LESSONS IN BOTANY.-III. SECTION IV.-STRUCTURE OF THE STEM OF VEGETABLES. THIS is a very important point, and helps to furnish us with a means of dividing plants, at least flowering plants, into two primary groups or divisions. Let us consider that which takes place during the growth of an oak from the acorn. The acorn, on being planted in the ground, sends down its root, and sends up its stem. At first this stem is a tiny thing of very inconsiderable diameter; year by year, however, it grows, until a gigantic tree results. If we now cut this tree across and examine the structure of its section, we shall recognise the following appearances. In the first place, commencing our examination from without, we shall find the bark, or cortex (Latin, cortex, bark), separable into two distinct layers, the outer of which is termed the cuticle (Latin, cutis, skin), or epidermis, (Greek enidepuis, pronounced ep-i-der-mis, the outer skin), and the inner one the liber, so called because the ancients occasionally employed this portion of the bark as a substitute for paper in the making of books-liber being the Latin for book. Passing onwards, we observe the woody fibre and its central pith. The woody fibre itself is evidently of two kinds, or at least is so put together that wood of two degrees of hardness results. The external portion of wood is the softer and lighter in colour, and termed by botanists alburnum, from the Latin word albus, white; the internal is the harder, and termed by botanists duramen, from the Latin durus, hard, although carpenters denominate it heart-wood. Lastly, in the centre comes the pith or medulla, from the Latin, medulla, the marrow, which traces its origin to another Latin word, medius, the middle, the marrow being in the middle of the bone. Regarding this section a little more attentively, we shall

13.

12.

16.

observe passing from the 10. HORIZONTAL SECTION OF AN EXOGEN.

pith to the bark, and

establishing a connexion between the two, a series of white rays, termed by

gress, the winter to which they are exposed being so short, that their course of growth is scarcely interfered with by any impediment. Under these circumstances, there is scarcely any winter pause sufficient to create a line of demarcation between ring and ring; the progress of deposition goes on continuously. However, the manner of deposition is not the less external be cause we cannot see the rings.

Very different from this method of increase is that by which another grand division of plants augments in size. For an example we must no longer have recourse to a section of a plant of our temperate zone, but must appeal to the larger tropical productions of this kind. If we cut a piece of bamboo, or cane (with which most of us are familiar), horizontally, we shall find a very different kind of structure to that which we recognised in the oak. There will be no longer seen any real bark, nor any pith, and the concentric rays will be also absent, but the tissue of which the stem is made up may be compared to long strings

11. HORIZONTAL SECTION OF AN ENDOGEN. 13. DOTTED VESSELS OF THE MELON. 14.

the botanist medullary rays, and by the carpenter silver grain. We shall also observe that the section displays a series of ring-like forms concentric one within the other. These are a very important characteristic. They not only prove that the trunk in question was generated by continued depositions of woody matter around a central line, or, in other words, by an outside deposition, but they enable us in many cases actually to read off the age of any particular tree-the thickness corresponding with one ring being indicative of one year's growth. Inasmuch as the formation of an oak tree is thus demonstrated to be the consequence of a deposition of successive layers of woody fibres externally or without it is said to be like all others subjected to the same kind of growth, an exogenous plant from two Greek words, tw (ex-o), without, and yevváw (genad-o, g hard, as in gun), I generate.

Fig. 10 represents the internal structure of an exogenous

stem.

It is true that the peculiar disposition of rings thus spoken of cannot always be recognised. For example, as a rule, trees which grow in hot climates are checked so little in their pro

VOL. I.

LACTIFEROUS VESSELS OF THE CELANDINE. 18. ANGULAR CELLS.

of woody fibre tightly packed together. These concentric rings, in point of fact, could not have existed; inasmuch as a cane does not grow by deposition of woody matter externally, but internally, or, more properly speaking, upwards. A young cane is just as big round as an old cane, the only difference between them consisting in the matters of hardness and of length. Hence, bamboos, and all vegetables which grow by this kind of increment, are termed endogenous, from two Greek words evdov (en'-don), within, and γεννάω (gen-na-o), Ι generate. The largest specimen of endogenous growth is furnished by palm trees-those magnificent denizens of tropical forests to which we are so much indebted for dates, cocoa-nuts, palmoil, vegetable wax, and numerous other useful products. Fig. 11 is a representation of the section of a palm tree, in which the peculiarities of endogenous structure are very well developed.

