CLASS III.-RHIZOGENS.

Khizantheæ, Blum. Fl. Javæ, (1828); Endlicher Meletemata, p. 10. (1832); Ed. prior, p. 389; Endl Gen. p. 72.

These are parasitical plants destitute of true leaves, in room of which they have cellular scales. Their stem is either an amorphous fungous mass, or a ramified mycelium, sometimes, perhaps always, appearing to be lost in the tissue of the plants on which it grows; and is very imperfectly supplied with spiral vessels, which in some instances seem to be wholly deficient. No instance of green colour is known among them; but they are brown, yellow, or purple. They are furnished with true flowers, having genuine stamens and carpels, and surrounded by a trimerous or pentamerous calyx, or absolutely naked. Their ovules appear to be constructed upon the same plan as in other flowering plants. The true nature of their seeds is in most species quite unknown; by some they are described as breaking up into a mass of spores, by others as consisting of a cellular nucleus abounding in grumous corpuscles (Endl.), and in general they may be regarded as too small for exact observation; but it is certain that in some instances they have a minute undivided embryo enclosed in mucilaginogranular albumen.

At this point of the Vegetable Kingdom we find a most curious assemblage, which, with many of the peculiarities of Endogens, seems to be an intermediate form of organisation between them and Thallogens. They have no relation to Acrogens, although they follow at this place, but they agree with Endogens in the presence of sexes, and sometimes in the ternary structure of their flower; they have, however, scarcely any spiral vessels, and their seeds appear, as far as they have been examined, either, as some say, to want the cotyledons and axis of other flowering plants, or to lose themselves in a mass of pulp, from which they are almost undistinguishable. In their amorphous succulent texture, in their colour, often in their putrid odour when decaying, in the formation of a mycelium or spawn, which is evident in Helosis, and is with good reason suspected to exist in others, and in their parasitical habits, these plants resemble Fungals, while in their flowers and sexes they accord with Arumworts, or similar Endogens.

Rhizogens all agree in being of a fungus-like consistence, and in their habits of living parasitically on the roots of other plants. They very generally stain water, or spirit, of a deep blood-red colour. Their forms are exceedingly diversified; some have the aspect of a Mushroom, or develop a head like that of a Bullrush (Typha): others push forth a thyrse of flowers, or an elegant panicle; while some have their bloom in a head like that of some Cynaraceous plant. In Helosis and Langsdorffia the rhizome, which is horizontal and branched, and which at intervals throws up perpendicular flowering stalks, is quite analogous to the spawn of

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The existence of a mycelium has also been adverted to by Dr. Brown. (Linn. Trans. xix. 232.) He suggests that in Rafflesia the earliest effort of the seed, after being deposited in its proper nidus, may consist in the formation of a cellular tissue extending laterally under the bark of the stock. He remarks that in Pilostyles and Cytinus, where the plants are closely approximated, their possible origin from a common base or thallus, is rendered the more probable by the parasites in the former genus, which is dioecious, being produced generally, perhaps always, in groups of the same sex, and by those groups, which are often very dense, not unfrequently surrounding the branch of the stock. He adds, however, that this view is not sustained by sufficient observation, but that there are circumstances in both genera favourable to the hypothesis, especially in Pilostyles.

RHIZOGENS.

