HAVING already, in the last chapter, explained the separate action of the stamens and pistils, I shall now confine myself to the consideration of their physical effect upon each other.

The duty of the stamens is to produce the matter called pollen, which has the power of fertilising the pistil through its stigma. The stamens are, therefore, the representatives, in plants, of the male sex, the pistil of the female sex.

The old philosophers, in tracing analogies between plants and animals, were led to attribute sexes to the former, chiefly in consequence of the practice among their countrymen of artificially fertilising the female flowers of the date with those which they considered male, and also from the existence of a similar custom with regard to figs. This opinion, however, was not accompanied by any distinct idea of the respective functions of particular organs, as is evident from their confounding causes so essentially different as fertilisation and caprification; nor was it generally applied, although Pliny, when he said that "all trees and herbs are furnished with both sexes," may seem to contradict this statement; at least, he indicated no particular organ in which they resided. Nor does it appear that more distinct evidence existed of the universal sexuality of vegetables till about the year 1676, when it was for the first time clearly pointed out by Sir Thomas Millington and Grew. Claims are, indeed, laid to a priority of discovery over the latter observer by Casalpinus, Malpighi, and others; but there is nothing so precise in their works as we find in the declaration of Grew, "that the attire (meaning stamens) do serve as the male for the generation of the seed." It would not be consistent with the plan of this work, to enter into any detailed account of the gradual advances which such opinions made in the world, nor to trace

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the progress of discovery of the precise nature of the several parts of the stamens and pistil. Suffice it to say, that, in the hands of Linnæus, the doctrine of the sexuality of plants seemed finally established, never again to be seriously controverted; for it must be admitted, that the denial of this fact, which has been since occasionally made by such men as Alston, Smellie, and Schelver, has carried no conviction with it. We know that the powder which is contained in the case of the anthers, and which is called pollen, must come in contact with the viscid surface of the stigma, or no fecundation can take place. It is possible, indeed, without this happening, that the fruit may increase in size, and that the seminal integuments may even be greatly developed; the elements of all these parts existing before the action of the pollen can take effect: but, under such circumstances, whatever may be the developement of either the pericarp or the seeds, no embryo can be formed. This universality of sexes in vegetables must not, however, be supposed to extend further than what are usually called, chiefly from that circumstance, perfect plants. In cryptogamic plants, beginning with Ferns, and proceeding downwards to Fungi, there are either no sexual organs whatever, or they are not analogous in structure to those of flowering plants.

In order to insure the certain emission of the pollen at the precise period when it is required, a beautiful contrivance has been prepared. Purkinge has demonstrated the correctness of Mirbel's opinion in 1808, that the cause of the dehiscence of the anther is its lining, consisting of cellular tissue, cut into slits, and eminently hygrometrical. He shows that this lining is composed of cellular tissue, chiefly of the fibrous kind, which forms an infinite multitude of little springs, that, when dry, contract and pull back the valves of the anthers, by a powerful accumulation of forces, individually scarcely appreciable: so that the opening of the anther is not a mere act of chance, but the admirably contrived result of the maturity of the pollen; an epoch at which the surrounding tissue is necessarily exhausted of its fluid, by the force of endosmose exercised by each particular grain of pollen.

That this exhaustion of the circumambient tissue by the

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 conformation 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 Asclepiadaceae 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).