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4. The Scalariform, which are extremely abundant in Ferns. These are angular tubes, whose sides are marked by transverse bars that scarcely reach the angles. (Fig. 9.)

Fig. 9.

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In all probability the spiral vessel is the type of all these; and the differences we perceive in them are owing to the various modes in which they are subjected to the force of developement. Thus the closed duct may be considered to be absolutely a spiral vessel, with little or no power of unrolling; the annular to be the same thing, but with the enveloping membrane growing more rapidly than the enclosed fibre, which is consequently broken into pieces that contract into rings. Reticulated ducts may in like manner be considered as spiral vessels, whose internal spire, instead of snapping into short lengths as the membrane extends, accommodates itself to the growth of the latter by separating its coils, which thus gain an irregular direction, and grow together at points of variable distance. I think this view of the nature of ducts was first taken by Mr. Solly. It is well illustrated by Slack in the paper already referred to, and it derives additional strength from the fact, which, I believe, has never before been mentioned, that ducts, common as they are in the Garden Balsam when full grown, are scarcely to be found in that plant in a young state.

Some anatomists have added to the varieties above enumerated, what they call strangulated vessels (vaisseaux en chapelet or étranglés, corpuscula vermiformia, vasa moniliformia). These are determined by Bischoff to be mere accidental forms, caused by their irregular compression, when growing in knots or parts that are subject to an interrupted kind of developement. They may be found figured in Mirbels Elémens, tab. x. fig. 15.; and in Kieser, fig. 56. and 57.; but the best view of their origin and true nature is in Slack's plate, fig. 33., in the Transactions of the Society of Arts, before referred to. Link defines them to be short spiral vessels with attenuated extremities, and regards them, in his latest works, as young spiral vessels, or as the commencement of spiral vessels, which, instead of lengthening, grow together by their ends.

Vascular tissue always consists of tubes that are unbranched. They have been represented by Mirbel as ramifying in some cases : but this opinion has undoubtedly arisen from imperfect observation. When forming a series of vessels, the ends of the tubes overlay each other, as represented in Plate II. fig. 15.

Slack states that the membrane is often obliterated at the place where two vessels touch each other, and that transverse bars only remain under the form of a grating: this appearance is produced by the remains of the spiral fibre, several of whose convolutions are partially uncovered by the absorption of the enveloping membrane. It would hence appear that ducts open into each other at their points of contact.

Sect. V. Of Laticiferous Tissue, or Cinenchyma.

The earlier anatomists were acquainted with the existence of milk vessels in many plants, but they gave no account of them sufficiently exact to distinguish them from other kinds of tissue, and, accordingly, they have been usually looked upon as forms of Pleurenchyma, or of Trachenchyma, or as intercellular cavities.

It was reserved for Prof. Schultz of Berlin to show the general existence of such vessels, and their real nature. The memoir upon this subject, in which his most recent views are stated, has not yet been published; but there is an abstract of it by M. Auguste de St. Hilaire in the Annales des Sciences, and upon this, and my own observations, the following account of them is founded.

Laticiferous tissue (Vital vessels, Vasa opophora) consists of branched anastomosing tubes, lying in no definite position with regard to other tissue, large and thick-sided when old, but so capillary and thin when young as hardly to be visible. The sides are not parallel as in other vessels, but often contracted and expanded at intervals, so that they may be described as partially closed up by strictures here and there: they are said to have a power of contraction, but there are no valves or dissepiments in their interior. The larger trunks Schultz calls vasa expansa, and the fine ramifications vasa contracta.

This kind of tissue has generally an undulatory direction (Plate II., fig. 19.). In their interior there is a quantity of granular matter, which sometimes fills them wholly, and sometimes is separated by empty spaces.

Their average size is too of an inch. Their sides, although they thicken by the successive deposit of new matter, never offer any marks, or pits, or other interruptions of continuity.

It is obvious from these characters that cinenchyma is quite different from every other form of tissue. Its constant, irregular branching and anastomosing would alone distinguish it.

As such vessels lie in no definite direction with respect to the rest of the tissue, they have been generally overlooked, and are often very difficult to find, although always present in the greater part of flowering plants. M. Schultz recommends maceration for five or six days, as affording a ready means of separating them from the surrounding tissue. It is, however, easy to find them in the liber of the Fig, in the roots of Dandelion, Scorzonera, Lettuce, and other milky Cichoraceæ, or in any of the parts of Chelidonium.

They are placed in great abundance in the innermost layers of the liber, across the Parenchyma of foliaceous

organs, in the bark of the root, in the pith, and probably in all other parts ; but their station seems to vary in different species. Sometimes they accompany the spiral vessels, forming a part of the bundle of tissue to which those organs belong. From the interior parts they proceed by finer and finer ramifications

until they enter the hairs of many plants, in which they form a plexus of excessively fine vessels.

In the lowest orders of plants, and in some others, they are wholly wanting. They are largest in plants having milky juice, and smallest in those whose juice is transparent.

Plate II. contains representations of several laticiferous vessels. Fig. 13. and 18. shows them in the capillary state in the hairs of plants. Fig. 16. shows them thin-sided and filled with latex. Fig. 17. represents them thick-sided and filled with latex. Fig. 19. shows them empty.

At fig. 12. a, and g, they are seen parallel with the other tissue, filled with latex, and a little contracted at intervals.

Sect. VI. Of spurious elementary Organs ; such as Air Cells,

Receptacles of Secretion, Glands, &c. &c.

The kinds of tissue now enumerated are all that have as yet been discovered in the fabric of a plant. There are, however, some other internal parts, which although not elementary, being themselves made up of some one or other of the forms of tissue already described, nevertheless have either been sometimes considered as elementary, or at least are not referable to the appendages of the axis, and can be treated of more conveniently in this place than elsewhere. These are, 1. Intercellular passages ; 2. Receptacles of secretion ; 3. Air cells ; 4. Raphides.

1. Of Intercellular Passages. As the elementary organs are all modifications of either the spherical or cylindrical figure, it must necessarily happen that when they are pressed together, spaces between them will remain, which will be more or less considerable in proportion as the tissue preserves in a greater or less degree the cylindrical or spherical form. When the pressure has been very uniforin, as in the case of the tissue of the cuticle, and in

many states of cellular substance, or when elementary mucus holds the tissue together completely, no spaces will exist. When

they do exist, they are called Intercellular passages. They necessarily follow the course of the tissue, being horizontal, vertical, or oblique, according to the direction of the angles of the tissue by which they are formed. Their size varies according to the size of the tissue and the quantity of sap. In plants of a dry nature, they are frequently so small as to be scarcely discoverable; while in succulent plants they are so large as to approach the size of cells, as in the stem of Tropæolum majus. They are remarkably large in the horizontal partitions which separate the air cells of water plants. In Limnocharis Plumieri they exist in the form of little holes at every angle of the hexagons of which the partitions in that plant consist ; and are, no doubt, there intended as a beautiful contrivance to enable air to pass freely from one cavity to another.

2. Of Receptacles of Secretion.

Fig. 10.

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But it frequently occurs that the simple intercellular passages are dilated by the secretions they receive, and either increase unusually in size, or rupture the coats of the neighbouring tissue; by which means cavities are formed, replete with the sap altered to the state which is peculiar to the particular species of tree producing it. Cavities of this nature are often called vasa propria. To this class also are to be referred the turpentine vessels of Grew; the réservoirs accidentels of De Candolle; and also the réservoirs

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