All the endogenous productions of temperate climes are small, though very important. In proof of the latter assertion it may suffice to mention the grasses; not only those dwarf species which carpet our lawns and our fields with verdure, but wheat, barley, oats, rice, maize, all of which are grasses, botanically considered, notwithstanding their dimensions. Indeed, size has little to do with the definition of a grass; for if we proceed to tropical climes, we shall there find grasses of still more gigantic dimensions. Thus the sugar cane, which grows to the elevation of fifteen or sixteen feet, is a grass, as in like manner is the still taller cane, out of the stem of which, when split, we make chairbottoms, baskets, window-blinds, etc., and which, when simply cut into convenient lengths, is also useful for other purposes; one of which will, perhaps, occur to some of our younger readers.

The reader will not fail to remember that we, a few pages back, divided vegetables into phænogamous and cryptogamic (we are sure we need not repeat the meaning of these terms). We may now carry our natural classification still further, and say that phænogamous plants admit of division into exogenous and endogenous ones. This division is quite natural, even if we

6

dmacy of the stem; but the and recognizable by other -!· when we come to consider • faves and seedia,

WAVES AND THEIR USES. teaching the nature of a thing; one - таре. Of these the latter is it the former is the more precise. Sommence by stating that in f definition as a thin dermis, containing between its two Ar tiene, nerves, and veins, and per -halation and respiration." Such is

Jaf. Probably the learner may stret. but a little contemplation, Vira the object of enabling him to Innose we go through its clauses one attened expansion of spidermis, -vient expression. The epidermis 17 stated, the outside bark-at least, Nation. Literally, the Greek word rare said above, and is also applied the ammal skin which readily peels „ction of a blister, and which, when nstitutes those troublesome pests on ms. As regards the epidermis of e seen in the birch tree, from which Vell, a leaf, then, consists of two e soove and the other below, closing ne meaning of which terms we have The word ascular means ** conand is derived from the Latin ee, which is derived from Lice or Cary, nears, consisting of * mang those ittle pipes or tubes quies Just 'ike arteries and veins

wich serve the purpose of conPrickit to another. In plants. weddingly smail that their tubular i by the aid of a microscope or who may be recognised by tik Mottie loubt that most

Still

an example the reader may refer to an orange, especially an orange somewhat late in the season. If the fruit be out, or, still better, pulled asunder, the cells will be readily apparent. more readily do they admit of being observed in that large species of the orange tribe to which the name shaddock, or forbidden fruit. is ordinarily given.

We must now inform the reader that not only do the cells of this cellular tissue admit of being altered in form, but occasionally they give rise to parts in the vegetable organisation which would not be suspected to consist of cells. The cuticle of which we have spoken is nothing more than a layer of cells firmly adherent; and the medullary rays, or silver grain, of exogenous stems, the appearance of which has been already described, is acthing more nor less than closely compressed ceilniar tissue.

We commenced by describing a leaf, but observations have been so often directed to matters collateral to the subject that the description appears somewhat rambling. Nevertheless, it cannot be helped. In Botany, above all other sciences, there oreur many curious names. They must be learnt, and the best way to teach them is to describe them as they occur.

A leaf, then, we repeat, is an extension of two flat surfaces of cuticie enclosing nerves and veins, vascular and cellular tissue. Al these terms have been pretty well explained. We may add, however, that when cellular tissue exists confusedly thrown together, as it does in the substance of a leaf, or as it appears in the range, then such cellular tissue is denominated parenhuma, from the Greek word zapévxvua (pronounced par-entuna anything poured out.”

