Ovule

The ovules ( ovule ) is an organ of seed plants, which corresponds to an area enclosed by a sheath megasporangium: It is the female reproductive organ that is located in the carpel or on the seed scale. From it after fertilization of the seeds developed.

• 4.1 angiosperms

Position in the ovary

Depending on the orientation in the ovary distinction is upright, pendant and horizontal ovules.

Hanging ovule

Horizontal ovule

Construction

The ovules are exposed at the gymnosperms, in the angiosperms are enclosed in the carpel.

With the stalk ( funiculus ), the ovule is attached to the carpel. It has usually one or two (rarely three) cases, the integuments; at a shell is called unitegmischen ovules in two of bitegmischen. Gymnosperms generally have only one integument, most angiosperms two. The integuments enclose the actual megasporangium, the nucellus. The transition zone between funiculus and nucellus is called chalaza. At the chalaza opposite side, where the integuments abut, they leave blank opening, the micropyle.

Ontogenetically the nucellus is first formed. Then grow from below the integuments up the nucellus. In bitegmischen species formed the inner before the outer. In the angiosperms, which are primarily bitegmisch, the rarer form a unitegmischen ovules arise in several ways:

• One of the two integuments is at an early stage of growth.
• It grows tall the base of the integuments by intercalary growth, so that only the tip consists of two integuments, but this is no longer clearly visible.
• It forms from the beginning only one integument.

In some parasitic plants the integuments completely absent, such as in the Loranthaceae.

Training of the nucellus

Depending on the location of the megaspore mother cell the following types of ovules can be distinguished:

• Crassinucellat: Here the nucellus, is strongly developed. The megaspore mother cell is separated by at least one cell, the parietal cell, or deck, of the epidermis. The amount of lateral and apical tissue is very different. According to another definition, it is a seed plant, in which the primary wall Archesporzelle cell, parietal cell forms. Thus, the megaspore mother cell is not subepidermal.
• Tenuinucellat: The nucellus is very weak and the time of fertilization mostly disappeared. The megaspore mother cell is located at the apical pole of the nucellus subepidermal. The embryo sac is then apically and is enclosed by the epidermis of the nucellus.
• Pseudocrassinucellat: Here are the megaspore mother cell from no parietal cell. The apical cell of the epidermis is divided periclinal itself, creating a Nucellarkappe arises.

In seed plants, which are fertilized by sperm ( Cycadophyta, Ginkgophyta ), located at the apical end of the nucellus a depression, which is called pollen chamber.

Orientation of the ovule

After the orientation of the longitudinal axis of the seed plants are divided as follows:

• In atropic or orthotropic ovule funiculus and micropyle are located on a straight line, the nucellus is not curved.
• In anatropen ovules the chalaza area of, integuments and nucellus is bent by 180 ° and the micropyle comes to lie adjacent to the funiculus. The nucellus itself is straight.
• Campylotrope ovules occupy an intermediate position, where the nucellus is considerably curved. The micropyle is adjacent to the funiculus.
• In amphitropic ovules, the curvature also reached the embryo sac.

When considering only those angiosperms anatrope, crassinucellate ovules with two integuments than originally.

Meiosis

In nucellus arises at the apical pole of a megaspore mother cell, which is called here the embryo sac mother cell. Through meiosis from this diploid cell four haploid cells: the megaspores, here called embryo sac cells. Usually they are arranged in a linear, T-shaped rare. The megaspores of gymnosperms have a cell wall with sporopollenin, but not those of angiosperms.

Gametophyte

The female gametophyte is called in the seed plant embryo sac. At its formation, only one megaspore is usually involved. This is called a mono -spore embryo sac. This usually develops the megaspore deeply lying, more rarely the outer. Rarely are two cells ( bisporer embryo sac ), as in some angiosperms, or all four (tetra sporer embryo sac ) involved, as in Gnetum, Welwitschia and some angiosperms.

In the gymnosperms of the gametophyte is quite clearly developed and resembles the prothallus of ferns. In all groups, with the exception of Gnetum, Welwitschia and the angiosperms the embryo sac at the apical pole archegonia, the female reproductive organs is formed. Ginkgo forms two or three archegonia per ovule, conifers up to 60 and Microcycas calocoma up to 100 A archegonium consists of an egg, which can be up to six millimeters ( Cycadopsida ) and from a large number of different neck wall cells. Neck canal cells always missing, sometimes a ventral canal cell or at least a ventral canal nucleus is formed. In the angiosperms the gametophyte is reduced to a mostly eight nucleate embryo sac.

Angiosperms

In the angiosperms, the core of the megaspore divides in most cases, in three free nuclear divisions in two, four and finally eight nuclei. The three of these nuclei migrate to the upper or lower end of the embryo sac. There, they surround themselves with their own plasma, then own cell membrane and finally a thin cell wall. The three upper cells form the egg apparatus. The middle of the three is the much larger egg cell, the other, the two synergids ( auxiliary cells ). Probably the two synergids are homologous to the neck canal cells of the archegonium. The three lower cells are the antipodes.

The two central cell nuclei do not set themselves apart from the embryo sac plasma and are called polar nuclei. The two polar nuclei fuse to the secondary embryo sac nucleus, which is thus diploid. This is done before or after the penetration of the pollen tube.

Development of this type there are a number of variations, depending on whether one, two or all four megaspores are involved in the development of the embryo sac, as well as the embryo sac is built up further. These types are according to the category in which they were first described denoted.