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. Biology and man. Biology; Human beings. Two sperm nuclei. Growth nucleus' Hugh Spencer POLLEN TUBE Under suitable conditions, pollen grains sprout like spores, the protoplasm growing out into a long thread. The haploid nucleus in the pollen divides into two. One of the nuclei seems to direct the growth of the tube. The other divides again: these final nuclei are the true sperm, or male, cells How Does Fertilization Take Place in a Flower? The Meeting of Gametes^ In most common plants stamens and pistils are borne in the same flower. Fertilization is nevertheless brought about in a very round

. Biology and man. Biology; Human beings. Two sperm nuclei. Growth nucleus' Hugh Spencer POLLEN TUBE Under suitable conditions, pollen grains sprout like spores, the protoplasm growing out into a long thread. The haploid nucleus in the pollen divides into two. One of the nuclei seems to direct the growth of the tube. The other divides again: these final nuclei are the true sperm, or male, cells How Does Fertilization Take Place in a Flower? The Meeting of Gametes^ In most common plants stamens and pistils are borne in the same flower. Fertilization is nevertheless brought about in a very round Stock Photo
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Library Book Collection / Alamy Stock Photo

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2789 x 896 px | 23.6 x 7.6 cm | 9.3 x 3 inches | 300dpi

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. Biology and man. Biology; Human beings. Two sperm nuclei. Growth nucleus' Hugh Spencer POLLEN TUBE Under suitable conditions, pollen grains sprout like spores, the protoplasm growing out into a long thread. The haploid nucleus in the pollen divides into two. One of the nuclei seems to direct the growth of the tube. The other divides again: these final nuclei are the true sperm, or male, cells How Does Fertilization Take Place in a Flower? The Meeting of Gametes^ In most common plants stamens and pistils are borne in the same flower. Fertilization is nevertheless brought about in a very roundabout way. The embryo sac remains inside the ovule, as the ovule remains inside the ovary. All the traveling is done by the pollen. When the pollen grain alights upon the stigma of a pistil, it absorbs some of the fluid on the latter. Then a very thin thread of protoplasm grows out of the pollen grain—the "pollen tube". It is comparatively easy to get pollen grains of many different kinds to sprout their pollen tubes in a drop of sweet- ened water, on a microscope slide, and to observe some of the changes that take place (see illustration above). The pollen tube normally grows through the style of the pistil into the hol- low of the ovary. Then it grows through a small hole in the ovule that reaches toward the embryo sac (see illustration opposite). Pollen tubes appear to be chemotropic. When the tip comes in contact with the embryo sac, the cell- wall melts away, and the two sexual nuclei combine. This is the essential fact of fertilization. The zygote, having the double, or diploid, number of chromo- somes, is the first cell of a new individual. It corresponds to the fertilized egg of a fern or moss—or, for that matter, of an animal. The New Individual^ After fertilization, the mass in the embryo sac absorbs food from the parent plant and grows into an embryo (see illustration opposite). The surrounding walls of the ovule become the seed coats. The ovule, with i

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