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Mounting objects in fluid for higher powers is a much more difficult operation, and one we should not advise any amateur to attempt until he is well able to mount opaque objects satisfactorily. It requires a long experience before the difficulties of mounting in balsam or fluid can be overcome. Different classes of objects also require different modes of treatment, and some a long course of preparation by soaking in liquor potassa;, spirits of turpentine, or other fluid. If we give an outline of the process of mounting in balsam, it will be merely with a view of briefly indicating rather than attempting to teach.
Canada balsam is the most common medium for mounting. Take a sheet of paper, and mark thereon with ink the outline of a glass slide, 3 in. by 1 in. Carefully, by drawing lines from one angle to its opposite, ascertain the middle of the space, which will be indicated by the crossing of the lines. Having cleaned a glass slide, lay it upon the ruled space; let fall a single drop of Canada balsam upon the slide in its centre, just over the intersecting lines, which will be seen through the glass. Insert the object— a tuft of hair, a scale, or whatever else it may be—in the drop of balsam. Having cleaned a cover of thin glass of the requisite size, say £ in. diameter, place it on the balsam; press the cover gently, and the excess of balsam will flow out around it. Care must be taken that no bubbles of air are left beneath the cover. Place the object on one side for a week to dry. At this period, if desired, a black line of cement, asphaltc, or varnish may be laid around the edge of the cover with a camel-hair pencil. Do not omit to attach a label, number, or name to the slide as soon as mounted, or it may be forgotten. Some persons prefer, and for some objects it is preferable, to place the object first upon the glass slide, in the centre, and let the drop of balsam fall upon the object. Some also prefer, instead of pure balsam, to mix old and thick balsam with chloroform until of such consistence that a drop will fall easily from a thin rod. Youngpersons without experience will find " hard picture copal varnish " better than either, being less liable to enclose little bubbles of air.
Some objects arc only seen to advantage whilst living, and a good variety may be obtained from ponds, ditches, or small pools of water, containing water-weeds. These may be found adhering to the duckweed, anacharis, myriophyllum, under surface of water-lily leaves, or floating freely in the neighbourhood, according to their habits. The Green Hydra will be found in such situations, adhering to weeds, and also the fresh-water Polyzoa and some Rotifers; the Crown animalcule, Floscules, Melicerta, Plumatella, &c, and, floating freely, the Water-Pleas, Cyclops, larva; of various kinds, Water-Mites, and small Annelids. Vegetable organisms of a very interesting kind are to be collected from floating scum, or coating the mud. Diatoms and Desmids, the ambition of microscopists in these days, are very widely and generally distributed, and green Conferva: abound in every stagnant ditch. The rolling Volvox will in itself afford amusement and instruction for many an hour. All these should be examined whilst still fresh and living.
Objects to be looked over at leisure are almost unlimited. All insects intended for preservation, wholly or in part, should be kept in spirit and water, about two parts of water to one of spirit. They may be taken out and dissected or examined at any time, and in this manner tongues and feet of flies, claws and mandibles of spiders, stings and mouths of bees and wasps, antennae and legs of beetles, and very many others, may be kept in store for wet and wintry seasons, so that the microscope never need be unemployed for want of objects. A long list of common objects, with short descriptions and a great many figures, under the name of " One Thousand Objects for the Microscope," is sold for one shilling by the publisher of this volume, and contains all that kind of information which it would be quite impossible to include within the limits of this article.
Finally, we would give two or three general precepts, which it is desirable should never be forgotten. Let your object be presented to the microscope in the best manner possible for ascertaining what you wish to observe; let your light be good, steady, not too glaring, but properly adjusted and directed; and always examine every object first with the lowest power, going gradually to the highest. These arc the first elements of success. In addition, we may suggest three things which should not be done. The microscope should not be left exposed to the dust and the chances of accident when not in use; the glasses must not be wiped with harsh, dirty, or gritty rags or leather, but with a soft clean piece of wash-leather, kept specially for that purpose; and the microscope should not be lifted or carried by the tube or body, but always by grasping the lower portion of the stand. Never forget to have plenty of light, of patience, and of perseverance.
