Fig. 1.

Measuring dimensions and growth rates of pollen tubes. Because circumference, C, and wall thickness, W, are constant over time, pollen tube growth rate (PTGR) can be measured as an increase in length, L over time, t. The synthesis of new plasma membrane and primary cell wall near the tip, and the subapical zone of secondary callose wall synthesis, track the forward movement of the tip, as does the leading edge of the elongating vacuole. As a result, total wall production rate, WPR, can be measured in mature tubes by multiplying circumference, C, by wall thickness, W, and PTGR. Because functional cytoplasm must be maintained between the elongating vacuole and tube tip, a doubling of cytoplasm volume after a whole genome duplication should cause up to a doubling of tube volume per unit of length, which results when tube diameter or circumference are increased by 41.4 %. Doubled tube volume doubles the volumetric growth rate (VGR) and requires a 41.4 % increase in WPR, if W and PTGR are held constant. Figure is not to scale.