on the structure of Gnetum, he shows that in reality the whole of the apparatus belongs to the ovulum. In that Memoir (dated Aug. 4, 1835), which unfortunately did not reach me till after the publication of the last edition of this work, there is the following description of the development of the parts of this extraordinary structure.

"At a period long before the exsertion of the anthers, the ovules, which lie upon the male flowers, are generally of an oblong form, and consist of a central cellular solid body, inclosed in two envelopes. The outermost of these is fibro-cellular and divided longitudinally on the upper face, or that nearest the axis; the fissure extending nearly to the base of the ovule. The inner or second envelope is cellular, and is divided irregularly towards its apex.

"This envelope does not at this period entirely inclose the nucleus: the points of some of the lacinia or divisions project occasionally beyond the apex of the outer envelope.

Fig. CLXIV.

2

The nucleus is an oval or oblong cellular body, rounded off at its apex, which is composed of lax cellular tissue.

The

"The next change consists in the commencement of the obliteration of the longitudinal fissure, existing along the posterior face of each outer envelope, and of an extension of the inner coat over the nucleus, the apex of which becomes more or less depressed: the centre of the depression, however, projecting in the form of a cone of a very slight elevation. At the time of flowering, or of the exsertion and dehiscence of the anthers, the fissure originally existing along the upper face of the outer coat has disappeared; with the exception of a small portion at the apex of the ovule, which remains unclosed throughout. The ovules are at this period in some species oblique. inner envelope is generally entirely inclosed within the outer; the points of its lacinia reach, however, to the opening existing in the apex of this latter, and occasionally, but by no means universally, project beyond it to a short distance. This coat has undergone scarcely any change, and corresponds in shape to the cavity of the outer envelope. The nucleus is completely covered by both integuments, and its apex, which continues of the same form, is occasionally tinged with brown. Within its substance, which is entirely cellular, and towards its centre, there exists a small cavity, lined with a membranous sac, attached apparently to the apex of the cavity, and containing a number of minute grumous-looking brown masses arranged without any obvious regularity. This sac I consider to be the amnios, with which it agrees in its development and subsequent disappearance; it exists at a rather early period, and is developed within a cavity formed by some excavating process.

"A short time after the fall of the male flowers an extraordinary change will be found to have occurred, consisting of the very rapid and apparently sudden development of a new membrano-cellular envelope between the second coat and the nucleus. This new formation, which I may term the additional coat, envelopes the nucleus pretty closely, and is continued upwards beyond the apex of the nucleus into a cylindrical tubular process; the mouth of the tube being laciniate or fimbriated. At the period now referred to, its apex barely projects beyond the outer envelope. During its development no particular change has taken place either in the original integuments or nucleus. At a somewhat later period, the ovules, except in the instance quoted in the note, hitherto concealed by the involucre, will be found exposed, and the outer coat to have become of a green colour.+

*** This division is perhaps similar to that which Brown states to take place in Dacrydium." "In one species, G. Brunonianum, the ovules are at an early period exposed, owing to the obsoleteness of the annulate involucre."

Fig. CLXIV.—Analysis of Gnetum, from sketches by Mr. Griffith; 1. a ring of and ¦ flowers; 2. a; 3. a perpendicular section of a Q, showing the 3 membranes overlying the nucleus; 4. an em. bryo extracted, with its long funicle.

The opening through its apex is distinct, and its direction vertical. The second envelope continues unchanged. The tubular prolongation of the additional or third envelope now projects through the openings in the original coats to a considerable distance. The mouth of the tube is also rather dilated, and the fringes of its margin spread out irregularly and to various extents. The whole of the tubular prolongation has become tinged with brown, in some cases approaching to black. It is to this stage or period that the descriptions of those authors who attribute a style and stigma to this genus apparently refer. Both Dr. Brown and Professor Lindley must likewise advert to this period when they state the nucleus to be surrounded with three envelopes."

There can be no doubt, then, that in reality Gnetum is as truly naked-seeded as Conifers themselves.

Independently of the singular organisation of its ovule, the genus Gnetum is remarkable for some other peculiarities. Its seed, which resembles a drupe, has within the outer fleshy integument, a layer of needle-like woody tissue of a very remarkable nature, freely separating when disturbed, and looking much like the hairs of Cowhage. The embryo, according to Mr. Griffith, is attached to an "enormously long tortuous and spirally but irregularly twisted cellular funiculus, the cells of which are much elongated and twisted. Its length varies, when moderately pulled out, from 3 to 5 inches, the whole length of the seed being about an inch. This funicle, as well as the extremely similar one of Cycas, has the property of contracting when immersed in water."-MSS. p. 15. Although belonging to the same category as Conifers and Taxads, the Joint Firs are very distinctly separated from them; for they have a calyx for the male flowers, and their anthers burst by pores, not longitudinally, to say nothing of the peculiarities of the ovule.

Natives of the temperate parts of Europe, Asia, and South America, and in the case of Gnetum, of the hottest parts of India and Guiana.

