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endosmose of the pollen is not a mere hypothesis, has been shown by Mirbel in a continuation of the memoir I have already so often referred to. He finds that, on the one hand, a great abundance of fluid is directed into the utricles in which the pollen is developed, a little before the maturity of the latter, while, by a dislocation of those utricles, the pollen loses all organic connection with the lining of the anther; and that, on the other hand, these utricles are dried up, lacerated, and disorganised, at the time when the pollen has acquired its full developement.
Morren has made some statistical observations upon the sexual organs of Cereus grandiflorus. He found that in each flower of this plant there are about 500 anthers, 24 stigmata, and 30,000 ovules. He estimates each anther to contain 500 grains of pollen; the whole number in each flower being 250,000; so that not more than an eighth of the whole number of pollen grains can be supposed to be effective. The distance from the stigma to the ovules he computes at 1150 times the diameter of the pollen grain.
The exact mode in which the pollen took effect was for a long time an inscrutable mystery. It was generally supposed that, by some subtle process, a material vivifying substance was conducted into the ovules through the style; but nothing certain was known upon the subject until the observations of Amici and of Adolphe Brongniart had been published. It is now ascertained, that, a short time after the application of the pollen to the stigma, each grain of the former emits one or more tubes of extreme tenuity, not exceeding the 1500th or 2000th of an inch in diameter, which pierce the conducting tissue of the stigma, and find their way down to the region of the placenta, including within them the molecular matter found in the grain. These pollen tubes actually reach the ovules. Brown states he has traced them into the apertures of those of Orchis Morio, and Peristylus (Habenaria) viridis, although this great observer adds that the tubes in those plants probably do not proceed from the pollen.
Be this as it may, it is quite certain that it is absolutely necessary for the pollen to be put in communication with the
foramen of the ovule, through the intervention of the conducting tissue of the style. In ordinary cases this is easily effected, in consequence of the foramen being actually in contact with the placenta. Where it is otherwise, nature has provided some curious contrivances for bringing about the necessary contact. In Euphorbia Lathyris the apex of the nucleus is protruded far beyond the foramen, so as to lie within a kind of hood-like expansion of the placenta : in all campylotropous ovules the foramen is bent downwards, by the unequal growth of the two sides, so as to come in contact with the conducting tissue; and in Statice Armeria, Daphne Laureola, and some other plants, the surface of the conducting tissue actually elongates and stops up the mouth of the ovule, while fertilisation is taking effect. Another case occurs in Helianthemum. In plants of that genus the foramen is at that end of the ovule which is most remote from the hilum ; and although the ovules themselves are elevated upon cords much longer than are usually met with, yet there is no obvious means provided for their coming in contact with any part through which the matter projected into the pollen tubes can be supposed to descend. It has, however, been ascertained by Adolphe Brongniart, that, at the time when the stigma is covered with pollen, and fertilisation has taken effect, there is a bundle of threads, originating in the base of the style, which hang down in the cavity of the ovary, and, floating there, are abundantly sufficient to convey the influence of the pollen to the points of the nuclei. So, again, in Asclepiadaceæ. In this tribe, from the peculiar conforination of the parts, and from the grains of pollen being all shut up in a sort of bag, out of which there seemed to be no escape, it was supposed that such plants must at least form an exception to the general rule. But before the month of November, 1828, the celebrated Prussian traveller and botanist, Ehrenberg, had discovered that the grains of pollen of Asclepiadaceæ acquire a sort of tails, which are all directed to a suture of their sac on the side next the stigma, and which at the period of fertilisation are lengthened and emitted ; but he did not discover that these tails are only formed subsequently to the commencement of a new vital action connected with fertilisation, and he thought that they were of a different nature from the pollen tubes of other plants: he particularly observed in Asclepias syriaca that the tails become exceedingly long, and hang down.
