Pollen Tube Growth and Early Seed Development in Eucalyptus regnans F. Muell. (Myrtaceae) in Relation to Ovule Structure and Preferential Outcrossing

Abstract

Pistils of Eucalyptus regnans were observed by bright field, fluorescence, phase contrast, Nomarski, and scanning electron microscopy from 4 weeks prior to anthesis to 16 weeks after self- and cross- pollination. The transmitting tissue of the style had a central canal which extended for half its length. The unfertilised ovary contained a mean of 30.5 ovular structures with 16.5 normal ovules, 2.5 abnor- mal ovules, and 11.5 sterile ovulodes. The ovules were arranged in two rows in three (occasionally four) locules. The ovulodes always occupied positions at the stylar end of the ovary, but with this exception there was no pattern to the occurrence of the abnormal ovules. Pollen tubes grew between the cells'of the transmitting tissue, and not in the stylar canal. Penetration of the ovules had commenced by 5 days after both self- and cross-pollination. The early fertilised ovule contained a zygote and free-nuclear endosperm, and embryos with between 1 and 16 cells were present at 16 weeks after pollination. Not all normal ovules were fertilised, despite adequate numbers of pollen tubes. Ovulodes were never observed to be penetrated by a pollen tube, but with this exception there was no relationship between position on the placenta and either the penetration of ovules by a pollen tube or the occurrence of fertilised ovules. There was no difference between self- and cross-pollination in either the number of ovules penetrated by a pollen tube, or the number of fertilised ovules up to 16 weeks following pollination.