The interior of the pericarp of Gnetum urens is lined with stinging hairs; the seeds are eaten; the stem exudes a transparent gum, and when cut across yields an abundance of clear transparent tasteless water, which may be drank.-Aubl. In Amboyna the seeds of Gnetum Gnemon are eaten roasted, boiled, or fried, and the green leaves form a favourite vegetable in lieu of Spinach; they are, however, very tasteless.-Rumph. The branches and flowers (Amenta Uvæ maritimæ, Off) of the Asiatic Ephedras were formerly kept in the shops as styptics. The fruit is said to be mucilaginous, eatable, sub-acid, and slightly pungent.

GENERA.
Ephedra, Linn.
Gnetum, Linn.
Thoa, Aubl.

Gnemon, Rumph.

Ula, Rheede.

NUMBERS. GEN. 2. Sp. 15.

Chloranthacea.

POSITION.-Pinaceae.-GNETACEA.- Taxaccw,

Casuarinacea.

N.B. C. A. Meyer enumerates 21 species of Ephedra alone. Endlicher makes out

7 species of Gnetum.

Dicotyledones, Juss. Gen. 70. (1789); Desf. Mem. Inst. 1. 478. (1796).-Exorhizeæ and Synorhizeæ, Rich. Anal. (1808).-Dicotyledoneæ or Exogenæ, DC. Theor. p. 209. (1813.-Phanerocotyledonea or Seminiferæ, Agardh. Aph. 74. (1821).-Anthophyte and Carpophyta, Oken. Dichorgana, Schultz. Phylloblastæ, Reichenbach. - Homogens, Lindl. in Bot. Reg. 1839.-Acramphibrya, Endl. Gen. p. 258. 1837)-Synechophyta, Schleiden.

By common consent the plants to which botanists formerly gave the name of Dicotyledons, and which now bear that of Exogens, are recognised as the most completely formed of all the Vegetable Kingdom. In the more highly organised species they possess a degree of vitality unknown except among Gymnogens. A century or two terminates the life of an Endogenous tree, unless in a few rare cases; while many Exogens may have been the monarchs of their forests even at the commencement of the Christian era. This arises from their peculiar manner of growth, which insures a renovation of their vigour with each succeeding year; and it is in allusion to this circumstance that their name has been contrived.

Exogens, or outward growers, are so called because, as long as they continue to grow they add new wood to the outside of that formed in the previous year; in which respect they differ essentially from Endogens, whose wood is constructed by successive augmentations from the inside. All the trees of cold climates, and the principal part of those in hot latitudes, are exogenous. In an Exogen of ordinary structure the embryo consists of a cellular mass, in which there is usually no trace of woody or vascular tissue; but as soon as germination commences fine ligneous cords are seen proceeding from the cotyledons towards the radicle meeting in the centre of the embryo, and forming a thread-like axis for the root. As the parts grow the ligneous cords are increased in thickness and number, and having been introduced among the cellular mass of the embryo, are separated from each other by a portion of the cellular substance, which continues to augment both in length and breadth as the woody cords extend. By degrees the plumule or rudimentary stem becomes organised, and having lengthened a little, forms upon its surface one, two, or more true leaves, which gradually expand into thin plates of cellular substance traversed by ligneous cords or veins converging at the point of origin of the leaves. If at that time the interior of the young plant is again examined, it will be found that more ligneous cords have been added from the base of the new leaves down to the cotyledons, where they have formed a junction with the first wood, and have served to thicken the woody matter developed upon the first growth. Those ligneous cords which proceed from the base of the leaves do not unite in the centre of the new stem, there forming a solid axis, but pass down parallel with the outside, and leave a small space of cellular tissue in the middle; they themselves being collected into a hollow cylinder, and not uniting in the middle until they reach that point where the woody cords of the cotyledons meet in order to form the solid centre of the root. Subsequently the stem goes on lengthening and forming new leaves: from each leaf may again be traced a formation of woody matter disposed concentrically as before, and uniting with that previously formed: a cylinder of cellular substance being always left in the middle. The solid woody centre of the root proceeds in its growth in a corresponding ratio, lengthening as the stem lengthens, and increasing in diameter as the leaves unfold and new woody matter is produced. The result of this is, that when the young Exogen has arrived at the end of its first year's growth it has a root with a solid woody axis,

T

and a stem with a hollow woody axis surrounding cellular tissue, the whole being covered in by a cellular integument. But as the woody cords are merely plunged into a cellular basis, the latter passes between them in a radiating manner, connecting the centre with the circumference by straight

passages, often imperceptible to the naked eye, but always present. The annexed diagram illustrates this.

Here we have the origin of pith in the central cellular tissue of the stem, of wood in the woody axis, of bark in the cellular integument, and of medullary processes in the radiating passages of cellular tissue connecting the centre with the circumference.

Fig. CLXV.

The woody axis is not, however, quite homogeneous at this time. That part which is near the centre contains vessels of different kinds, particularly dotted vessels (bothrenchyma); the part next the circumference is usually destitute of vessels, and consists of woody tissue exclusively of these two parts that with the vessels belongs to the wood, properly so called, and serves as a mould on which future wood is added; the other belongs to the bark, separates under the form of liber, and in like manner serves as a mould within which future liber is disposed.

