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outer face to the lining of the cell of the anther, and by its inner face to the tissue formed by the pollen-cells. Three and a half or four lines of length in the flower-bud corresponded with a phenomenon altogether unexpected. At first the thick and succulent wall of each pollen-cell dilated, so as to leave a void between its inner face and the granules, not one of which separated from the mass, which proved that a force of some kind held them together. Shortly after four appendages, like knife blades, developed at equal distances on the inner face of the pollen-cell, aud gradually directed their edge towards the centre, so that they began by cleaving the granular mass in four different lines, and finished by dividing it into four little triangular masses; and when the appendages met in the centre they grew together, and divided the cavity of the pollen-cell into four distinct cavities, which soon after rounded off their angles, and in a short time the little granular masses became spherical, like melted lead run into the hollow of a bullet-mould. The partition of the mass thus brought about by the appendages seems to me to indicate that at this period the mass was not protected by a special integument, and that the mutual adhesion of the granules was very weak.

"When things had arrived at this point, the portion of the tissue formed by the pollen-cells separated itself from the surrounding parts, and each pollen-cell became loose, generally in the form of a square parallelopiped with rounded angles; each little mass of granules gained a smooth, colourless, transparent membrane, which was at first membranous, but afterwards became thick and succulent, and soon began to take on the characters peculiar to the pollen of the Gourd. The integument began to bristle with fine conical papillæ; several roundish lids were traced out here and there on its surface; it hardened, became opaque, assumed a yellow colour, ceased to grow, and attained its perfect maturity." Mirbel adds to this highly interesting statement, that he finds in the generality of plants that the mode of forming the pollen is much the same as in the Gourd.

Observations upon the same subject by Professor Mohl were published at Berne, in 1834. The principal points of

novelty in regard to the developement of the pollen are, that, 1. The union of pollen-grains in fours is sometimes permanent, sometimes very temporary. 2. That the four are sometimes placed upon the same plane, sometimes in the same relation to each other as the four angles of a cube. 3. That the original number of cohering grains is in most species of Inga, Acacia, and Mimosa, from eight to sixteen. 4. That the external coat of the pollen-grain is not an uniform membrane, analogous to that of a simple cell, but an organ composed of numerous cells like the integument of an ovule, although it appears in some cases to be simply granular, and in others to be almost homogeneous. This last idea is sharply attacked by Mirbel (Ann. Sc.; n. s., IV. 1.), who insists upon the external skin of the pollen-grain being simple in all cases.

Mr. Griffith, in November 1836, published some curious observations upon this subject, the result of which is, that in Pardanthus chinensis the pollen is developed in the midst of a solid grumous semiopaque mass, forming at a very early stage the contents of one of the four cells of the anther; That subsequently the grumous mass becomes cellular, the cells having undergone some separation, and consisting of a hyaline membrane containing a smaller mass of granular molecular matter. Later still, each cell, which has acquired an orange colour, presents traces of division into four, often into three, very rarely into two portions, the division being more distinct towards the circumference of each cell, and the smaller masses being each enclosed in a proper cell, but without having undergone any separation. Eventually each of the divisions becomes a pollen-grain. "The young grains are oblong-ovate, flattened on their contiguous or inner faces, and open along the centre throughout the whole length of their outer faces. They are even at this period reticulated, and have rather a papillose appearance, and are lined by an inner membrane in the form of a hyaline sac, which bulges out slightly along the opening just mentioned."

According to Schleiden, the difference between ordinary pollen and that found in masses in Asclepiadaceæ and Orchidaceae consists in this, that the enveloping cells in common

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cases are, and in the two others are not, absorbed. same condition may be seen as a temporary stage in the developement of Picea and Abies, in the months of January and February, in Pinus in February and March, in which a loose waxy pollen mass may be found embedded in each division of the anther. At a somewhat later period we may see the four cells in Picea and Abies, in which the four pollengrains lie closely united; and it offers a pleasing spectacle under the microscope to observe each grain expand itself by the absorption of water until it bursts its case in order to escape, leaving the four cells emptied of their contents."

Link supposes the cellular substance in which pollen is generated to be semiorganic, and calls it collenchyma, considering with me that it is what forms the appendage of the · pollen masses of Orchidaceæ. But it can hardly be called semiorganic, especially if it is examined in Polystachya ramulosa.

