en cæcum of the latter, which are the clavate vessels of oil found in the coat of the fruit of Umbelliferæ, and which are commonly called vittæ. Although the receptacles of secretion have no proper coat, yet they are so surrounded by cellular tissue, that a lining or wall is formed, of perfect regularity and symmetry. The tissue of this lining is generally much smaller than that of the neighbouring parts. When filled with a fluid having a different refractive power from that of the surrounding parts, they give a semitransparent dotted appearance to the organs in which they occur, as may be seen by holding up the leaf of an orange tree against the light.

While, however, many kinds of receptacles of secretion are mere cavities in the tissue, others are little nuclei of cells, as in the Dictamnus (fig. 10. c). These are of the nature of glands, and are called internal glands by Meyen.

Numerous modifications of these parts have been described by the German anatomists, especially by the last-mentioned author, but they only relate to the refinements of the subject. In figure, the receptacles are extremely variable, most commonly round, as in the leaves of the Orange and of all Myrtaceæ, where they are called crypta, or glandulæ impressæ, or réservoirs vesiculaires, or glandes vesiculaires, or receptacles of oil. In the Pistacia Terebinthus the receptacles are tubular; in Coniferæ they are very irregular in figure, and even position, chiefly forming large hollow cylindrical spaces in the bark. Those in the rind of the orange and lemon are little oblong or spherical cysts; their construction, which is easily examined, gives an accurate idea of that of all the rest.

3. Of Air Cells.

Besides the common intercellular passages, and the receptacles now described, there is another and a very remarkable sort of cavity among the tissue of plants. This is the air cell; the lacuna of Link. Like the receptacles of secretion, the air cells have no proper membrane of their own, but are built up of tissue; and this sometimes takes place with a wonderful degree of uniformity and beauty. Each

cell is often constructed so exactly like its neighbour, that it is impossible to regard it as a mere accidental distension of the tissue: on the contrary, air cells are, in those plants to the existence of which they are necessary, evidently formed upon a plan which is uniform in the species, and which has been wisely contrived by Providence in the manner best adapted to the purpose for which they are destined.

They differ from receptacles of secretion in containing air only, and not the proper juice of the plant; a peculiarity which is provided for by a curious contrivance of Nature. In receptacles, the orifices of the intercellular passages through which the fluid that is to be deposited drains, are all open; but, to prevent any discharge of fluid into the air cells, the orifices of all the intercellular passages that would otherwise open into them are closed up, except in the partitions that divide them from each other.

Air cells are very variable in size, figure, and arrangement. In the stem of the Rush (Juncus articulatus), they consist of a number of tubular cavities placed one above the other, and separated by membranous partitions composed of a combination of minute bladders; in some aquatic plants they are very small, as in Butomus umbellatus. In form they are either cylindrical, or they assume the figure of the bladders by which they are formed, as in Limnocharis Plumieri (Plate III. fig. 1. and 2.), in which the structure of the air cells and their coats forms one of the most beautiful of microscopical objects. In the green parenchymatous parts of plants, such as the leaf, the cortical integument, &c., where they always abound, they are irregular spaces among the tissue, communicating freely with each other. They are represented in Plate I. fig. 2.

The inner surface of the air cells, when those parts are essential to the life of a plant, is smooth and uniform; but in grasses, umbelliferous plants, and others where air cells are not essential, they seem to be caused by the growth of the stem being more rapid than the formation of the air cells; so that the tissue is torn asunder into cavities of an irregular figure and surface. Kieser was the first to observe that in many plants in which the air cells of the stem are regularly

separated by partitions, the intercellular passages of the bladders forming the partitions are sometimes left open, so that a free communication is maintained between all the tiers of air cells.

