacteria, really a branch of botany, we have dealt with in [another chapter], so here we will omit these interesting though lowly plants. By far the number of botanical objects for the microscope consist of sections—exceedingly thin slices of whatever portion of the plant is being examined, cut either with a sharp razor or a special instrument called a microtome. Section cutting, though not a difficult accomplishment, requires a considerable amount of practice and cannot be learned from a book; all our descriptions, therefore, will be confined to objects from the plant world which may be studied without the assistance of razor or microtome.
One cannot help being struck with the fact that green is the prevailing colour among plants and the reason is not far to seek. If we take a cabbage leaf and carefully tear off the skin, we shall find green spongy matter below. A little of this green material may be examined under the microscope and will show us rounded green bodies composed of a substance called chlorophyll. Now chlorophyll is absolutely necessary to all plants, except the fungi and to one or two parasitic plants. It is necessary because, by its aid, plants can build up raw food material into food which will be useful to them. It is not formed in darkness; that is why a board, a roller or any similar object left on a lawn, causes the grass below to turn yellow; it is the reason also why certain parts of plants, not usually green, turn that colour when exposed to the light. Chlorophyll does not always occur in round globules, sometimes it is found in bands.
One of the most interesting botanical studies for the microscope is furnished by the leaf of the American water-weed. This plant, which was introduced into the country from North America some years ago, has now spread far and wide and is easily obtained. A leaf which is slightly yellowed with age is the best to take for the experiment. It should be cut from the plant, placed at once in a small bottle of water and kept warm for a few hours; this may be accomplished by keeping the bottle in one’s pocket. After a sufficient interval, put a drop of water in a clean slide, put the portion of leaf in the drop of water, cover with a coverslip and examine with a moderately high power. If the experiment has been properly carried out a wonderful sight will reward us. We shall see that the leaf is divided into a number of divisions called cells; this name has been handed down from the very early days of the microscope, because of a supposed resemblance to the cells in a bee’s honey comb. In each cell we shall see signs of activity, the little round grains of chlorophyll are there, but instead of being stationary, as in the cabbage leaf, they are slowly moving round the walls of each cell. In reality they are carried along in the stream of living matter within the cell. It is a wonderful sight and brings home to the observer very forcibly a fact which is liable to be forgotten, that the plant is just as much a living being as an animal. Perhaps our experiment will not succeed at the first attempt, then we must try again; maybe we have been too rough in detaching the leaf or we have not kept it sufficiently warm. Sometimes the movement may be started by slightly warming the slide over a flame; too much heat, of course, will kill the leaf.
We shall see this green colouring matter over and over again in our botanical studies, in fact it is found in all manner of situations, in leaf and stem. Very often its colour is hidden by sap of another colour, as for instance in copper beech leaves or in the brown seaweeds. Chlorophyll dissolves in alcohol, however, and this affords us a ready means of detecting its presence though we cannot see its green colour. If we boil any leaf, suspected of containing chlorophyll, in alcohol we shall obtain a solution with rather peculiar properties because, when held up to the light it appears green, but when light is reflected from it, it appears reddish.
From the under side of the cabbage leaf whence we obtained our first specimen of chlorophyll, we must now take another piece of skin. If we perform the operation properly the skin will be colourless, like a piece of thin parchment; any green colour will show that we have torn off more than the skin and we must make another attempt. Having secured our piece of skin we place it in a drop of water on a clean slide and examine it under the microscope. We first notice that the skin is divided up into a number of small areas called cells and dotted here and there amongst the cells are several oval bodies, containing chlorophyll. These oval bodies are the pores through which the leaf breathes, amongst other things. In the centre of each pore there is a hole, at least there is if the pore is open, for the two cells comprising the pore have the power of opening and closing.
It is interesting to try the same experiment with a fern leaf and to notice that there are pores, very similar to those of the cabbage, but that the walls bounding the cells of the leaf are irregular and that they contain chlorophyll. We may try several other leaves and also the upper and lower surfaces of leaves, then we shall soon learn that, in leaves with distinct upper and lower surfaces, there are far more pores on the lower than the upper surface; leaves like those of the iris have almost the same number on each side, and floating leaves, like those of water lily have all their pores on the upper side. There is a reason for this; the pores are likely to become filled with dust, being on the lower side they are protected somewhat; flat leaves, by their shape, afford no protection and floating leaves must have their pores on the upper surface to obtain air.
There are many other interesting things we may learn about leaves, with the help of our microscope. The cabbage leaf is quite smooth, but if we are observant we shall have noticed that sometimes each leaf appears as though it had been powdered, it has a decided bloom. The bloom does not appear on the leaf for ornament but for a purpose. It is a waxy substance and it prevents the leaf from losing moisture too quickly in dry weather. This is very important for the plant; if the moisture taken up from the soil were lost in the air too quickly by the leaves, the plants would wither and eventually die. It is not all plants which can wear a protective covering when danger threatens, most plants have either no protection or are permanently protected. There is a large class of plants with folded or rolled leaves; heather and marram grass belong to this class. We must examine some of these leaves and we shall find that all the pores are on the inside of the leaf whether it be folded or rolled. The reason for this is that moisture also escapes through the pores and, when they are thus protected, it is not carried off too quickly by drying winds.
Many plants are protected, as far as their leaves are concerned, at anyrate, by hairs. They take the place of the bloom in such plants as the cabbage. There are thousands of plants with hairy leaves and they will provide as many interesting objects for our microscope. Let us examine as many as we can for the hairy covering of each plant will be a little different to the one we examined previously. There are simple hairs, quite ordinary affairs, forked hairs, branched hairs, T-shaped, star-shaped and club-shaped hairs. If we are clever with our microscope we shall notice that, however complex each hair may be it is really nothing but one cell of the skin of the leaf which has assumed a peculiar shape.
The leaves of the nettle are armed with ordinary and stinging hairs; the latter are worth examining and we shall notice that there is one great difference between all the other hairs we have examined and the stinging hairs of the nettle. The former are of one cell only, the latter of several cells. A high magnification will show that the stinging hair of the nettle is not quite so simple as it appears at first sight.