Fungals. In Cynomorium, Seybalium and Balanophora, this part is wanting, and in its room the roots of those genera emit roundish deformed tubers collected in a circle upon the roots of other plants, and growing into them by some unknown process. Blume says, of Balanophoreæ there is produced from the roots of the Fig on which they that at the period of germination grow an intermediate body, of a fleshy nature and intimately combined with its superficial woody layers, and that this intermediate body is penetrated by their spiral vessels, which render it woody." He moreover adds, that "several seeds of Balanophoreæ germinate on nearly the same points of the Fig-root; hence this woody body, or luxuriant product of the juices that are sucked out, has generally an irregular form, and the plants proceeding from such tubers grow out in different directions, much in the same manner as the tubers of a Potato generate their offsets: with this difference, however, that in a Potato the eyes of the plant are in the circumference, while in Balanophora they are placed in the centre, and on that account the intermediate body where the offsets break out, has necessarily a conical extension." Something of the same kind occurs in Seybalium, whose tubers are expanded in an irregular form about the root of some unknown tree, are fleshy, and composed even in the substance of the stalk of somewhat irregular cells and no spiral vessels. In the room of leaves these plants have scales, which differ from true leaves in the want of colour, a character common to all other plants parasitical on roots. by a solitary head of flowers, sometimes bearing several heads variously A vertical stalk (stipes), sometimes terminated arranged upon the stalk, is found in all the genera of Balanophoraceae; which moreover agree in this that the flower-heads, which at first are sessile on the rhizome and concealed by many rows of imbricated scales, resemble the leafy rosette of a Sempervivum without colour, or rather the very small bud of a Rafflesia. The genuine species of Helosis show on their rhizome roundish conical buds seated on a very short stalk, or altogether sessile, enclosing the rudiments of the future head within a very thin involucre, as a fungus within the volva; this latter after a time splits into three or more segments, and emits the flower-head enlarged and furnished with a stalk, which is altogether naked except at the base, where it is surrounded by the scale-like segments of the withering involucre. This is the most simple form of involucre, which in the other genera becomes more and more complicated, and finally runs into numerous series of imbricated scales which clothe the stipes more or less completely. In those genera which grow upon the bark of the stems of trees, there are some diversities of structure in the organs of vegetation that are very remarkable. Blume tells us that Rafflesia Patma appears upon the creeping roots or stems of Cissus scariosa in the form of solitary or clustered hemispherical dilatations, which look like excrescences or expansions of the root. These excrescences are something of the nature of leaf-buds, consisting of layers of scales and a more solid centre. As the latter increase in size they burst through the wrapper by tearing it irregularly from the apex towards the base, and develop themselves in the form of numerous scales, at first flesh-coloured, then brownish, and finally deep purple, which surround the flowers. As soon as these parts are exposed, richly nourished as they are by the humid air that surrounds them, they grow with such rapidity that it is reported that Rafflesia, which, when full-blown, is a yard across, and when unexpanded, is as large as a middle-sized cabbage, only takes about three months for its complete formation. Brugmansia has a similar mode of development.

At one time it was believed that Rhizogens agreed with Fungals in the

total want of spiral vessels. That, however, was a mistake. Spiral vessels do not exist among them. Brown says that he has discovered them in Rafflesia, in which he originally failed to perceive them, and in several other cases. Martius also found them in Langsdorffia, in the form of bundles lying in the rhizome, stem and branches, and Mohl in similar parts of Helosis, but in small quantity compared to the mass of the plants. Brown adds that "the vascular system of all these parasites is uniform and more simple than that of the far greater part of Phænogamous plants; that the spiral, or slight modifications of it, is the only form of vessel hitherto observed in any of them; and that the large tubes or vessels with frequent contractions, corresponding imperfect diaphragms, and variously marked surface, which have received several names, as vasa porosa, punctata, vasiform cellular tissue, dotted ducts, &c., and which are so conspicuous in the majority of arborescent Phænogamous plants, have never been observed in any part strictly belonging to these parasites. (Linn. Trans. vol. xix. 231.) He, however, does not attach systematical importance to this curious fact.

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The flowers are in general formed upon some symmetrical plan, the portions varying from genus to genus. But in a singular deformed genus called Sarcophyte the flowers are not reducible to symmetry, as far as has yet been observed. It has not, however, been examined in a philosophical

manner.

The seeds of many Rhizogens have escaped the observation of those who have had the best opportunities of examining them. Even the seeds of the common Cytinus Hypocistis of the South of Europe are unknown. But if there has been a want of facts concerning this part of the structure there has been speculation in abundance for which the reader is referred to the last edition of this work. I can positively confirm the statement of the elder Richard (Mem. Mus. viii. t. xxi.), who gives to Cynomorium coccineum an embryo. I find in that plant that the seed consists of a mucilaginous mass filled with angular particles, which are doubtless loosely cohering cells. They contain starch in a very minutely globular state, but are chiefly composed of gum. On one side of this seed is a globular embryo, looking like a speck, but found, when properly examined, to be a globose mass of cells, destitute of starch, enclosed within the albumen, and apparently undivided on any part of its surface. It is, however, difficult to speak positively upon this point, on account of its smallness, and I am not sure that it is not very slightly 2-lobed. Francis Bauer too ascertained the ovules of Rafflesia Arnoldi to have the ordinary structure, a strong indication that the seeds would not be so anomalous as has been represented, and he found an undivided embryo in the seed of the same plant, (Linn. Trans. xix. t. xxv.), a circumstance confirmed by the observations of Brown. Ferdinand Bauer found in Hydnora Africana what seems to be a central embryo (Ibid. t. xxx.) of the same nature, and the researches of Weddell and Dr. Hooker leave no further room for doubting that all Rhizogens are truly embryonate.