11

4. Of Raphides.

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Among the tissue are found certain needle-shaped transparent bodies, lying either singly or in bundles, and called raphides. They were first discovered by Rafn, who found them in the milky juice of Euphorbia; afterwards they were met with by Jurine, in the leaves of Leucojum vernum, and elsewhere; and they are now well known to all vegetable anatomists. If a common Hyacinth is wounded, a considerable discharge of fluid takes place, and in this myriads of slender raphides (fig. 13.) are found floating; or if the cuticle of the leaf of Mirabilis Jalapa is lifted up, little whitish spots are observable, which are composed of them; all these are acicular in form, whence their name. In Cactus peruvianus (fig. 11.) they are found in the inside of the bladders of cellular tissue, and, instead of being needle-shaped, have the form of extremely minute conglomerated crystals, which, according to Turpin, are rectangular prisms with tetraedral summits, some with a square, others with an oblong base. Crystals of a similar figure have been remarked by the same observer in Rheum palmatum (fig. 12.); and their presence, according to him, is sufficient to distinguish samples really from China and Turkey, from those produced in Europe. The former abound in these crystals, the latter have hardly

any. They are insoluble in alcohol, water, and caustic potash, but are dissolved by nitric acid.

Raphides are found solitary in the cells of Papyrus antiquorum, Epidendrum elongatum, &c., scattered in considerable numbers in the cells of Musa paradisiaca and collected firmly into bundles which are a little shorter than the cells in which they lie. They are in most instances formed in the cells of Merenchyma and Parenchyma without order; but Meyen has observed that in the bark of Viburnum Lantana they are principally stationed in the interior of thin-sided cells, clustered in cavities of thicker sided tissue.

Link compares the raphides in plants to calculi in animals. Raspail asserts that raphides are never found either in Cactus or elsewhere in the inside of the bladders of cellular tissue, but are exclusively placed in the intercellular passages. The slender kind (fig. 13.) he states to be crystals of phosphate of lime, from to of a millimetre in length, and to be in reality six-sided prisms, terminated at each end by a pyramid with the same base. The crystals found in the Cactus and Rhubarb (figs.11.and 12.), he says are composed of oxalate of lime; and he represents them to be right-angled prisms, terminating in a four-sided pyramid. (Nouv. Syst. de Ch. Org. p. 522.) According to Marquart the raphides of Aloe arborescens consist of phosphoric acid combined with lime and magnesia. Mohl says that raphides are never sixsided prisms, as Raspail asserts; but that they are rightangled four-sided prisms, which gradually vanish into points; and he declares that Meyen is right in asserting that the raphides are constantly formed inside the bladders, and never in the interstitial passages of cellular tissue (Anat. Palm. p. 28.); about which there is no sort of doubt. In Liparis pendula, in which the tissue is very thin, the raphides may be seen in situ without disturbing the surrounding parts, and they then form dense bundles of acicular crystals lying in the centre of cells.

The same circumstance is particularly visible in the oval cells found in the leaves of Caladium esculentum, Dieffenbachia Seguina, and some other Araceæ. Here the acicular raphides are not only collected in bundles inside the cells, but

are expelled from them by an opening at each end of the cell, on which account Turpin calls such cells Biforines. Morren found the power of emitting their raphides preserved in these bodies after having undergone 6° of cold of Reaumur (18° Fahr.), and he therefore concludes that the phenomenon is, as Turpin supposes, a mere physical action produced by endosmose, and not a vital action.

(For further remarks on raphides see the Appendix to this Work.)

SECT. VII. Of amylaceous and other granular matter contained in Tissue.

INSIDE the tissue of plants, are found various kinds of particles, some of which give colour or its peculiar turbid appearance to the fluid, others their nutritive quality to particular species.

Of these some are turned blue by iodine, and are therefore regarded by chemists as composed of amylaceous matter or starch; others are rendered olive brown by that agent, and many are dissolved by alcohol, whence they are considered of the nature of resins: all are decomposed by cold, and appear to be connected with the function of nutrition.

The following kinds may be distinguished :—

1. Amylaceous granules.-These are so extremely common that no plant can be said to be destitute of them, and many have the cells of their roots and some parts of their stem filled quite full of them. In the rhizoma of Equisetum the tubes are so crowded with them, that when the tubes are wounded, the granules are discharged with some force, apparently by the contraction of the membrane, so that they appear as if in voluntary motion so long as the emptying the tissue continues to take place. These particles are perfectly white, semitransparent, generally irregularly oblong, sometimes compound, and marked with oblique concentric circles; they are extremely variable in size, some being as fine as the smallest molecular matter in pollen, that is, not more than 3000