BOTANY WITH BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VIII. BASICS OF PLANT MORPHOLOGY AND ANATOMY

Features of reproduction in angiosperms

Structure of the ovule, megasporogenesis.

Development of the female gametophyte

Structure OF THE ovule. The ovule of angiosperms is located within the Ovary of the pistil. It consists of two main parts: the nucellus (megasporangium) and the surrounding integument (Fig. 85). At the apex of the ovule, the edges of the integument do not fuse, leaving a small opening called the micropyle. The lower part of the ovule, which attaches it to the ovary wall, is termed the funiculus, while the region where the ovule transitions into the funiculus is known as the chalaza.

The nucellus (megasporangium) consists of sporogenous tissue and the megasporangium wall. The nucellus is enclosed by two (rarely one) integuments, distinguishing bitegmic ovules (with two integuments) from unitegmic ones (with a single integument). In certain parasitic dicotyledonous plants, the integument disappears entirely, leaving the ovule naked (ategmic).

Based on their structural features, shape, and orientation, several types of ovules are distinguished. Ovules attached perpendicularly to the Placenta, with their micropyle aligned on the same axis as the funiculus, are termed orthotropous (straight). Ovules curved downward so that the micropyle faces the placenta and lies adjacent to the funiculus are called anatropous (inverteed).

Ovules oriented at a right angle so that the micropyle lies parallel to the placenta are termed hemitropous (semianatropous). Ovules in which one side grows faster, causing them to bend so that the micropyle is positioned near the chalaza, are called campylotropous. In this case, both the integument and the nucellus undergo bending. Ovules that, due to uneven growth, curve to the extent that their nucellus appears horseshoe-shaped in longitudinal section are called amphitropous.

Depending on The structure of the nucellus (megasporangia), ovules are classified into two types: crassinucellate (with a massive nucellus) and tenuinucellate (with a poorly developed nucellus).

Typically, a single diploid megaspore mother Cell (archesporial cell) forms within the nucellus. It undergoes Meiosis to produce a linear tetrad of haploid Cells. Subsequently, one of these cells—upper, lower, or middle—enlarges and develops into a functional megaspore, while the remaining cells degenerate and break down.