are referred to. This places the Oak, upon whicn the Mistletoe is rare, very low down in his table, so as to suit his hypothesis very well. But it must not be forgotten that the Oak, like the Beech (with which it is bracketed), and unlike the Chestnut (with which it is also bracketed), possesses two kinds of rays, large and small. In the Oak the larger rays may measure one or two inches, or sometimes more in depth, and, say 1/30th or 1/40th of an inch in thickness something very different from the depth of 1/120th, and thickness of 1/2500, as stated (for the small rays no doubt) in Dr. Harley's table.

With regard to the physiological relations subsisting between the parasite and its nourishing plant, Dr. Harley concludes, "that the roots of the Mistletoe stand to the nourishing plant in the relation of an hypertrophied medullary system, and one which induces an excessive flow of the sap of the branches, resulting at first in the local hypertrophy of its tissues, but subsequently, the supply of sap, or the power of transmitting it, failing, the central portion of the wood becomes exhausted and dies, involving in its death that of its destroyer also." But we are not without some misgivings as to the soundness of this conclusion. For admitting that the root-processes of the parasite do, in one way or other, become buried in the wood of the prey so as to occupy the situation normally occupied by medullary rays, this may be simply to ensure the most intimate contact possible with the wood-cells of the nurse-plant, which cells, it is generally conceded, perform the function of carrying upward the fluids absorbed by the root. We have no reason to assume that the Mistletoe is directly dependent upon the elaborated juices of its prey, which juices, there is evidence to show, occupy the normal medullary rays, at least in autumn. The Mistletoe has a cooking-apparatus of its own, surely sufficing to render it independent of elaborated sap, which, indeed, could not be afforded to it, owing to the 'atrophied' condition of the stem above its attachment. Structurally, Dr. Harley may, perhaps, be right in regarding the roots of the parasite as standing in the relation of an hypertrophied medullary system to the nourishing plant; physiologically, we think him wrong.

243

XXXVI.-DIMORPHIC FLOwers.

(1.) EINIGE BEOBACHTUNGEN UEBER DIMORPHE BLUETHEN.-By H. von Mohl. Botanische Zeitung, 1863, pp. 309, 321. (2.) TREVIRANUS, UEBER DICHOGAMIE. Bot. Zeit. 1863, pp. 1, 9. UEBER DIE BEFRUCHTUNG EINIGER ORCHIDEEN. 1

(3.)

Ib. p. 24

(4.) ALEFELD, UEBER LINUM. Ib. p. 284. /

(5.) SCOTT. EXPERIMENTS ON THE FERTILISATION OF ORCHIDS IN THE ROYAL BOTANIC GARDEN OF EDINBURGH. Trans. Edinb. Bot. Soc. vii. 543. Also Rev. Horticole. 1863, p. 216.

(6.) HILDEBRAND. DIE FRUCHTBILDUNG DER ORCHIDEEN, EIN BEWEIS FÜR DIE DOPPELTE WIRKUNG DES POLLENS.

1863. pp. 329, 337.

Bot. Zeit.

(7.) A. GRAY. STRUCTURE AND FERTILISATION OF CERTAIN ORCHIDS. Am. Journ. Science, Ser. ii. xxxvi, p. 292.

(8.) TRIMEN. ON THE FERTILISATION OF Disa grandiflora. Journ. Proc. Linn. Soc. Bot. vii. 144.*

PROFESSOR VON MOHL's observations refer principally to those flowers of abnormal structure which occur upon plants bearing, besides abnormal flowers, others which present the structure usual in the genera to which they belong. These abnormal' dimorphic flowers are structurally hermaphrodite; and necessarily so, since a prominent character which they present is the total, or almost total, occlusion of their sexual organs, so that if these flowers be fertilised at all, they must necessarily, it would seem, be self-fertilised. In a previous number of this Journal (1862, p. 235), we have described the general features of these dimorphic flowers, and indicated the different groups of plants in which they occur. At the time our communication was penned, some of the best marked and most easily accessible instances had never been accurately described, so that we could indicate only their general features. The chief value of von Mohl's essay is the account which it gives of his own recent examination of a few of these cases, thus supplying a gap which much needed to be filled, and which no one could be more competent to supply than this accomplished and experienced observer. The

The papers above cited, suggested by the researches of Mr. Darwin, were all published in 1863.

general characters of the flowers to which we refer are briefly these: they are usually very small; they remain unopened, and often hermetically sealed up, at least until after fecundation has taken place; the corolla is more or less arrested or wholly suppressed; the stamens are often fewer than in the normal flowers, and the quantity of pollen which their anthers contain is very small indeed; their fruit is developed like that of the normal flower, or sometimes even more freely. They may either precede or be contemporary with, or follow the flowers of normal structure borne by the same plant; they are developed either above or below the surface of the ground; they may either be produced annually with the normal flowers, or at a different period, or, according to von Mohl, in some cases to the exclusion of the ordinary flowers.

Von Mohl gives copious references to what has been already written about these curious flowers; instances of which were not unobserved by Dillenius and Linnaeus. As in our previous communication referred to above, we made some reference to their literature, we shall content ourselves here with noting the points in their structure which von Mohl's observations have now finally set at rest. He proves that in Oxalis, Viola, etc. pollen-tubes are really developed from the pollen-grains while still contained in the anthers, which, in order to get access to the stigma (to which the anthers are closely applied) develope their tubes through the wall of the anther. We previously entertained no doubt but that pollen-tubes were actually developed, binding the anthers to the stigma like so many Lilliputian cables, but we had satisfied ourselves of their existence by direct observation, only in the genera Campanula and Krascheninikovia.

