2

3

Mirbel states that "the sides of the cellules are sometimes riddled full of holes (fig. 2.), the aperture of which does not exceed the 3 of a millimètre (or of half a line); or are less frequently pierced with transverse slits, which are occasionally so numerous as to transform the cellules into a real articulated tissue, as in the pith of the Nelumbium (fig. 3.)." This statement is now so well known to have

been founded upon inaccurate observation, and such pores or slits are so universally admitted to be small portions of amylaceous matter sticking to the walls, that no additional disproof seems necessary. A good microscope is alone sufficient, generally, to show the real nature of these supposed pores, or if not, the test used by Dutrochet, and mentioned at page 2., is in all cases sufficient.

It may also be observed, that cellules often contain airbubbles, which appear to have no direct means of escape, and that the limits of colour are often very accurately defined in petals, as, for instance, in the stripes of tulips and carnations, which could not be the case if cellular tissue were perforated by such holes as have been described; for in that case colours would necessarily run together.

One of the most striking instances with which I am acquainted, of cellular tissue having the appearance of pores is in Calycanthus, where it was pointed out to me by Mr. Varenne. (Plate I. fig. 1.) But even in this, a careful examination with glasses of different magnifying powers shows that the apparent pores are certainly not so, but composed of a solid subsance which may be distinctly seen by varying the direction of the rays of the transmitted light with which it is viewed. Sometimes they appear like luminous points; by a little alteration of light they acquire a brownish tint; and if seen with the highest powers of a compound microscope, where there is a great loss of light, they become perfectly opaque.

Cellular tissue is always transparent and colourless, or at most only slightly tinged with green. The brilliant colours of vegetable matter, the white, blue, yellow, scarlet, and other hues of the corolla, and the green of the bark and leaves, is

not owing to any difference in the colour of the cellules, but to colouring matter of different kinds which they contain. In the stem of Impatiens balsamina, a single cell is frequently red in the midst of others that are colourless. Examine the red cellule, and you will find it filled with a colouring matter of which the rest are destitute. The bright satiny appearance of many richly coloured flowers depends upon the colourless quality of the tissue. Thus, in Thysanotus fascicularis, the flowers of which are of a deep brilliant violet, with a remarkable satiny lustre, that appearance will be found to arise from each particular cellule containing a single drop of coloured fluid which gleams through the white shining membrane of the cellules, and produces the flickering lustre that is perceived. The colouring matter of the cellular tissue is frequently fluid, but is in the leaves and other parts more commonly composed of granules of various sizes; this is particularly the case in all green parts; in which the granules lie amongst greenish liquid, the latter of which, as they grow older, dries up, while the granules themselves gradually change to olive green, and finally to brown.

Kieser distinguishes three sorts of globules among tissue: -1. Round extremely transparent bodies, of a more or less regular figure, found principally in young plants and in cotyledons, and soluble in boiling water: it is these that constitute starch or fæcula. 2. Globules of a small size, a more irregular figure, and coloured either green or some other tint. They are not soluble in water, but are so in alcohol; but when dissolved, their matter is not precipitated by the addition of water, on which account they are distinguishable from resinous substances. 3. Extremely small round bodies, varying in colour, and found floating in the proper juices of vegetables.

The green granules are what M. Turpin calls Globuline. He believes them to be young cellules, and that it is from them that new tissue is developed. There does not, however, appear to be any evidence of this, which must be considered, at present, a gratuitous hypothesis, if, indeed, it were not rather said to be an untenable one. No one has ever seen the granules passing through the sides of the cellules; no

rupture of the sides of the cellules, caused by such á passage, has ever been detected; and yet it seems inconceivable how the granules are to be constantly developing as new tissue, without some such trace of their passage being observable. Those who are curious to know the exact nature of this speculation, should consult the memoir of the author, in the Mémoires du Muséum, vol. 18. p. 212. The mode in which cellular or any other tissue is really formed, is buried in mystery. It has been suspected by Mr. Valentine, and I believe the same idea has also occurred to Mr. Bauer, that it may be caused by the extrication of gaseous matter among mucus; but it is obvious that there are many difficulties in the way of this supposition. Amici says the cellules pullulate. The cellules develope, in some cases, with great rapidity. I have seen Lupinus polyphyllus grow in length, at the rate of an inch and a half a day. The leaf of Urania speciosa has been found by Mulder to lengthen at the rate of from one and a half to three and a half lines per hour, and even as much as from four to five inches per day. This may be computed to equal the developement of at least 4000 or 5000 cellules per hour. But the most remarkable instances of this sort are to be found in the mushroom tribe, which in all cases develope with surprising rapidity. It is stated by Junghuns, that he has known the Bovista giganteum, in damp warm weather, grow in a single night from the size of a mere point to that of a huge gourd. We are not further informed of the dimensions of this specimen; but supposing its cellules to be not less than the diameter, and I suspect they are nearer the fairly estimated to have consisted, when full grown, of about 47,000,000,000 cellules; so that, supposing it to have grown in the course of twelve hours, its cellules must have developed at the rate of near 4,000,000,000 per hour, or of more than sixty-six millions in a minute.