Such being the supposed facts that have been ascertained with regard to these singular parasites, it only remains to notice some of the views entertained regarding them by systematic botanists. Dr. Robert Brown, who, aided by the microscopical drawings of the two Bauers, has had peculiar advantages for considering the question, appears to be opposed to the idea of regarding Rhizogens as a distinct class. He considers Rafflesiads as being unquestionably allied to Birthworths, and therefore as a form of Exogens. His objections to regarding Rhizogens as a distinct class are as follows.

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RHIZOGENS.

He denies the absence of spiral vessels, which he himself and others once supposed to be a characteristic of some at least among them, and asserts that the vascular texture of Rhizogens is not essentially different from that of any perfectly developed Phænogamous plants. But, as was stated in the last edition of this work, the true question to be considered is, not as to the presence or absence of spiral vessels, but as to their abundance. In Exogens or Endogens equally developed they would be most copious, and would exist in all the foliaceous organs; and it is no argument against the importance of this circumstance, to say, that spiral vessels have no existence in certain Endogens, as Lemna, for instance; for in that and similar cases the small degree in which such plants are developed, may be considered to account for the absence of spiral vessels; just as in a common Exogen, the spiral system does not make its appearance until the general development of the individual has made some progress.

So, indeed, in Ferns and other Acrogens of high degree, we have no right to say that the vascular system is absent; on the contrary, in the centre of the stem of Clubmosses, and in the soft parts of that of Ferns, either spiral or scalariform vessels exist in abundance; but they do not make their appearance in the foliaceous organs as in more perfect plants.

Brown also attaches no importance to the supposed homogeneity of the embryo of Rhizogens, because the same structure, he says, exists in Orobanche and Orchids. But with regard to Orobanche, that plant has a slightly two-lobed embryo lying in a mass of albumen, so that I do not see how it can be brought into comparison with that of Rhizogens; and as to Orchids we have no right to say that their embryo is essentially different from that of common Endogens, except in its smallness.

The late Mr. Griffith adopted the views of Brown, and endeavoured, by new arguments, to show that Rhizogens cannot be regarded as a peculiar class in the Vegetable Kingdom. He asserted that these plants are not similar in their parasitism, and that in those he had examined there would appear to be two remarkably different types of development of the embryo. And he was persuaded that Rhizogens are an entirely artificial class, not even sanctioned by practical facility.-(Proceedings of the Linnean Society, No. XXII., p. 220., where this author's views are given in detail.)

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Arguments like those of both Brown and Griffith never appeared satisfactory to me. Most of the species brought together to constitute the class of Rhizogens seem to have little relation to other parts of the system. true that the genera differ much from each other in the details of their fructification; though not more than the genera of some other classes; but the character of the order does not depend upon the fructification. It depends upon the great peculiarity in the manner of growth, already pointed out; and the fructification is connected with questions of a subordinate degree. All the classes of plants depend equally upon such considerations; and, therefore, Rhizogens are logically a class. a botanist as Griffith should not have perceived how much his position was It was indeed singular that so acute weakened by comparisons like the following. He particularly directed attention to the resemblance between the pistil of Balanophorads and that of Mosses, or more especially to that of some evaginulate Liverworts, and to the effects produced by the action of the pollen on their styles. "Indeed," he observed, "in the development of the female organ, in the continuous surface of the style before fecundation, and in its obvious perforation after, Balanophora presents a direct affinity to a group of plants with which otherwise it has not a single analogy." In another genus, called Phæocordylis,

he found that the hairs in which the fruits are imbedded present a remarkable analogy with the paraphyses of Drepanophyllum and certain Nockeræ, and also with bodies which he suspected to be the male organs of Ferns. Surely this is a class of peculiarities which should indicate a group of lower rank than Exogens or Endogens.

Dr. Hooker, without adopting Griffith's views, is of opinion, after a most minute examination of Balanophorads, that that order at least has no claim to be separated from Exogens, but that it has a plain affinity to Onagrads. In order that the arguments adduced in support of this view of a very difficult question may be exactly stated, I have requested my acute friend, who has had ample opportunities of examining Balanophorads, to favour me with his own statement; and the reader will find it in the succeeding page.

There is an account of Rhizogens by Endlicher in his Meletemata, which contains a summary of all that was in 1832 known concerning them. For further information the reader is referred to Blume's Flora Javæ ; Martius' Nova Genera, &c., vol. 3; Brown's Observations on Rafflesia, in the 13th and 19th volumes of the Linnean Society's Transactions; Griffith, in the Proceedings of the same learned body, the various works quoted at the head of the following natural orders, and in a note by Dr. Hooker upon Cynomorium in Webb's Histoire Naturelle des Canaries, iii. 431.