We observe in a recent number of the Bulletin of the Botanical Society of France (Vol. x. p. 194), an interesting account of the flowering and fruiting of Leersia oryzoides by M. Duval-Jouve. It would seem as though this grass might be classed along with the other species cited by von Mohl as bearing dimorphic flowers. From what M. Duval-Jouve tells us, it may, perhaps, prove to be a plant with self-fertilising flowers only. The Leersia is an inconspicuous grass, growing in a few of our southern counties, especially liable to be overlooked, from the circumstance that the flowering-panicle is rarely protruded from the sheath of the upper leaf of the stem. Occasionally it is exserted, and when this is the case the flowers borne upon the exserted portion are sterile, those which are included in the sheath being fertile. So much Schreber pointed out in his u. Beschreibung d.

Gräsern, ii. p. 8. In the exserted flowers the glumes spread out so as to allow the stamens and stigma to get free. These organs appear normally developed, nevertheless they do not produce any seed. In the included flowers, fertilisation is found by M. Duval-Jouve to take place at an extremely early period. The fertile flowers are found not only in the uppermost sheaths, but in the sheaths of the lower leaves as well. The glumes of the included fertile flowers are so closely applied that it is almost impossible to separate them without tearing, and the cavity which the envelopes of the flower enclose is constantly filled with a transparent and slightly viscous fluid which bathes the essential organs. Curiously enough the anthers of the fertile flowers are described as being very small, containing but very little pollen. The pollen-grains are extremely delicate and easily torn, and look as though they were abortive.

The very small number of pollen-grains in the anthers of the closed dimorphic flowers, not only of Leersia, but also, as stated above, of Oxalis and Viola, is another interesting point and singularly in contrast with their profusion, and often apparent waste in many flowers of normal structure. In the latter, however, as we learn especially from Mr. Darwin's observations, foreign and adventitious aids are of essential importance, and often absolutely necessary in transferring the pollen from flower to flower, and the supply is no doubt proportioned to the average risk of its miscarriage or careless delivery by the casual or indifferent agents employed.

There can be no doubt but that the imperfect dimorphic flowers described by von Mohl are as evidently designed for self-fertilisation as are those of normal form very often designed to insure constant or partial crossing with other flowers. But, von Mohl says, we must not forget "that these dimorphous flowers are not the only ones designed for self-fertilisation; there occur other plants which bear only homomorphic flowers, the structure of which is such that they can only be elf-fertilised." In the present state of our knowledge it is quite out f our power to indicate, with any fair confidence, what role the nall dimorphic flowers occupy in the general economy of plants. We find them occurring in species widely removed from each other their natural affinity, although in infinitely the largest proportion flowering plants they do not occur at all, so far as we know. Von ohl does not venture an explanation of the part which they supply. a cannot help thinking, however, that he ascribes an undue ortance to them, especially when he proceeds to couple with

them the self-fertilising homomorphic flowers referred to above, and opposes them collectively to the alleged law of nature,' that both in the vegetable and animal kingdoms there shall be an occasional intercross of distinct individuals.

But what are these cases of self-fertilising homomorphic flowers? Will they bear investigation? The instance of self-fertilising homomorphic flowers specially adduced by von Mohl is that of the Fumitories. He says, "in all the living species examined by me, selffertilisation must necessarily take place, and the transport of pollen from one flower to another appears to me to be impossible, owing to the intimate union of the inner petals which enclose the anthers and stigma."

Now without having forlorn recourse to some casual cross once in a century or a chiliad,† we have reason to believe that, in the case of the Fumitories, fertility is favoured and increased by the not unfrequent visits of insects. We venture to state this upon the authority of the admirable observer to whom we already owe so much in this direction, and whose accuracy is unimpeachable. Mr. Darwin has not overlooked the Fumitories in relation to the question of their being self-fertilising or otherwise, and he has been led to the opinion that although they are fertile to a large degree without insect aid, yet they "are all (or nearly all), manifestly adapted to visits of insects." When a case like that of the Fumitories in which the possibility of a cross has been apparently most vigilantly guarded against breaks down, we feel but little hope of discovering any hermaphrodite flower habitually self-fertilising. We do not, indeed, recall any plant in which self-fertilisation has been shown to obtain with greater general certainty than the Bee Orchis, so well described in Mr. Darwin's work on the "Fertilisation of Orchids."

Professor von Mohl in conclusion, recommends to further examination those plants of warm countries, which, when removed to colder climates, develop their fruit without the flower first expanding in the normal way. Experiments are also needed upon the influence which temperature exercises upon the development of the

* Darwin. Origin of Species, 101.

† “Gänzlich unzulässig ist es aber, zu Gunsten der angeblichen Allgemeinheit eines Naturgesetzes, in welches sich bestimmte Thatsachen nicht fügen wollen, zu verlangen, dass da und dort einmal, wenn auch nur in Jahrhunderten oder Jahrtausenden, Ausnahmen von dem gewöhnlichen Gange der Functionen der Organe vorkommen, welche bei normaler Ausbildung nicht vorkommen können und für deren wirkliches Vorkommen keine Beobachtung spricht."