At the commencement of a second year's growth the liber separates spontaneously from the true wood; a viscid substance called cambium is secreted between them; and the stem again lengthens, forming new leaves over its surface. The ligneous cords in the leaves are prolonged into the stem, passing down among the cambium, and adhering in part to the wood and in part to the liber of the previous year, the former again having vessels intermingled with them, the latter having none. The cellular tissue that connected the wood and liber is softened by the cambium, and grows between them horizontally while they grow perpendicularly, extending to make room for them, and consequently interposed between the woody cords of which they each consist, forming in fact a new set of medullary processes terminating on the one hand in those of the first year's wood, and on the other in those of the first year's liber. This addition of new matter takes place equally in the stem and in the root, the latter extending and dividing at its points, and receiving the ends of the woody cords as they diverge from the main body. The following figure illustrates this, and shows, when compared with the last, what difference there is in the appearance of the stem of an Exogen one and several years old.

And thus, year after year, the Exogen goes on, forming zone upon zone of wood, which is permanent, and zone within zone of bark which perishes at the outside, but is renovated at the inside, as the stem increases in diameter.

If this account is compared with what has already been stated concerning Endogens, it must be obvious that the stem of these two great classes is formed from the very beginning in an essentially different manner. Endogens have no cylindrical column of pith; their woody arcs are never collected into a cylinder, through the sides of which the cellular tissue passes

Fig. CLXVI.

Fig. CLXV.-Ideal section of the root and stem of an Exogen one year old.

Fig. CLXVI.- Real section of the stem of an Exogen six years old.

in the form of medullary processes; and the woody matter of their bark, so to call their cortical integument, is not parallel with that of the wood and spontaneously separable from it. The only way in which the growth of the stem of Exogens corresponds with that of Endogens is that in both classes the woody matter is connected with the leaves; and in both a cellular substance is the foundation of the whole structure. Nevertheless, attempts have been made by some modern physiologists to identify the two, and to show that the one is very little different from the other.

It is not, however, to be supposed that the manner of growth in Exogens, is in all cases exactly what has been thus described as its normal condition. On the contrary, a great variety of modifications has been found to exist, dependent in part upon an excessive development of cellular matter, and in part upon the formation of angles, lobes, or sinuosities, upon the loss of concentric rings of wood for which a great homogeneity of structure is substituted, and upon the production of irregular zones of cellular matter resembling bark, between the zones of wood. Cases of this kind have attracted the attention of most modern botanists. Several have been noticed in my Introduction to Botany, in the Penny Cyclopædia, art. Exogens, and by Decaisne, Adrien de Jussieu, Schultz, Gaudichaud and Schleiden; but they have not been applied successfully to systematical purposes. In a sketch of a possible plan of extending the classes of plants at the expense of Exogens (Bot. Reg. 1839. Misc. p. 76), I have suggested the formation of a group to be called Homogens, to which it has been proposed to unite Birthworts, Nepenths, Lardizabalads, Menisperms, Peppers, and several other Orders. The character upon which reliance was placed was the remarkable nature of the wood of these plants, which never have more than one zone of woody matter, to whatever age they may have arrived. M. Decaisne has however shown (Mémoire sur les Lardizabalées), that although this peculiarity is extremely striking in some cases, as for example, in Aristolochia labiosa, yet that it is not constant in even the same Order, A. Clematitis having annual zones; and that in Menispermads, while there is a great departure from the ordinary structure of Exogens, except Aristolochia so far as regards the liber, the wood is regularly zoned in many instances, although the dotted vessels are wanting.

Nevertheless, although from the very imperfect state of information concerning the true structure of the stems of plants, I am unable to offer, for retaining this division, such reasons as would be satisfactory, yet I think it will be recognised hereafter, either wholly or in part; at least I am persuaded that the time will come when the internal structure of the stem will be far more extensively consulted than it now is, and be made the basis of good and important systematic divisions. Schultz preceded me in this attempt, in preparing his Synorgana dichorganoidea (Naturliches System des Pflanzenreichs, p. 319, 1832), to which he referred Piperaceæ, Saururaceæ, Chloranthacere, Nyctaginaceae, Callitrichaceæ, Hippuridacea, Myriophyllaceae, Amaranthaceæ, Cycadeaceæ, Nymphæaceæ, Nelumbiacea, and Diphylleiaces, and his proposition, like mine, has fallen to the ground. But although the genera he collects under Diphylleiaceæ, namely, Diphylleia, Podophyllum, and others, are in no wise different from the ordinary state of herbaceous Exogens, yet it must be admitted that Hippurids and several of the others offer less resemblance to that plan of organisation. It is difficult to say whether Schleiden contemplates the possibility of any similar division; but it is worthy of notice that he, in his paper On the Anatomico-physiological Differences in the Structure of Stems, translated in the Annals of Botany (iv. 240)