. Introduction to botany. Botany. 136 INTEODUCTION TO BOTANY egg supposed that the pollen tube is nourished by the cells that are broken down in the path of the tube. The time required for the pollen tube to reach the ovule varies in different plants, rangmg from a few hours to more than a year. Usually the tube finds its way into the ovule through a minute opening known as the inicro- pyle (fig. 119, m), but in some plants it grows directly through the substance of the ovule. 127. Fertilization. On en- tering the ovule, one of the male cells unites with the egg nveleus of the embryo sac, whic
. Introduction to botany. Botany. 136 INTEODUCTION TO BOTANY egg supposed that the pollen tube is nourished by the cells that are broken down in the path of the tube. The time required for the pollen tube to reach the ovule varies in different plants, rangmg from a few hours to more than a year. Usually the tube finds its way into the ovule through a minute opening known as the inicro- pyle (fig. 119, m), but in some plants it grows directly through the substance of the ovule. 127. Fertilization. On en- tering the ovule, one of the male cells unites with the egg nveleus of the embryo sac, which is within the ovule, and the other male cell in some cases, or perhaps usually, unites with the central nu- cleus of the embryo sac to form the endosperm micleiis (fig. 119, e«.). The nature and function of the latter union are not as yet per- fectly understood. The fusion of egg nucleus and male cell is, however, a very common and most important phenom- enon, oceurrmg in many of the simpler plants as well as in the higher ones. In general it may be said thutfertilhution consists in the imion of tin' nuehnis of a iriaJe cell with the nuclevs of an egg cell. Other illustrations of this will be given in Chapters XV-XVIII. After fertilization the egg rapidly grows into the embryo of a new plant. The ovule also grows. Fig. ll'.i. Diagram to illustrate course of the pollen tube during fertilization p, pollen grains; t, pollen tube; n, nucel- lus, or body of the ovule ; a, atiiipixhd cells of embryo sac; en, endosperm nu- cleus of embryo sac; egg, the egg ap- paratus, consisting of the egg cell and two cooperating cells; m, the micropyle, or small opening, through which, in most ordinary flowering plants, the pollen tube makes its way to the egg at the tip of the embryo sac. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the orig
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