Kost and colleagues Given the rapidity of cell elongation and the necessary synthesis, transport, and secretion of cell wall components, it can be appreciated that an abundant supply of energy is essential for pollen-tube growth. The bulk of the cell wall material is secreted as methyl-esterified pectins. Elevated calcium may then retard growth. Several investigators report a region of high intracellular calcium in the apex of the pollen tube, immediately adjacent to the plasma membrane, where growth is maximal figure 4top row; Holdaway-Clarke and HeplerHepler et al. Lovy-Wheeler and colleagues
Observations support the importance of these phosphoinositides to pollen tube growth.
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Taken together, these lines of evidence make it clear that the pollen tube has overlapping components that respond to elevated calcium and restore the basal concentration. The cell was first subjected to rapid freeze fixation and freeze substitution. Inevitably the question arises as to why pollen tube growth oscillates. The embryo and endosperm are packaged as a seed, which becomes encased in a fruit formed from the ovary and, in some instances, from additional floral parts. Meiosis does not directly produce gametes. In experiments with pollen tubes, the culture medium must be acidic, with growth being optimal at pH 5 to pH 6 Holdaway-Clarke et al. Preliminary data, summarized in a recent review Holdaway-Clarke and Heplerfail to note a similar phase relationship between the increase in calcium and the increase in secretion wall thickness.