Oenothera hookeri L. pollen germination and pollen tube growth proceed in very similar ways in the pistil and in vitro. A characteristic feature of pollen grain germination is the emergence of one or more pollen tubes from 1, 2 or 3 colpi at the same time. The second specific event is the different way pollen tubes branch during their growth. The present study compares germination and pollen tube growth in fresh and frozen pollen grains. Pollen grain viability was estimated by fluorescein diacetate staining and tested by germination on medium. The localization of nuclear and plastid DNA was detected after DAPI staining. Sperm cell and vegetative nucleus positions were observed in only one of the pollen tube branches and pollen tubes germinating from the same pollen grain. The vegetative nucleus was weakly stained and its fluorescence was dim. The nuclei of sperm cells were very well seen and were found at different distances from the pollen tube tip. After a period of incubation the pollen tube containing sperm cells was always longer than other tubes or branches germinating from the same pollen grain. In some tubes the sperm cell nuclei seemed to change position from one branch to another. Sperm cells probably can move within the pollen tube growing in the ovary to the branch closest to the embryo sac.
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