. Botany for agricultural students. Plants. MALE GAMETOPHVTE AND FEirilLlZATlON 465 The pollen tube lives as a parasite on the structures tlu-ough which it passes, using their tissues as food for growth and mak- ing a passageway for itself at the same time. The growth of the pollen tube is directed by the tube nucleus which maintains a position near the end of the tube. Soon after the pollen tube is well started, the generative nucleus passes from the pollen grain into the tube and later divides, forming two sperms which are carried along with the con- tents of the tube to the embryo sac. The
. Botany for agricultural students. Plants. MALE GAMETOPHVTE AND FEirilLlZATlON 465 The pollen tube lives as a parasite on the structures tlu-ough which it passes, using their tissues as food for growth and mak- ing a passageway for itself at the same time. The growth of the pollen tube is directed by the tube nucleus which maintains a position near the end of the tube. Soon after the pollen tube is well started, the generative nucleus passes from the pollen grain into the tube and later divides, forming two sperms which are carried along with the con- tents of the tube to the embryo sac. The male gametophyte, consisting of tube nucleus and two sperms, is now complete. In some plants, however, the formation of the sperms occurs before the development of the tube is begun. When the tube reaches the embryo sac and comes in contact with its contents, the mem- brane enclosing the tube is destroyed, and the tube nucleus, sperms, and other contents of the tube flow into the embryo sac. The con- tents of the embryo sac apparently destroy the tube nucleus, for it soon disappears, while the sperms apparently thrive. Since there are no cell walls in the embryo sac, the sperms are free to move about. As to how they are moved is not known, for they have no cilia, but one very soon reaches the nucleus of the egg and the other the polar nuclei or the primary endosperm nucleus, with which they come in contact and fuse. Since there are two fusions, one with the egg nucleus and the other with the polar nuclei or the primary endosperm nucleus, there are two fertilizations or double fertilization, and this also is a notable feature of Angiosperms {Fig. 411). Of course fertilization is difficult to follow and has been seen in only a comparatively few Angiosperms. It is therefore possible that many times the second sperm does not fuse with the polars or the primary endosperm nucleus, but double fertilization has been found so generally in the Angiosperms whose fertilization has been stu
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectplants, bookyear1919
