Pollen-tube growth pattern and chalazogamy in Casuarina equisetifolia (Casuarinaceae)

For a better understanding of pollen-tube guidance in relation to pollen-pistil interaction, we investigated the mode of pollen-tube growth in pistils of Casuarina equisetifolia, a monoecious, wind-pollinated species that undergoes chalazogamous fertilization. The pistil is bicarpellate, but only one of the two carpels develops with two ovules. One of these ovules develops more than four embryo sacs. Pistils usually require more than 1 month to reach maturity after pollen grains have been deposited on the stigmas. During that period, pollen-tube growth proceeds discontinuously in five distinct steps that lead up to fertilization: (1) from the stigma to the upper region of the style, (2) from the upper region of the style to a septum in the ovary, (3) from the septum to the surface of the funiculus, (4) from the funiculus to chalaza in the ovule, and (5) from the chalaza to an egg apparatus. Probably because of competitive interaction between male and female gametophytes (or ovules), one pollen tube is selected from among many during the first step (just before the second step), one ovule from the two during the second and third steps, and one embryo sac from more than four during the fourth and fifth steps. On the basis of our results, erroneous drawings and explanations reported in earlier publications on chalazogamy in Casuarinaceae should be brought into question.