There is no royal road to any of the sciences, by means of which the student can acquire all the knowledge necessary to become its master, and avoid all the trouble. Neither is it desirable that knowledge should be acquired without labour, for that which costs but little is less valued, and commonly that which entails on us but little labour to learn is soon forgotten. Those who are in earnest in their desire to know something about plants will soon find their difficulties vanish one by one, and at last will have the gratification of looking back with pleasure on what they have achieved. How often have we heard grown-up men and women say, " I wish that I had learnt something of botany when I was young;" meaning thereby that they are sorry at having missed opportunities and advantages for the study of plants in a regular and methodical manner. All sciences must be studied in a regular and methodical manner, for there can be no science without method. It shall be our endeavour, as briefly and plainly as we can, to show how something may be learnt about plants, by explaining their structure, and the method employed in their classification. The space at our disposal will not permit us to do more than introduce the subject, and prepare the student to take up a book specially devoted thereto, and read it with understanding and profit.
Let us go into the lane, and pluck the first flower that we sec blooming on the hedgerow—it matters not what so long as it is in bloom. It will not answer our purpose to have the blossom only, with just a few inches of the stalk, but the whole plant must be taken up, the fragments of mould carefully shaken from the roots, and the plant laid before us on the tabie. Hence we do not require anything very large: it may be a daisy, or a buttercup, or a primrose, a speedwell, or a dead-nettle, but we must have the whole plant. An oak tree is a plant, but we require nothing so large; duckweed is a plant also, but we desire nothing so small. Nevertheless, what we have to say of one will to a large extent be true of all, for our first object is to learn something which applies to all flowering plants.
The plant lying before us has a root, a stem—perhaps also branches, whiclj are only offshoots or parts of the stem—leaves, and flowers. This is the enumeration of the parts of the plant which anyone unacquainted with botany would give. The stem may be very short, as in the daisy and primrose, or long as in the buttercup and dead-nettle, but to all there is a very decided and palpable root. It is often difficult to determine exactly where the stem ends and the root begins. The root is, in fact, only the low er and underground portion of the stem. Some botanists call the root and stem together the axis of the plant; whatever name is adopted, there is a very close connection between them. It will be better for us to regard them as distinct parts in the following remarks.
Before proceeding any further, we must at once assure our reader that we are no believers in botany without hard names or technicalities. Hard they are not, to any except the ignorant, and may be easily remembered if their meaning is understood. All our writing, if we avoided technicalities, would be useless, since any other book on the same subject, in which the terms employed in the science are used, would be just as unintelligible as though we had not written at all. The one great object, therefore, which we have in view is to make the workman acquainted with his tools, their names, and how to use them. The apprentice will learn to distinguish a plane from a hand-saw before he constructs a writing-desk or a chest of drawers.
After this brief digression we come back to our root again, and must ask our reader to think of all the different forms of root which he has ever seen, and class them together in his own mind under two groups, namely, those which are simple, or are merely single continuations downwards of the stem, and those which are compound, or composed of two or more parts starting from the same point. As, for example, the radish, the carrot, the turnip, the dandelion, and the speedwell have all single roo'.s. They may be branched as they go down into the soil, but they are only single continuations of the stem. On the contrary, the dahlia, the onion, and many of the grasses possess a bundle of roots starting from the same point, which are sometimes branched, and sometimes are not. Following the plan which we have suggested, our pupil will soon arrive at the conclusion that, although the kinds which we have suggested, and many more which he may think of, are all roots, they have a very different appearance; and whilst it is quite correct to call them all roots, if we would distinguish one kind from another, we must have a name for each which would indicate its character, without giving us the trouble of making a drawing of the root, or using a long description. Here, then, we sec the first necessity for the use of words or terms which all botanists will understand. Being agreed that for the different forms of roots different words should be employed, we will enumerate the most common.
A carrot or a parsnip are familiar examples of a kind of root which is thick and fleshy above, gradually tapering downwards to a point, like an inverted cone. Hence such a one is appropriately called a conical root. But if the root, instead of being largest at the top, thickens towards the middle and then diminishes again downwards, so that it decreases in both directions, like the roots of many varieties of radish, it becomes spindle-shaped, and is called a fus1form root. The turnip has a root, however, which resembles neither of these, and when well grown is nearly the shape of a boy's top. This may be called a turnip-shaped root, but the term generally employed is napiform, the word "napus" being the Latin for "a turnip." The common form of simple root, which proceeds downwards as a continuation of the stem, without enlarging, but becoming gradi al y thinner and thinner, often much branched, nccasionally with only thread-like rootlets issuing from its sides, is known as a tap-root. It is not distinctly conical as in the root of the carrot, and is the commonest form of root amongst herbaceous plants.
