56

ELEMENTARY MEMBRANE AND FIBRE.

In general, however, it is quite sufficiently transparent to allow the colour of fluids in contact with it to be distinguished on the other side; and accordingly, though itself colourless or nearly so, it often appears tinged, in consequence of the cells or vessels which it forms being filled with coloured fluid. Thus the cells of leaves appear green, those of the parts of flowers yellow, blue, red, &c.; not because that colour exists in the membrane of which they are composed (which, if they could be emptied, would appear almost colourless) but on account of the minute colouring particles diffused through their contained fluids. One of the most remarkable properties of vegetable membrane is its power of allowing fluids to pass slowly through it, even though no visible pores or apertures can be detected in it. Sometimes the appearance of such apertures exists, when membrane is highly magnified; but this appearance is sometimes produced by grains of semi-transparent matter sticking to it; and is sometimes due to that portion of the membrane being thinner than the rest, through the deposition of new matter upon certain points, subsequent to the first formation, of which several examples will be presently given.

70. Elementary Fibre may be compared to hair of extreme delicacy; its diameter often not exceeding the 1-12000 of an inch. It is generally transparent and colourless, and is usually disposed in a spiral direction. Its peculiar property is elasticity, combined with a degree of firmness which, for its diameter, is very considerable. Accordingly we find its chief use to be the keeping open, like an interior spring, the delicate membranous tubes through which air is to pass, and the preventing these from being pressed together by the growth of neighbouring parts. Not unfrequently, however, it seems less elastic than usual, and is broken during the processes of growth into several smaller fragments, which then exhibit a peculiar tendency to grow together in various irregular forms. In this way several peculiar kinds of tissue are produced, which will now be noticed.

71. The one most universally present, no kind of plant being without it in some form or other, and many being entirely composed of it, is that called cellular tissue, from its being made up

of a number of separate cells or minute bags adherent together. These, when first formed, are usually nearly globular, or of a figure

Fig. 11. Various forms of cellular tissue; a, separate vesicles of an egg-shaped form; b, section of cubical cellular tissue of pith; c, section of muriform cellular tissue,

resembling an egg; so that, if cut across, their walls would exhibit a series of circles touching each other at certain points. Afterwards, however, they are gradually pressed against each other, and their sides become flattened. Their form will then depend upon the amount of the pressure on the respective sides. If it have been equal in all directions, the cell will sometimes be cubical, as it is often found in pith; or it will have the form

termed the dodecahedron, which is a solid having twelve equal sides. But if it be pressed more on one side than another, it will be narrowed in that direction and elongated in the other. Thus the original form of the cell may become greatly modified during the growth of the plant. In general, the greatest elongation takes place in the direction of most rapid increase; but this is not always the case; for in the stems of most trees in this climate, there is a peculiar set of cells extending from cells, the walls of which, the pith towards the bark, which have when cut across in any their greatest length in a horizontal didirection, present hexarection; and the cells being of an oblong flattened form, and arranged like bricks in a wall, this kind of structure has been called muriform (wall-like) cellular tissue.

Fig. 12.

Section of irregularly compressed cellular tissue; the honey-comb appearance of the greater part is due to

the 12-sided form of the

gons or 6-sided figures.

72. From what has been said of the permeability, or power of giving passage to fluids, which vegetable membrane possesses, it

58

PROPERTIES OF CELLULAR TISSUE.

may be inferred that this power is also possessed by the simple modification of it just described. Accordingly we find this to be the case, fluids being conducted through it very readily from one part to another: but still it affords a sufficient degree of resistance to cause the transmission of fluids most readily in the direction of the greatest length of the cells, where, of course, there will be the fewest partitions in a given space. Thus, therefore, fluids absorbed at the bottom of a stem, will pass upwards through its cellular tissue more readily than in any other direction, except in the case of the muriform cellular tissue, which conducts fluids horizontally with the greatest readiness; and the object of this peculiar adaptation is to convey the nutritious sap which is passing down the bark into the interior of the trunk. It will be more fully described in Chap. VI. where the structure and offices of the different parts of the stem will be severally detailed.

