cells below the stomates of Pinus sylvestris (fig. 1.b), and there are sufficient traces of it to be found elsewhere to justify the opinion that it is a common mode of increment in thickness. Turpin has remarked that this thickening of the membranous sides of cells by means of a hard sedimentary matter, called by him Sclerogen, is what causes the grittiness of the pear, and the boniness of the stone of the peach and plum, in all which the osseous parts were originally membranous. It is, however, by no means in old or woody parts that a thickening of the membrane takes place: it may be observed distinctly in the cells of the corolla of Convolvulus tricolor, and in all probability occurs in any part containing fluid matter exposed to decomposition.

Elementary membrane generally tears readily, as if its component atoms do not cohere with greater force in one direction than another; but I have met with a remarkable instance to the contrary of this in Bromelia nudicaulis, in which the membrane of the cuticle breaks into little teeth of nearly equal width when torn. (Plate I. fig. 6.) Hence it may be conjectured, that what we call primitive membrane is itself the result either of primitive fibres completely consolidated, or of molecules originally disposed in a spiral direction, as Raspail supposes. (Chim. Org. p. 85.)

In the membrane of certain plants, as in the liber of the Oleander, in Vinca minor, and others belonging to the families of Apocynaceæ and Asclepiadaceæ, an appearance is discoverable of spiral steep ascending lines, some of which turn to the right, others to the left, thus dividing the surface into a number of minute rhomboidal spaces. Mohl, however, who has made this observation, does not therefore consider with Grew that the membrane is woven together of fibres, but that their appearance is owing to a small difference in the thickness of the cellular membrane: "Perhaps a different arrangement of the molecules at various points, perhaps a small difference in the thickness of the membrane, causes a different refraction of light, precisely in the same way as fibres are visible in badly melted glass." Valentin confirms Mohl's views, and regards all such appearances as caused by the process of lignification.

It is in all cases destitute of visible pores; although, as it is

readily permeable by fluids, it must necessarily be furnished with invisible passages. An opinion to the contrary of this has been held by some botanists, who have described the existence of holes or pores in the membrane of tissue, and have even thought they saw a distinct rim to them; but this idea, which originated in imperfect observation with illconstructed glasses, is now generally abandoned. Different explanations have been given of the nature of the supposed pores. Dutrochet asserted them to be grains of semi-transparent matter sticking to the membrane: he found that boiling them in hot nitric acid rendered them opaque, and that treating them with a solution of caustic potash restored their transparency, a property incompatible with a perforation. Slack believed them to be, in other cases, thin spaces in the sides of tissue, such as might be produced by the adhesion and separation at regular intervals of a thread developed spirally within a membranous sac (Trans. Soc. Arts, xlix.). A nearly similar opinion was previously offered by Mohl, who considers the dots on the membrane of tissue to be thinner portions of it. He says it may be distinctly seen by the aid of a powerful microscope that the little circles which are visible on the surface of the tissue of Palm-trees are passages (meatus) in the thickness of the membrane, opening into the cavity of the cells, and closed externally by the membrane itself. He adds, that when dotted tissue is in contact, these passages are placed exactly opposite to each other. (Martius Palm. Anat. v. col. 2.) The latter is undoubtedly the general cause of the appearance of dots, as has now been ascertained by repeated observations. If a thin section of any vessel or cell, the sides of which appear to be dotted, is placed under a good microscope, it will be found to have the matter deposited on its sides, pierced with short passages, which give the appearance of dotting, because the sides of the membrane are thinner where they are stationed than any where else. (See Plate II. fig. 2.) They are therefore not dots, but pits.

Should the observer fail in seeing the pits in their natural state, the application of tincture of iodine to the subject under examination will enable him to discover them readily, with a magnifying power of 350 diameters. But it is

by no means to thin transparent tissue that these passages are confined; they are universally present in the sides of the thickest sided tissue, where they form minute cul de sacs often branched, and always opening into the interior of the cell. They may be readily found in the gritty tissue of the pear (fig. 2. a), the stone of the plum b, and the compact albumen of seeds. Fig. 2. c represents them in the albumen of Alströmeria, where they are about 7500 of an inch in diameter.

fig. 2.

*

By what power the sedimentary matter, left on the sides of such tissue as this, is prevented from choking up the pits is at present unknown.

