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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 are considered to be caused by the compression or extension of such spheroids.

When a mass of spheroidal bladders 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, a variety of forms is produced, of which the following are the most worth noticing:

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 Adolphe Brongniart's memoir upon the organization 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 I. fig. 6.)

5. The cylindrical (Plate I. fig. 8. a); in Chara; this has been seen by Amici so large, that a single vesicle 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 the membrane that surrounds the seed of a Gourd. (Plate I. fig. 5.)

7. The muriform ; in the medullary rays. This consists of prismatical bladders compressed between woody tissue 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. Here the bladders 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 sinuous ; in the cuticle, and also sometimes beneath it, as in the leaf of Lilium candidum. (Plate III. fig. 5.)

10. The stellated; where the cells are so deeply lobed at the angles as to leave open passages between them, as in the stem of Eriophorum vaginatum. Plate III. fig. 2. is an approach to this structure.

11. The tabular; as in the epiphlæum of many plants.

Cellular tissue is frequently called Parenchyma. Professor Link distinguishes Parenchyma from Prosenchyma ; referring to the former all tissue in which the bladders (Plate I. fig. 1, 3. 6, 7, &c.) have truncated extremities; and to the latter, forms of tissue in which the bladders taper to each end, and, consequently, overlap each other at their extremities.

Meyen has Merenchyma, for ellipsoidal and spheroidal cells; Parenchyma for angular cells; and Prosenchyma as above described.

FIBRO-CELLULAR TISSUE is that in which the sides are composed either of membrane and fibre together, or of fibre only.

It is only lately that this kind has been recognised. The first observation with which I am acquainted is that of Moldenhauer, who, in 1779, described the leaves of Sphagnum as marked by fibres twisted spirally. (Fig. 3. a, p. 9.) In November, 1827, I described the tissue of Maurandya Barclayana as consisting of bladders formed of spiral threads crossing each other, interlaced from the base to the apex, and connected by a membrane. A few other solitary cases of this kinds of tissue had subsequently been observed when the investigations of a modern anatomist suddenly threw an entirely new light upon the subject.

Instead of being very rare, cellular tissue of this kind appears to be found in various parts; it has been already mentioned as existing in the leaves of Sphagnum ; it is also found in the pith of Rubus odoratus. I originally discovered it in the parenchyma of the leaves of Oncidium altissimum, and in the coat of various seeds. Mr. Griffith has detected it abundantly in the aërial roots of Orchidaceous plants, where in fact it is extremely common in numerous species, and Purkinje has shown, by a series of excellent observations and drawings, that it constitutes the lining of the valves of almost all anthers. The forms under which it exists in these parts are far more various than those of membranous cellular tissue. The principal varieties are these :

A. Membrane and Fibre combined.

1. Fibres twisted spirally, adhering to a spheroidal or angular membrane, and often anastomosing irregularly, without the spires touching each other. (Plate I. fig. 12.) This is what is found in Oncidium altissimum leaves, in the aerial roots of some Orchidaceous plants, in the lining of many anthers, and is what Mohl has figured (Ueber die Poren, 8c. tab. i. fig. 9.), from the pith of Rubus odoratus. It approaches very nearly to the nature of spiral vessels, hereafter to be described, and appears only to be distinguishable by the spires of the fibres not being in contact, being incapable of unrolling, having no elasticity or tenacity; and by the bladders not being cylindrical and tapering to each end, but spheroidal. It is easily examinable in Pleurothallis ruscifolia, and forms upon the side of the cells elevations which give them a beautifully pitted appearance when cut across. In the subcutaneous parenchyma of the leaves of this plant the fibres of one cell are placed exactly opposite those of the next cell, so that sections of the walls exhibit double depressions and elevations all along the line, so regular that, unless a very good microscope is used, they appear to form open passages from one cell to the other.

2. Fibres crossing each other spirally, and forming a reticulated appearance by their anastomosing within oblong blad

ders. Of this nature are the reticulated cells of the seed-coat of Maurandya Barclayana, Wightia gigantea, and the like. (Plate I. fig. 11.)

3. Fibres running spirally close together, except at certain places where they separate and leave between them small spaces, which appear like dots.

4. Fibres running spirally, but completely grown together, except at certain spaces where they separate and leave small dot-like spaces. This and the last have been noticed by Mr. Valentine in Orchidaceous plants, and have been extremely well figured by Slack. (Trans. Soc. Arts, vol. xlix. t. 6. f. 5, 6.)

5. Fibres running straight along the sides of truncated cylindrical cells in the anthers of Richardia africana (Calla æthiopica) and many other plants. (Plate I. fig. 13.)

6. Fibres running transversely in parallel lines round three of the sides of prismatical right-angled cells, in the anthers of Nymphæaceæ, &c.

7. Fibres very short, attached to the sides of cells of various figures, to which they give a sort of toothed appearance, as in the anther of Phlomis fruticosa and other Labiatæ. (Plate I. fig. 15.)

The last three were first noticed by Purkinje.

8. The fibre twisted spirally, in the membranous tubes that form the elaters of Jungermannia, apparently constitutes another form of tissue of this order (Plate I. fig. 17.), and has recently been found by Corda among Fungi in the genus Trichia.

B. Fibre without Membrane.

It is not improbable that this form is always in the beginning of its growth composed of membrane. Mirbel has shown that the curious cells which line the anther of the common gourd are continuous membranes till just before the expansion of the flower, when they very suddenly enlarge, and their sides divide into narrow ribands or threads, curved in almost elliptical rings which adhere to the shell of the anther by one end; these rings are placed parallel with each other in each

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cell, to which they give an appearance like that of a little gallery with two rows of pilasters, the connecting arches of which remain after the destruction of the roof and walls. According to the observations of Dr. Schleiden, the formation of fibre never takes place independently of membrane, but occurs in the interior of cells, whose membrane was originally quite simple. He regards Corda's statements to the contrary (Ueber Spiralfaserzellen, 7. and 8.), as formed upon imperfect observations. He says that cells always attain their full size before the fibre appears, and he regards its formation as a part of the process of lignification. In the beginning he states that each cell is filled with starch, rarely with mucus or gum. By degrees the starch is always converted into the latter; this becomes changed, and, as it would seem, always froin without inwards, into jelly. This jelly changes at its surface into a spiral fibre of variable width, which either does or does not adhere to the sides of the cells, and which may be supposed to owe its spiral direction to the course taken by a current setting between the side of the cell and the central mass of jelly

The following are the more important varieties :

1. Spiral fibres repressed by mucus, but having sufficient elasticity to uncoil when the mucus is dissolved, and then breaking up into rings. (Plate I. fig. 16.) These are what are found in the seed-coat of Collomia linearis. They approach spiral vessels so very nearly, that when I originally discovered them I mistook them for such. They are known by their depressed figure when at rest, by the want of an inclosing membrane, and by their brittleness when uncoiled.

2. Fibres short, straight, and radiating, so as to form little starlike appearances, found in the lining of the anthers of Polygala Chamæbuxus, &c. by Purkinje. (Plate I. fig. 19.)

3. Fibres originating in a circle, curving upwards into a sort of dome, and uniting at the summit, observed by the same anatomist in the anthers of Veronica perfoliata, &c.

4. Fibres standing in rows, each distinct from its neighbour, and having its point hooked, so that the whole has some resemblance to the teeth of a currycomb, in the anthers of Campanula; first noticed by Purkinje. (Plate I. fig. 18.)

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