This is a single pollen grain, as in most of the species from Oenothera genus pollen grains are triporate. However, some species (e.g. Oenothera brevistylis)  have tetraporate pollen grains. It is also worth mentioning that two pollen tubes may germinate from one pollen grain, and these tubes can split into many branches, especially near the ovules.

Dear Adlolfo,

The family Onagraceae include Jussiaea, Oenothera, Fuchsia, etc. A general pollen description: grains monad, isopolar, radiosimetric, triporate, occasionally tetraporate in Oenothera.  Hausser (1971), said are colporate, but the picture in this link, show clearly porated grains.

http://commons.wikimedia.org/wiki/File:Oenothera_fruticosa_pollen01.jpg