Before we quite inish with our remarks relative to the substances when enter into leaves, it is necessary to observe that the green colouring matter of leaves is termed by botanists and by nemists Moronhyi, from the two Greek words xλwpós (pronounced tons), yellowish green, and puλλov (pronounced nm), a leaf. This chlorophyl is subject to become siennared in autumn, as we all know, but the cause of this alteration has not yet been explained.

de we orces, and raw from rand a which meet in B, then the haha tha C4Ong Prink thus formed is the minud kad duction, of o P and o q Now, I shall not here give you the strict mathematical proof of this proposition: it is too complicated, and involves so much close reasoning, that to force it on a student in the beginning of a treatise on me mary difficulty in his way you have become more and then return to it. In Ives that it is true by a mments, one derived from

...;hta, U V W, be attached to knotted together at o; and the third, with their at

130

The had bes nove wer de ingani, mi mes OFLY a te sme me that I porosis moved over the sides. Tuet ud not to i te resitant of two frames was not repreenter a magle and firection by the ingetal Instruments are itel i for enteroms by when me experiment can be made and the rent ways is as I have stated.

Ling the principle, then, as estabushed, ist as observe its consequences. In are gre two furous, seting at a point, and you want their want a you will mimediately say, a let gram of the two formes, and me goal is the require l Nut so fast. 7 med not describe the whole of that igre, a part will suffer. Now, I brom the end a of o A, you irava a parald and equal to 3, it is tear you do not want to draw 32 at all. A pvUS YOU the far end of the resuitant, and all you have to do then is to join B with o, and your object is a gained Thus your para!lelogram of forces suddenly becomes a triangle of forces; and you may lay this down as your rule in future for compounding two forces.

Fig. 5.

Draw from the extremity of one of the forces a line equal, and parallel to, the other force; and the third side of the triargle so formed by joining the end of this line with the point of application is the resultant.

There is great advantage in this substitution of the triangle for the parallelogram, for it saves the drawing of unnecessary

lines, which, as you will see, when many forces have to be compounded, would cause much confusion in your figures.

R

R2

Let us apply this principle now to compound any number of forces acting on a point. Let there be five, and that will illustrate the rule as well as a thousand could. Suppose forces, o A, O B, O C, O D, O E, applied to the point, o. By the triangular rule, if I draw A R equal and parallel to o B, the line joining o with R is the resultant of the first two forces. I shall not actually draw this line, o R; let us suppose it drawn. Now, if I compound this resultant with o c, I have the resultant of three of the forces. But that, by

E

Fig. 6.

R3

the same rule, is got by drawing from R a line R R, equal and parallel to o c. The line o R is this resultant of three. Again we shall not draw it. The resultant of this and o D, for the same reason, would be o R, got by drawing R, R, parallel and equal to o D, and, lastly, the resultant of this and o E would be o R, the line, R, Rg, being equal and parallel to o E. We have thus exhausted all the forces, and evidently o R, is the resultant of the whole five. There was here no confusing ourselves with parallelograms; all we had to do was to draw line after line, one attached to the other, carefully observing to keep their magnitudes and directions aright. A kind of unfinished polygon was thus formed, and the line o R,, which closes up the polygon, joining the last point B, with the point of application, is the resultant in magnitude and in direction. Thus you have made another step in advance, and arrived at the Polygon of Forces. You have learned how, by the mere careful drawing of lines, to determine the resultant of any number of forces. All you require is paper and pencil, a rule, compasses, a scale, and a pair of parallel rulers.

Now, there is one point about this polygon I wish you carefully to note. You will observe that the arrows on its sides, representing the directions of the forces you have compounded, all point from left to right, as you go round the figure, turning it with you so as to bring each side in succession to the top. The resultant, however, points in the opposite direction, from right to left, when that side is uppermost. This is as it should be; the direction of the resultant, as you go round the figure, must be opposite to those of the components. The use of this you will see in the next lesson.

