II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

The plants of Phylum Magnoliophyta (the flowering plants) vary greatly in size, shape, texture, form, and longevity. The phylum includes, for example, the tiny duckweeds that may be less than 1 millimeter long, all the grasses and palms, many aquatic and epiphytic plants, and most shrubs and trees, including the huge Eucaliptus regnans trees of Tasmania that rival the redwoods in total volume.

Like the gymnosperms, the angiosperms are heterosporous (produce two kinds of spores), and the sporophytes are even more dominant than in the gymnosperms. The female gametophytes are wholly enclosed within sporophyte tissue and reduced to only a few cells. At maturity, the male gametophytes consist of a germinated pollen grain with three nuclei.

While the flower is developing in the bud, a diploid megasporocyte cell differentiates from all the other cells in the ovule and undergoes meiosis, producing four haploid megaspores. Soon after they are produced, three of these megaspores generate and disappear, but the nucleus of the fourth undergoes mitosis, and the cell enlarges. While the cell is growing larger, its two haploid nuclei divide once more. The resulting four nuclei then divide yet another time. Consequently, eight haploid nuclei in all are produced (without walls being formed between them). By the time these three successive mitotic divisions have been completed, the cell has grown to many times its original volume.

At this stage there are eight haploid nuclei in two groups, four nuclei toward each end of the large cell. One nucleus from each group then migrates toward the middle of the cell. These two central cell nuclei may become a binucleate cell, or they may fuse together, forming a single diploid nucleus. Cell walls also form around the remaining nuclei. In the group closest to the micropyle, one of the cells functions as the female gamete, or egg. The other two cells, called synergids, either are destroyed or degenerate during or after events that occur later. At the other end, the remaining three cells, called antipodals degenerate too. The large sac, usually containing eight nuclei in seven cells, constitutes the female gametophyte (megagametophyte), formerly known as the embryo sac.

Usually while the megagametophyte is developing, a parallel process that leads to the formation of male gametophytes takes place in the anthers. As an anther develops, four patches of tissue differentiate from the main mass of cells. These patches of tissue contain many diploid microsporocyte cells, each of which undergoes meiosis, producing a quartet (also referred to as tetrad) of microspores.

After meiosis, the haploid microspores in the pollen sacs undergo several changes more or less simultaneously, the processes usually taking from a day to a couple of weeks. The following three changes are the most important: (1) the nucleus in each microspore divides only by mitosis; (2) the members of each quartet of microspores separate from one another (in some species, the separation does not occur, but this is unusual); (3) a two layered wall, whose outer layer exine is often finely sculptured, develops around each microspore. When these events are complete, the microspores have become pollen grains.

Pollination. Those who consider pollination to be the equivalent of fertilization are mistaken. Pollination is simply the transfer of pollen grains from an anther to a stigma. Fertilization involves the union of egg and sperm, and it may not occur until days or weeks or months after pollination has taken place, or it may not follow pollination at all.

Most pollination is brought about by insects or wind, but in many species, water, birds, bats, other mammals, and gravity act as agents or pollinators. In some instances, self pollination may occur, with the pollen grains germinating on the stigma of the same flower in which they were produced.

After pollination, the dense cytoplasm of the pollen grain absorbs fluids from the stigma and bulges out through one of the apertures in the form of a tube. This pollen tube then grows down between the cells of the stigma and style until it reaches the micropyle of the ovule. As the tube grows, most of the contents of the pollen grain are discharged into it. The vegetative nucleus stays at the tip, while the generative nucleus (cell) lags behind and divides by mitosis, usually in the tube, producing two nuclei that become sperm cells; no flagella develop. Sometimes, the generative nucleus (cell) divides before the pollen tube has formed. The germinated pollen grain with its vegetative nucleus and two sperms constitutes the mature male gametophyte (microgametophyte).

When the pollen tube reaches the micropyle, it continues on to the female gametophyte (megagametophyte), which it enters, destroying now degenerating synergid in the process; it then either bursts or forms a pore, discharging its contents. Next, an event unique to angiosperms (and a few gnetophytes), called double fertilization (or double fusion) takes place. One sperm migrates from the synergid to the egg, losing most of its protoplasm along the way. The sperm cell nucleus then unites with the egg nucleus, forming a zygote.

The other sperm cell also migrates from the synergid, and, upon reaching the central nuclei, unites with them, producing a 3n (triploid) endosperm nucleus. The endosperm nucleus becomes exceptionally active and divides repeatedly by mitosis. This nutritive 3n tissue, called endosperm, may eventually have hundreds of thousands of nuclei; it surrounds the embryo that develops from the zygote. At the conclusion of theses various events, the ovule has become a seed, and at the same time, the ovary matures into a fruit.

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I. Match each word on the left to its correct definition on the right	\|	Vocabulary notes

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