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

VIII. FUNDAMENTALS OF PLANT MORPHOLOGY AND ANATOMY

Features of Reproduction in Angiosperms

Double Fertilization in Angiosperms

The Fertilization process in angiosperms is preceded by the germination of a pollen grain on the stigma of the pistil. It begins with pollen Swelling and The Emergence of a pollen tube from a furrow or pore in its outer coat.

The tip of the pollen tube acts as a haustorium, actively carving its way toward the ovule through the softened Tissues of the stigma, style, and Ovary walls. In most cases, the pollen tube grows into the nucellus and embryo sac through the micropyle of the ovule (porogamy).

Less commonly, the pollen tube penetrates the embryo sac through other Regions of the ovule (aporogamy), known as chalazogamy and mesogamy. In chalazogamy, the pollen tube enters the nucellus through the chalaza, grows upward, and reaches the embryo sac near the egg apparatus; in mesogamy, the pollen tube enters the nucellus laterally, between the chalaza and the micropyle.

During the growth of the pollen tube, the vegetative Cell Nucleus moves ahead of the generative cell nucleus in two-celled pollen, or ahead of the sperms in three-celled pollen. The vegetative cell nucleus is the first to reach the embryo sac, where it disintegrates, though it often degenerates while still in the pollen grain. In two-celled pollen, the division of the generative cell into two sperms occurs during the growth of the pollen tube, although cases are known where sperms are formed already within the pollen grain. When the pollen tube reaches the embryo sac, it discharges its contents there.

At the same time, as first discovered in 1898 by the prominent Ukrainian cytologist and embryologist S. G. Navashin, one of the sperms fuses with the egg cell, while the other fuses with the secondary Nucleus of the central cell of the embryo sac or with the unfused primary haploid polar nuclei of the dikaryon. As a result of egg cell fertilization, a diploid zygote is formed, which develops into the embryo. The fusion of the other sperm with the primary haploid polar nuclei of the dikaryon, or with the diploid secondary nucleus of the central cell—triple fusion—leads to The formation of a central cell with a triploid nucleus. From this cell, a specialized nutritive tissue, the endosperm, is formed. This process is known as double fertilization.

However, triple fusion cannot strictly be called fertilization, because the central cell is not a gamete, nor does this fusion result in a zygote capable of giving rise to a new Organism. These are two distinct biological processes conventionally united under the common term - double fertilization.