It

Now, let us suppose that, in determining the resultant after this method, as we come to the end of the operation, the end, B of the last line, R, R,, chanced to coincide with, or fall upon the point of application, o. The polygon would close of itself without any joining line; what is the meaning of this? means that there is no resultant; the line, 0 R, is nothing, and therefore the resultant is nothing, and the forces produce equilibrium. What a valuable result we have arrived at! A method by which we can, by rule and compass, tell at once whether any number of forces make equilibrium at a point or not. have to do is to describe the polygon of forces, and if it closes up of itself, there is equilibrium; if it does not, there cannot be equilibrium, and the resultant is in magnitude the side which is necessary to close the figure.

All we

Deferring the further expansion of this subject to the next lesson, I shall now turn back and apply these principles to a few elementary examples.

First Example.-Three equal forces act at a point in different directions what condition should they fulfil in order to be in equilibrium? Get your ruler and compass, and commence constructing the figure by which their resultant may be found. From the end of one of the forces you are to draw a line equal and parallel to the second equal force, and from the end of that another line, equal and parallel to the third. You will thus have three lines strung together, all equal to each other. But if the forces are in equilibrium, the end of the last line must fall on the point of application, that is to say, the polygon of forces must close up, and form a triangle. Your construction will then give you a triangle of three equal sides, commonly called an equilateral triangle. But such a triangle must have

all its angles equal; also the angles between the sides of the triangle, or of the polygon of forces, are the angles between the forces themselves, since they are parallel to these forces. This is evident from the properties, 1 and 2, of the parallelogram referred to above; therefore, in the case we are considering, the three equal forces must act at equal angles, as I showed otherwise must be the case at the close of the last lesson.

P

Second Example.-Let a weight hang from the ceiling by means of two cords of unequal length, as in Fig. 7. It is evidently at rest. Whatever be the forces called into action, they produce equilibrium. Is there nothing further to ascertain? There is; it may be desirable to know by how much each cord is strained. Our assurance that the cords will support the weight depends on this knowledge. Let P and Q be the two points of support of the strings which meet at o. Now, whatever be the strains on the cords, O P, o Q, they make equilibrium with w, the weight. Therefore, if we suppose a length, o A, of O P to represent the strain on o P, and from A draw a line, A R, parallel to o Q, equal to the strain, o B, on o Q, then, since the three forces are in equilibrium, the line, R O, closing up the triangle must be equal to, and be in the direction as, the third force, or weight, w. This, then, tells us what to do. Measure on o R upward as many inches as there are pounds in w; and from R then draw RA parallel to the cord o q to meet the cord o A. The number of inches in o A will represent in pounds the strain on o P, and those on RA the strain on o Q. All, therefore, that we desire to know is determined.

Fig 7.

Third Example.-A horse pulls a roller up a smooth inclined plane or slope; what is the force he must exert when he just keeps the roller at rest? And by how much does the roller press on the plane?

B

A

Let the horse pull in any direction, o A. Then there will be three forces acting on the roller; namely, its own weight right downwards, the horse's pull, and the resistance of the plane or slope, perpendicular to itself. There must be this third force, for the other two, not being opposite to each other, cannot make equilibrium. The roller is somehow supparted by the plane; and that it cannot be unless by its resistance; and a plane cannot resist except perpendicularly to itself. This third force, you thus see, must be

Fig. 8.

taken perpendicular to the plane. It is represented in the figure by o B. Now apply the polygon of forces. Let o c represent the weight of the roller, and from c suppose a line, c R, drawn equal and parallel to o A, the horse's pull. Then, since there is equilibrium, the polygon of forces should close up and become a triangle-that is, the line joining R with o should be the pressure, and therefore should be perpendicular to the plane. What then are we to do? Take o c, equal in inches to the number of pounds in the roller, draw then from c a line c R parallel to the horse's pull, to meet the line drawn from the centre o of the roller perpendicularly to the plane; c R so determined will in inches tell the pounds in the horse's pull, and o R the amount by which the roller presses the plane. You can easily see from this that as the slope increases the pull will increase and the pressure diminish. This is what naturally we should expect. The plane I have supposed to be smooth; for, where there is friction against the roller caused by roughness in itself or in the plane, or in both, the question is much altered, as in due time you will see.