Of compound roots, or those in which a bundle of little rootlets proceed from the b:\se of the stem, a tuft of grass, or, still better, a stem of wheat or barley, affords an example. These rootlets, or little roots, being thin and thread-like, the tuft is called a fibrons root. When the rootlets are thickened, so as not to be thread-like or fibrous, but are still clustered together in a kind of bundle, it is called a fasciculated root, from the Latin word "fasciculus? which is often employed in botany, and means "a little bundle." There are modifications of form in toe rootlets which compose the fasciculated root, as in the dahlia, in which each rootlet is thick, fleshy, and of a fusiform shape; in some others a portion only of the rootlets are thickened or swollen cither once or several times throughout their length.
It will not be out of place here to remark that the bulb of the onion, the white lily, and many similar plants, is not a root, but a kind of bud composed of scales closely overlapping each other, and growing upon a button-shaped stem, from the under surface of which the fibrous root is produced. The potato (that portion which is cooked as a vegetable) botanists do not class as a root, but as a tuber, or swelling of the underground stem. Some roots last only one year, and arc said to be annual; others last two years, and are called biennial; whilst others continue in vigour a longer period of time, and are said to be perennial.
The roots of plants serve a twofold purpose — to attach the plant to the soil, and to furnish it with the means of sustenance. For the latter purpose, the extreme ends of the thin fibres of the rootlets are of a more delicate and spongy texture, and by their means water, and the materials diffused through water, are taken up and conveyed to the plant. These spongy ends of the rootlets are called the spongioles.
Certain plants possess the power of producing additional roots, or organs having some of the functions of roots, according as they may be required for the purposes of the plant. These organs are termed adventitious roots, which, in the ivy, arc like suckers growing from the stem to attach it more firmly to the tree or wall which supports it. In a species of Indian fig called the banyan. these adventitious roots droop like ropes from the branches till they reach the ground, when the lower, or growing point, buries itself in the soil, makes a true root, and that which was at first but a swinging rope becomes an additional stem, scores of which may be produced by one tree, furnishing it with fresh means of support.
From the root of our plant proceeds the stem. This is another of the parts to which we have referred as essential to flowering plants. Sometimes the stem is so short that it can scarcely be distinguished, but it is commonly a very prominent feature. Whether this stem stands erect, or supports itself by twining around or clinging to another, or lies prostrate upon the ground, it is still a stem. If we cut across any stem, branch, or twig of a woody plant, such as a tree or shrub, we shall find, amid a great variety in detail, a uniformity in plan in all British trees and shrubs. The outer circle or circumference wfll be the bark; the inner or central point, the pith; and between this pith or medulla and the outside bark, the woody portion is deposited in layers, which appear as rings when a section of the stem is made, with lines called medullary rays cutting them from the centre to the circumference.
This is the structure of all exogenous plants; and as all British trees and shrubs are exogenous, it is the structure of all British trees and shrubs; another type of structure exists in the stems of endogenous plants, as in the palms of hot climates; and a third type in tree-ferns. Although the latter are not natives, we cannot on that account omit from a sketch of botany (though but a brief one) the principal features in the stems of the two groups or classes of flowering plants which are known botanically as exogens and cndoget1s.
Having given figures of the section cut across a stem of each kind, it will be seen that their appearance is very different. There is no true external bark in that called the endogen, no defmite central pith, no rings of wood, and no medullary rays traversing the wood from the centre to the circumference.
But why are they called exogens, and why endogenst
The circles of wood shown in the section of an exogenous stem disposed one outside the other, represent periods of growth and repose. In temperate climes like our own each circle represents the wood which the tree has produced during the growth of one year, and which is deposited outside the growth of the previous year. Hence the outer circle is always the last formed, so that the newest wood is on the outside. Additions to the circumference of the stem, therefore, arc made in an outward direction.
Exogen is a word of Greek origin, meaning "to grow outwards," and is applied to those plants which make additions to the thickness of their stems by layers of wood deposited the one outside the other at every period of active growth. In such stems there is an outer bark which may be separated from the wood, and between this bark and the wood all additions to the bulk of the stem arc made.
Endogen is also a word of Greek origin, and its meaning is "to grow inwards." It was given to the class of plants to which it is still applied upon the assumption that all addit'ons to the bulk of the stem were made from the centre. The exterior of such stems have no separable bark, and the interior consists of a soft cellular substance, in which bundles of a harder and more woody texture are produced; these are at first directed to the centre, but after