73. In the fabric of the lowest tribes of Plants, such as Seaweeds, Lichens, the Fungi (or Mushroom tribe,) Liverworts, and Mosses, little besides cellular tissue and its simple modifications can be found; and it forms a large proportion of that of even the highest tribes. Thus in every Plant, the leaves, flowers, bark, pith, and fruit, consist almost entirely of cellular tissue; and it is even found in the woody part of the stem and roots, besides forming the largest proportion of those soft succulent stems which are only of short duration, dying as soon as the fruit they bear has ripened. The whole of the young plant, too, even of the highest tribes, consists, like the permanent forms of the lower, of this kind of structure. It is only when the true leaves have been unfolded and are actively performing their functions, that the other kinds of tissue show themselves. In all newly-forming parts, also, the foundation, as it were, is laid with this tissue, in which the others subsequently appear. So universally is it present, even in the adult fabric, that, if it were possible to abstract all the others from it, the original form would still be retained, except where it would give way with its own weight.

74. But, although cellular tissue is, in its regular state (of which the pith of young twigs, or the pulp of fruits are character

MODIFICATIONS OF CELLULAR TISSUE.

59

istic examples) soft and spongy in its character, it does not always remain so, but often acquires considerable hardness. This is the case, for example, in the prickles of the Rose and other plants, which are merely connected with the cuticle and are not prolonged from the wood beneath. It is the case also in the stones of the Plum, Peach, Cherry, &c.; and in the gritty matter in the centre of the Pear. In all these parts, the processes of vegetation are no longer going on; but the power of firm resistance

is required in their place. This is effected by the deposition of solid matter within the cells. Sometimes the new product lies in regular layers, one within another, covering the whole membrane; sometimes it is deposited in what appears a less regular manner, certain points Round cells thickened of the membrane being left uncovered by it. by internal deposits arranged regularly in In this last case, however, an additional object is attained; for the cells, though the

circles.

Fig. 13.

www

Fig, 14. Sections of cells strengthened by internal matter irregularly deposited; the shaded portion indicates the remaining cavities: a, cells from the gritty centre of the pear; b, cells from the stone of the plum.

greatest part of their walls is so much thickened, are still in a degree permeable to fluid, through the spots of the membrane on which no deposit has taken place. These spots in the walls of contiguous cells generally correspond with each other; so that fluids can find their way from one cell into the cavities of the neighbouring ones; though so large a proportion of their contents has become solid. When the walls of cells have been thus strengthened in particular parts, the membrane has a dotted appearance, the thinnest portions seeming almost like perforations.

75. The size of the cellules of this tissue is extremely variable;

they are usually from 1-300 to 1-500 of an inch in diameter; but may be found of all sizes, from 1-30 to 1-3000 of an inch. One of the most interesting modifications of it is found in the Sphagnum or Bog-Moss; and in the coverings of some seeds. This consists in the presence, within the membranous wall of the cell, of a spiral fibre, coiling from one end to the other. In some of the seed-coats in which these spiral cells exist, the membrane of the cells is so delicate as to be easily dissolved away; so that, if a portion be put into water, the fibres spring out very beautifully by their own elasticity.

76. The next form of elementary tissue to be described is that

called Woody Fibre. It has received the name of fibre, because it always exists in an elongated form, and several of the tribes of which it consists adhere together continuously so as to form cords. This is seen in the common flax thread, for example. If the finest thread that could be separated with the naked eye were submitted to a microscope, it would be seen to consist of several other fibres adhering together; none of these have any great length; but by the manner in which they adhere, side by side, and end to end, a continuous cord is produced. Each of these minute fibres, when more closely examined, is seen to consist of a slender transparent tube, tapering to a point at each end. It thus resembles a greatly of Woody elongated cell. It differs from cellular tissue of similar Fibre. form, in the much greater strength of the membrane forming the walls of the tubes, though it is at the same time thinner. There are many intermediate forms, however, between one and the other. Woody fibre is evidently destined to convey fluid in the direction of its length, and is easily permeated by it. Minute openings have sometimes been detected in the points of the tubes, so as to connect one cavity with another, and thus to render the passage of fluid more easy. It is, however, especially destined to give firmness and elasticity to the parts of the structure which require support; and we almost constantly find vessels protected by it wherever they exist.

Fig. 15.
Bundle