of an inch in do, it may be

The cellules of cellular tissue are always very small, but are exceedingly variable in size. The largest are generally found in the gourd tribe (Cucurbitaceæ), or in pith, or in aquatic plants; and of these some are as much as the diameter; the ordinary size is about the bo or they are sometimes not more than the

of an inch in the 3, and

Kieser has

computed that in the garden pink more than 5,100 are contained in half a cubic line.

Cellular tissue is found in two essentially different states, the membranous and the fibrous.

MEMBRANOUS CELLULAR TISSUE is that in which the sides consist of membrane only, without any trace of fibre; it is the most common, and was, till lately, supposed to be the only kind that exists. This sort of tissue is to be considered the basis of vegetable structure, and the only form indispensible to a plant. Many plants consist of nothing else; and while numberless vegetables are destitute of all other kinds of tissue, the membranous cellular tissue is never absent. It constitutes the whole of Mosses, Algæ, Fungi, Lichens, and the like; it forms all the pulpy parts, the parenchyma of leaves, the pith, medullary rays, and principal part of the bark, in the stem of exogenous plants, the soft substance of the stem of endogenous plants, the delicate membranes of flowers and their appendages, and both the hard and soft parts of fruits and seeds.

It appears that the spheroid is the figure which should be considered normal or typical in this kind of tissue; for that is the form in which cellules are always found when they are generated separately, without exercising any pressure upon each other; as, for example, is visible in the leaf of the white lily, and in the pulp of the strawberry or of other soft fruits, or in the dry berry of the jujube. All other forms of the cellules are considered to be caused by the compression or extension of such spheroids.

When a mass of spheroidal cellules is pressed together equally in all directions, rhomboidal dodecahedrons are produced, which, if cut across, exhibit the appearance of hexagons. (Plate I. fig. 12.) This is the state in which the tissue is found in the pith of all plants; and the rice paper, sold in the shops for making artificial flowers, and for drawing upon, which is really the pith of a Chinese plant, is an excellent illustration of it. If the force of extension or compression be greater in one direction than another, various other forms are produced, of which the following have been observed: —

1. The oblong; in the stem of Orchis latifolia, and in the inside of many leaves. (Plate I. fig. 9.)

2. The lobed (Plate I. fig. 2. f); in the inside of the leaf of Nuphar luteum, Lilium candidum, Vicia Faba, &c.: in this form of cellular tissue the vesicles are sometimes oblong with a sort of leg or projecting lobe towards one end; and sometimes irregularly triangular, with the sides pressed in and the angles truncated. They are well represented in the plates of M. Adolphe Brongniart's memoir upon the Organisation of Leaves, in the Annales des Sciences, vol. xxi.

3. The square; in the cuticle of some leaves, in the bark of many herbaceous plants, and frequently in pith. (Plate I. fig. 13.)

4. The prismatical; in some pith, in liber, and in the vicinity of vessels of any sort. (Plate 1. fig. 6.)

5. The cylindrical (Plate I. fig. 8. a); in Chara; this has been seen by Amici so large, that a single cellule measured four inches in length and one third of a line in diameter. (Ann. des Sciences, vol. ii. p. 246.)

6. The fusiform or the oblong pointed at each end; in wood, and in the membrane that surrounds the seed of a Gourd. These are what Dutrochet calls clostres. (Plate II. fig. 19. 8.; Plate I. fig. 5.)

7. The muriform; in the medullary rays. This consists of parallelopiped cellules compressed between woody fibre or vessels, with their principal diameter horizontal, and in the direction of the radii of the stem. It is so arranged that when viewed laterally it resembles the bricks in a wall; whence its name. (Plate I. fig. 7.)

8. The compressed; in the cuticle of all plants. The cellules are often so compressed as to appear to be only a single membrane. (Plate I. fig. 2.a; Plate III. fig. 3, 4, &c.)

9. The irregular; in the testa of many seeds, as Casuarina: here the form of the cellules is so very irregular that they can be reduced to no certain form.

10. The sinuous; in the cuticle, and also sometimes beneath it, as in the leaf of Lilium candidum. (Plate III. fig. 5.)

Cellular tissue is frequently called Parenchyma. Professor Link distinguishes both Parenchyma and Prosenchyma; referring to the former all tissue in which the cellules are applied by their plane faces (Plate I. fig. 1. 3. 6, 7, &c.), and