Reproduction in Multicellular Plants - 221 



gametophyte lies near the center of the ovule, 

 protected and nourished by the surrounding 

 nucellar tissues, and is not exposed to the 

 hazards of an external environment (Fig. 12- 

 14). Finally the single egg cell matures and 

 must await fertilization before further devel- 

 opment can occur. 



Meanwhile the pollen grains (microspores), 

 after liberation from the anthers, may light 

 on the stigma (Fig. 12-14) of the same flower 

 (self -pollination), or more frequently, they 

 are carried by wind or insects to the stigma 

 of another plant (cross-pollination). The pol- 

 len grains (microspores) are haploid cells, 

 which are formed in the anthers as a result 

 of meiosis, from diploid microspore mother 

 cells (microsporocytes). Each pollen grain is 

 destined to grow into a gametophyte — in this 

 case, a male gametophyte. Such a develop- 

 ment normally occurs only when the micro- 

 spore falls into the sugary secretion of a 

 stigma of the same species, although pollen 

 grains can often be induced to germinate in 

 artificial solutions. The male gametophyte, 

 which is commonly called the pollen tube, 

 now grows downward through the tissues of 

 the stigma and style, deriving nourishment 

 and protection from these tissues (Fig. 12- 

 14). At maturity, when it penetrates an ovule 

 and approaches the egg cell, the male game- 

 tophyte consists of only three cells — or rather 

 consists of a trinucleate syncytium (Fig. 12- 

 19). One of the three nuclei is the tube 

 nucleus, which regulates the growth of the 

 pollen tube; and the other two are sperm 

 nuclei. One of these sperm nuclei unites 

 with the egg, forming the zygote, while the 

 other unites with both endosperm nuclei, 

 forming a triploid endosperm cell (Fig. 

 12-19). 



Significance of the Gametophytes. The in- 

 trinsic nature of the pollen tube cotdd not 

 be appreciated, were it not for the occur- 

 rence of equivalent stages in lower plants. 

 The pollen tube must be regarded as a male 

 gametophyte because (1) it arises from a 

 spore, (2) consists of haploid cells, and (3) 

 gives rise to sperm. On similar grounds, the 



small female gametophyte of the seed plant 

 is also identified as such. In seed plants, the 

 vigorous land-adapted sporophyte harbors 

 and nurtures the gametophytes inside the 

 ovule and pistil), where they are protected 

 from the hazards of independent life; and 

 the gametophytes are reduced to the simplest 

 condition, although they are still capable of 

 fulfilling their essential functions. In this 

 way the seed plants have adapted their lile 

 cycle to conditions as they exist in the land 

 environment. The male and female gameto- 

 phytes develop in close proximity to each 

 other, protected by the flower tissues of the 

 parent sporophyte. Under these conditions 

 fertilization can occur even in very dry re- 

 gions, where the water available in the 

 external environment is so scant that free- 

 swimming sperm would not be able to reach 

 the eggs. Accordingly, seed plants have been 

 able to spread to drier parts of the earth, 

 where more primitive plants, handicapped 

 by the aquatic type of fertilization, cannot 

 survive. In the life cycle of the seed plants, 

 the burden of carrying the paternal chromo- 

 somes to the egg is shifted from the sperm, 

 which are ill adapted to land conditions, to 

 the spores, which even in primitive plants 

 are very resistant to the dry conditions of the 

 atmosphere. The thick-walled pollen grains 

 are produced in very large numbers, and this 

 makes it likely that some will be carried, 

 usually by air — but sometimes by insects or 

 water — from an anther to the stigma of the 

 same species. 



The Embryo Sporophyte, the Seed, and 

 the Fruit. Fertilization occurs when a sperm 

 nucleus leaves the pollen tube (male gameto- 

 phyte) and fuses with an egg nucleus (Fig. 

 12-19). The penetration of the sperm into the 

 egg is facilitated by the fact that the inter- 

 vening cell walls are "digested away" when 

 the end of the pollen tube comes in contact 

 with the egg cell. As in other plants, the 

 zygote soon develops into a new sporophyte. 

 The zygote multiplies by mitosis, and, grow- 

 ing at the expense of the surrounding endo- 

 sperm tissue, it produces a mass of diploid 