It also appears from Mr. Francis Bauer's observations, that the masses of pollen of both Asclepiadacea and Orchidaceæ, in the most solid state, are truly cellular, the grains of pollen being contained in cavities, the walls of which are either separable from each other, as in some Orchidaceæ, or are ruptured without a separation of the cavities, as in Asclepiadaceæ. (See the Observations on Orchidacea and Asclepiadacea, before referred to.) It does not however follow that because pollen is engendered in the interior of cells, its grains must therefore adhere originally by an umbilicus; and in fact the part so described by Turpin has no existence.

When they are fully formed the granules are generally discharged at once, upon the dehiscence of the anther. But in some Araceæ, which emit their pollen by a hole in the apex of their anther, the formation or developement of pollen must be going on for a considerable time after the first emission. A single anther continues to secrete and discharge pollen, till, as Brown remarks, the whole quantity produced greatly exceeds the size of the secreting organ.

The surface of the pollen is commonly smooth. In some plants it is hispid, as in the Gourd and Ipomoea purpurea;

in others it is covered with strong points, as Hibiscus syriacus; in Jatropha panduræ folia it is granular; in many Labiatæ, banded; in Passiflora, reticulated; in Geranium sylvaticum, crested; in Armeria vulgaris, polygonal, with crested angles, among which are some of the most beautiful microscopical appearances in the vegetable kingdom. In all cases, where there are asperities of the surface or angles in the outline, pollen is asserted by Guillemin to have a mucous surface, which was first observed in Proteacea by Brown. But Mohl finds that the presence of mucosity upon pollen is a constant character, at least when the grains first quit the anther; and that a power of secreting a viscid substance is one of their functions when perfectly smooth, as well as when covered with points and inequalities. He, however, admits that hispid pollen is generally more viscid than that which is smooth.

The figure of the granules is various; most frequently it is spherical or slightly oblong. Many other forms have, however, been described. The cylindrical exists in Anethum segetum, and in a very remarkable degree in Tradescantia virginica, where the grains become curved: in Colutea arborescens, they were observed by Guillemin to be nearly square; in Lavatera acerifolia to be oval, much attenuated to each end; in Enothera they are triangular, with the angles so much dilated as to give the sides a curved form; in Jacaranda tomentosa I have remarked them to be spherical, with three projecting ribs tapering to either apex; in Cichoracea the granules are spherical with facettes; in Dipsacea a depressed polyedron; in Scabiosa caucasica, patelliform and angular. In numerous plants it is oval, with a furrow on one side, like a grain of wheat; in Thunbergia fragrans, Mimulus moschatus, &c., it is strongly ribbed, as if formed of many folds; in Morina persica, cylindrical, with a narrow neck rising abruptly from each side; in Scolymus, Scorzonera, &c., it is a polygon, with crested angles; and of all these there are numerous modifications, some of which are represented in Plate IV.

In consequence of the great diversity of forms observable in pollen, it has been supposed that it might be employed in

the definitions required for systematic botany; and Messrs. Guillemin and Adolphe Brongniart have stated that plants of the same family have similar pollen, adducing as instances Graminacea, Cyperaceae, Thymelaceæ, Proteaceæ, &c., &c. But Mohl, who has inquired into this part of the subject in a most elaborate manner, declares that pollen varies extremely in form, not only in genera of the same family, but also in species of the same genus; and that it even occurs in some plants that the same anther contains grains, " de formation assez diverse." The more or less complex structure of the pollen is not in relation to the more or less elevated station of a plant in the scale of developement; but the same form is found in families so different, that they are separated by every other point of structure.

The shell of the pollen-grain appears to the observer who examines it with low magnifying powers, as if it were simple. But it has been ascertained to consist in the greater part of plants of two or even three membranes, of which the outer (extine) is thicker than the inner (intine), the latter being hyaline, extensible, and of extreme tenuity, not colourable by iodine, and destructible by concentrated sulphuric acid. Mohl considers the extine to be in all cases composed of minute grains, or cells, held together by organic mucus that it is often cellular there is no doubt; he refers, in proof of the correctness of this opinion, to cases where, as in Pitcairnia latifolia, the coating is manifestly cellular in the middle of the pollen-grain, but becomes granular at the extremities. He also states, that in other cases the points forming granulations become less and less, till, at last, the membrane becomes almost entirely smooth and uniform, and is extremely like the membrane in the common cells of plants; as in Allium fistulosum, Araucaria imbricata, &c. Mirbel, however, disputes the cellularity of the extine; and Fritzsche, in his latest work, asserts that it unquestionably is sometimes a simple membrane (p. 30.). The intine has the power of absorbing water with great force, so that immediately upon being exposed to the action of a fluid it swells, and eventually bursts, discharging its contents; in general the extine extends as well as the intine, and then the organic difference between

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