It is, no doubt, very common for the pits of the membrane of one cell to be placed exactly opposite those of the next cell, as is seen in the irregular half gelatinous tissue of Cereus grandiflorus (see Plate II. fig. 1. a a), so that it may be supposed that they are passages to allow of permeation from one cell to another; but this arrangement is by no means uniform (see same fig. b).*

Elementary Fibre may be compared to hair of inconceivable fineness, but it is extremely variable in size. In Pleurothallis ruscifolia, where it is large, I find it 3, in Crinum amabile, where it is middle-sized, 730 of an English inch in diameter. It has frequently a greenish colour, but is more commonly transparent and colourless. It appears to

*For the supposed chemical difference between elementary membrane and fibre, see Book II. Chapter 1.

be sometimes capable of extension with the same rapidity as the membrane among which it lies, and to which it usually adheres; but it occasionally elongates less rapidly, when it is broken into minute portions, and is carried along by the growing membrane. In direction it is variable (Plates I. and II.); sometimes it is straight, and attains a considerable length, as in some fungi; sometimes it is short and straight, but hooked at the apex, as in the lining of the anther of Campanula; occasionally it is straight, and adheres to the side of membrane, as in the same part in Digitalis purpurea; but its most common direction is spiral. Whether it is solid or hollow is not quite settled; Purkinje asserts that it is hollow, as will be hereafter mentioned; but there can be no doubt that it is also, at least sometimes, solid, as in the fibrous utricles of the leaf of Oncidium altissimum; and I have every reason to believe that it is always so, an opinion equally entertained by Valentin, Schleiden, and Morren. Elementary Fibre has a constant tendency to anastomose, in consequence of which reticulated appearances are frequently found in tissue. Slack adds that it sometimes branches. Like membrane it is increased in thickness by the deposit of sedimentary matter on that part which does not adhere to the membrane, as has been proved by some beautiful microscopico-chemical experiments of Schleiden.

Of the organic mucus, membrane, and elementary fibre thus described, all the elementary organs of plants are constructed. For the convenience of description, they may be considered as of five different kinds, 1. Cellular tissue, or Parenchyma; 2. Pitted tissue, or Bothrenchyma; 3. Woody tissue, or Pleurenchyma; 4. Vascular tissue, or Trachenchyma; 5. Laticiferous tissue, or Cinenchyma.*

* Professor Morren has proposed the following nomenclature of tissue, which has some advantages over that now more commonly in use. I. PARENCHYMA; 1. merenchyma, or sphærenchyma, spherical; 2. conenchyma, conical, as in hairs; 3. ovenchyma, oval; 4. atractenchyma, fusiform ; 5. cylindrenchyma, cylindrical; 6. colpenchyma, sinuous; 7. cladenchyma, branched; 8. prismenchyma, prismatical. II. PERENCHYMA, amylaceous granules. III. INENCHYMA, fibro-cellular tissue. IV. ANGIENCHYMA, vascular tissue; 1. pleurenchyma, woody tissue; 2. trachenchyma, spiral vessels; 3. modified trachenchyma, ducts; 4. cinenchyma, laticiferous vessels.

There is no doubt that all these forms are in reality modifications of one common type, namely, the simple cell, (according to Morren of an amylaceous granule) however different they may be from each other in station, function, or appearance. For, in the first place, we find them all developed in bodies that originally consisted of nothing but cellular tissue; a seed, for instance, is an aggregation of cells only; after its vital principle has been excited, and it has begun to grow, woody tissue and vessels are generated in abundance. We must, therefore, either admit that all forms of tissue are developed from the simple cell, and are consequently modifications of it; or we must suppose, what we have no right to assume, that plants have a power of spontaneously generating woody, vascular, and laticiferous tissue in the midst of the cellular. Mirbel has lately reduced the first of these suppositions to very nearly a demonstration; in a most admirable memoir on the development of Marchantia he speaks to the following effect. I at first found nothing but a mass of tissue composed of bladders filled with little green balls. Of these some grew into long slender tubes, pointed at each end, and unquestionably adhering by one of their ends to the inside of the sac; others from polygons passed to a spherical form in rounding off their angles. As they grew older, other very important changes took place in certain cells of the ordinary structure, which had not previously undergone any alteration: in each of these there appeared three or four rings placed parallel with each other, adhering to the membrane, from which they were distinguished by their opaqueness; these were altogether analogous to annular ducts. The cells become tubes did not at first differ from other cells in any thing except their form; their sides were uniform, thin, colourless, and transparent; but they soon began to thicken, to lose their transparency, and to be marked all round from end to end with two contiguous parallel streaks disposed spirally. They then enlarged, and their streaks became slits, which cut the sides of the tubes from end to end into two threads, whose circumvolutions separated into the resemblance of a gun-worm.' In these cases there can, I think, be little doubt that the changes witnessed by