In 1831, the subject was resumed by Brown in this country, and by Adolphe Brongniart in France, at times so nearly identical that it seems to me impossible to say with which the discovery about to be mentioned originated: it will therefore be only justice if the Essays referred to are spoken of collectively, instead of separately. These two distinguished botanists ascertained that the production of tails by the grains of the pollen was a phenomenon connected with the action of fertilisation; they confirmed the existence of the suture described by Ehrenberg; they found that the true stigma of Asclepiadaceæ is at the lower part of the discoid head of the style, and so placed as to be within reach of the suture through which the pollen tubes or tails are emitted; they remarked that the latter insinuated themselves below the head of the style, and followed its surface until they reached the stigma, into the tissue of which they buried themselves so perceptibly, that they were enabled to trace them, occasionally, almost into the cavity of the ovarium; and thus they established the highly important fact, that this family, which was thought to be one of those in which it was impossible to suppose that fertilisation takes place by actual contact between the pollen and the stigma, offers the most beautiful of all examples of the exactness of the theory, that it is at least owing to the projection of pollen tubes into the substance of the stigma. In the more essential parts these two observers are agreed: they, however, differ in some of the details, as, for instance, in the texture of the part of the style which I have here called stigma, and into which the pollen tubes are introduced. Brongniart both describes and figures it as much more lax than the other tissue; while, on the other hand, Brown declares that he has in no case been able to observe “the slightest appearance of secretion, or any differences whatever in texture between that part and the general surface of the stigma” (meaning what I have described as the discoid head of the style).
I have remarked that, in Morrenia odorata, an Asclepiadaceous plant, the emission of tubes takes place to such an extent as to give the head of the stigma altogether the appearance of a mass of tow. (See Botanical Register, 1838, Misc. No. 129.)
The first act of fecundation in plants is, therefore, the emission of a tube by a pollen grain; but the impregnation of the ovule must necessarily be a subsequent process, in consequence of the distance which the pollen tube must travel through the stigmatic tissue before it reaches the ovule; a distance computed by Morren to amount to 1150 times its own diameter in Cereus grandiflorus. This botanist states that, in that plant and the Vanilla, impregnation does not in fact occur till some weeks after contact between the pollen and stigma has taken place.
It is, however, worthy of remark, that the first act of fecundation produces an immediate effect upon the floral envelopes. In Orchidaceæ, a flower artificially fecundated will change colour and begin to fade in twenty-four hours at the latest after this has happened, although the same flower would have remained in beauty some days if not impregnated.
It would, therefore, seem that actual contact between the pollen and the stigma is indispensable in all cases. Orchidaceous plants have, however, been thought to offer an exception; for in them nature has, on the one hand, provided special organs, in the form of the stigmatic gland and the caudicle of the pollen masses, to assist in the act of fertilisation; and on the other appears to have taken great precautions to prevent contact, by so placing the anther that it seems next to impossible for the pollen to touch the stigma unless artificially applied to it. Nevertheless, it is represented by Adolphe Brongniart, in a paper read before the Academy of Sciences at Paris, in July, 1831, that contact is as necessary in these plants as in others, and that, in the emission of pollen tubes, they do not differ from other plants.
These statements have been followed up by Brown, in an elaborate essay upon the subject, in which the results that are arrived at by our learned countryman are essentially to the same effect. On the other hand, the observations of
Mr. Bauer, and the general structure of the order, seem at variance with the probability of actual contact being necessary; and, as Brown is obliged to have recourse to the supposition that the pollen of many of these plants must be actually carried by insects from the boxes in which it is naturally locked up, it would seem that the mode of fertilisation in Orchidaceæ is still unsettled. I must particularly remark that the agency of insects, to which Brown has recourse in order to make out his case, seems to be at variance with his supposition that the insect forms, which in Ophrys are so striking, and which, he says, resemble the insects of the countries in which the plants are found, “are intended rather to repel than to attract.” But although such arguments are objectionable, it is, nevertheless, now certain that Orchidaceæ require that contact between their pollen and stigma should take place in order to insure fertilisation. This has been shown by Professor Morren, and has now become in gardens a matter of notoriety.
The most interesting and precise accounts of the process of impregnation yet given are those by Mr. Griffith, in the Trans. of the Linn. Soc. vol. xviii., with numerous explanatory plates. This excellent observer describes the impregnation of the ovulum of Santalum album as taking place by a pollen tube first coming in contact with the sac of the amnios, with which it becomes blended, without perforating the membrane. The molecular matter has at this time lost its locomotivity, and becomes aggregated into a grumous line reaching from the apex of the sac to its base. Then a globular vesicle, containing mobile granules, appears at the apex of the sac, in communication with the grumous molecular line. About the same time a distention of the base of the sac occurs, and a central cell is formed in it; by degrees the space intervening between the latter and the apex of the sac becomes cellular, and changes to a suspensor, having an embryo at that end which is next the base of the sac.
In Loranthus and Viscum the ovulum is not formed till some time after the stigma is impregnated by the pollen. Subsequently to that event, a slender sphacelated line, not previously discernible, reaches from the stigma to the interior of