The development of the ovule, which represents the embryo- Gymnosperms; when mature it consists of one or two sac. coats surrounding the central nucellus, except at the apex where an opening, the micropyle, is left.
There are five, or sometimes fewer, carpels, which unite to form an ovary with as many chambers, in each of which are one or two, rarely more, pendulous anatropous ovules, attached to the central column in such a way that the micropyle points outwards and the raphe is turned towards the placenta.
In Casuarina, Juglans and the order Corylaceae, the pollen-tube does not enter by means of the micropyle, but passing down the ovary wall and through the placenta, enters at the chalazal end of the ovule.
Such a method of entrance is styled chalazogamic, in contrast to the porogamic or ordinary method of approach by means of the micropyle.
a pro-embryo - a cellular row of which the cell nearest the micropyle becomes attached to the apex of the embryo-sac, and thus fixes the position of the developing embryo, while the terminal cell is projected into its cavity.
In some Monocotyledons, Pistil and macrosporangium, is very similar to the process in Fertiliza- to the opening of the micropyle, into which the pollen- tion.
The primary root of the embryo in all Angiosperms points towards the micropyle.
Brown's views as to the structure of the unimpregnated ovule (with the introduction of the term "sac embryonnaire"); and in that it shows how nearly Brongniart anticipated Amici's subsequent (1846) discovery of the entrance of the pollen-tube into the micropyle, fertilizing the female cell which then develops into the embryo.
The ovary is small, rounded to elliptical, and one-celled, and contains a single slightly bent ovule sessile on the ventral suture (that is, springing from the back of the ovary); the micropyle points downwards.
A young ovule consists of a conical nucellus surrounded by a single integument terminating as a two-lipped micropyle.
In Araucaria and Saxegothaea the nucellus itself projects beyond the open micropyle and receives the pollen-grains direct.
The female flower is enveloped in a closely fitting sac-like investment, which must be regarded as a perianth; within this is an orthotropous ovule surrounded by a single integument prolonged upwards as a beak-like micropyle.
A complete and functional female flower consists of a single ovule with two integuments, the inner of which is prolonged into a narrow tubular micropyle, like that in the flower of Gnetum.
- Vertical section of the ovule of the Scotch Fir (Pinus sylvestris) in May of the second year, showing the enlarged embryo-sac b, full of endosperm cells, and pollen-tubes c, penetrating the summit of the nucellus after the pollen has entered the large micropyle.
n, Nucellus; i, inner; o, outer integument in section; m, micropyle.
- Orthotropous ovule of Polygonum in section, showing the embryo-sac s, in the nucellus n, the different ovular coverings, the base of the nucellus or chalaza ch, and the apex of the ovule with its micropyle m.
The micropyle m is very wide.
The integuments do not completely invest the apex of the nucellus, but an opening termed the micropyle is left.
The micropyle indicates the organic apex of the ovule.
This embryo-sac increases in size, gradually supplanting the cellular tissue of the nucellus until it is surrounded only by a thin layer of it; or it may actually extend at the apex beyond it, as in Phaseolus and Alsine media; or it may pass into the micropyle, as in Santalum.
When the ovule is so developed that the chalaza is at the hilum (next the placenta), and the micropyle is at the opposite extremity, there being a short funicle, the ovule is orthotropous.
I12), Cistaceae, and most gymnosperms. In such an ovule a straight line drawn from the hilum to the micropyle passes along the axis of the ovule.
I I 1) is the commonest form amongst angiosperms. In this ovule the apex with the micropyle is turned towards the point of attachment of the funicle to the placenta, the chalaza being situated at the opposite extremity; and the funicle, which runs along the side usually next the placenta, coalesces with the ovule and constitutes the raphe (r), which often forms a ridge.
The ovule is curved upon itself, so that the micropyle is near the funicle.
- Anatropous ovule of Dandelion (Taraxacum), nucellus, which is inverted, so that the chalaza ch, is removed from the base or hilum h, while the micropyle f is near the base.
112), through the conducting tissue of the style when present, and reach the interior of the ovary in angiosperms, and then pass to the micropyle of the ovule, one pollen-tube going to each ovule.
In angiosperms when the pollentube reaches the micropyle it passes down into the canal, and this portion of it increases considerably in size.
The analogies with chief difference is in the micropyle, In one great division of the genus - the Eusigillariae - the stems are ribbed, each rib bearing a vertical row of leaf-scars; the ribbed Sigillariae were formerly divided into two sub-genera - Rhytidolepis, (From a drawing by Mrs D.
The neck of the flask-shaped pollen-chamber projected a little from the micropyle and no doubt received the pollen directly.
This is a large seed, with a very long micropyle; it has a beaked pollen-chamber, and a complex integument made up of hard and fleshy layers, closely resembling the seed of a modern Cycad; the nucellus, however, was free from the integument, each a sketch after Kidston.
Upper part of seed, in longitudinal section; i, integument; mi, micropyle; n, remains of nucellus; p.c, pollen-chamber (containing pollen-grains), with its canal extending up to the micropyle; pr, part of prothallus; ar, archegonia.
15, 4 and 7), as investigated in English, French, Italian, and American specimens, may be briefly described as a short lateral shoot or peduncle, arising in a leaf-axil and terminating in a bluntly rounded apex, bearing numerous linear bracts enclosing a central group of appendages, some of which consist of slender pedicels traversed by a vascular strand and bearing a single terminal ovule enclosed in an integument, which forms a distal canal or micropyle.
Where, by more rapid growth on one side than on the other, the nucellus, together with the integuments, is curved upon itself, so that the micropyle approaches the hilum,and ultimately is placed close to it, while the chalaza is at the hilum, the ovule is campylotropous (fig.
Sometimes the micropyle lies close to the base of the style, and then the pollentube enters it at once, but frequently it has to pass some distance into the ovary, being guided in its direction by various contrivances, as hairs, grooves, &c. In gymnosperms the pollen-grain resting on the apex of the nucellus sends out its pollen-tubes, which at once penetrate the nucellus (fig.
The megasporangium is enclosed in an a A"Ã‚° cu integument, which completely envelopes it, leaving only a narrow micropyle at the distal end (fig.
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