mitive condition of vegetable tissue; although it has long been known as a substance existing in Algaceous plants, prior to the appearance of organization, as in Protococcus nivalis, &c. It has been found by Brongniart, Henslow, &c. in the form of a thin homogeneous membrane, applied to the cuticle of the leaves of some plants, and only separable after maceration; it is probable that it constitutes the whole exterior surface of all plants, and that it is even drawn over the sacs which constitute hairs; I have found it distinctly on the petals of Hydrotænia meleagris (see Bot. Reg. 1838. misc. No. 128.), but its extreme tenuity and firm adhesion to the tissue below it renders it difficult to detect it; and there is no doubt that it occurs very generally in the interior of plants between their cells, filling up the intercellular spaces, and gluing together all the parts. Mohl, with his usual skill, has shown that this substance is found so frequently, that we cannot refuse to acknowledge its presence as a constant fact. The Box, and the young annual shoots of Sambucus nigra, are especially noticed as well suited to show this structure; it will be seen to form a considerable part of the mass of the albumen of Alstromeria salsilla, see fig. 2. c. where it is 3300 of an inch in diameter. Valentin has measured the thickness of the intercellular organic mucus in several instances, and gives the following table of the proportion between it and the cells of certain plants, calculated in Paris inches.

Meyen admits the fact of the presence of this intercellular

mucus, but considers it a secretion from the sides of the cells. He particularly refers to its condition in the petiole of Beta cycla, in proof of the correctness of that view.

It is the opinion of some anatomists that of membrane and fibre, the latter only is the basis of the tissue of plants: fibre itself being a form of membrane. But we find both the one and the other developed in many of the most imperfectly organized plants, such as Scleroderma and other fungi, and it is difficult to conceive how that can be a mere modification of membrane which is generated independently of it, which has no external resemblance to it, and which in many cases is obviously something superadded.

Membrane varies in its degree of transparency, being occasionally so exceedingly thin as to be scarcely discoverable, except by the little particles that stick to it, or by its refraction of light, but in ferns, some fuci, and other cryptogamic plants, it is brown from its first birth: according to Röper it is green in Viscum album; Link says it is green in the leaves of Ruellia Sabiniana and the petiole of Cycas revoluta; and Meyen mentions its being orange coloured in the petiole of many tropical Orchidaceæ. It is always excessively thin when first generated; and whatever thickness it afterwards acquires must be supposed to be owing to the incorporation or incrustation of secreted matter. This was first observed by Mohl in Palm-trees, where he found a successive addition of strata to the lining of the cavities of the cells; and is apparently an universal occurrence where membrane becomes thickened. But the matter added to membrane is often so homogeneous as to offer no trace of its being deposited concentrically, even when examined by the most powerful microscopes, and I am by no means able to discover the regular lines upon its section which are represented so uniformly by the German anatomists. There can, however, be no doubt that the membrane of the woody tubes of the

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 73 of an inch in diameter.

fig. 2.

a

b

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 so in Crinum amabile, where it is middle-sized, 723 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.