When a pollen grain lands on the stigma of a compatible pistil, it germinates. Germination, for a pollen grain, is the de-
velopment of a pollen tube (Figure 39.4). The pollen tube either traverses the spongy tissue of the style or, if the style is hollow, grows downward on the inner surface of this female organ until it reaches an ovule. The pollen tube may grow millimeters or even centimeters in the process.
The rapid growth of the pollen tube requires calcium ions, which are taken up by the growing tip of the tube, as well as cell adhesion proteins. The downward growth of the pollen tube is believed to be guided by a long-distance chemical signal from the synergids within the ovule. If one synergid is destroyed, the ovule still attracts pollen tubes, but destruction of both synergids renders the ovule unable to attract pollen tubes, and fertilization does not occur.
Tube cell nucleus
Initially the pollen tube contains two haploid cells, the generative cell and the tube cell.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.