Cytological mechanisms of 2n pollen formation in Populus × euramericana (Dode) Guinier (2n = 2× = 38) and in P. × popularis (2n = 2× = 38). (a) Normal anaphase II, simultaneous cytokinesis; (b) normal tetrads; (c) anaphase II, parallel spindle; (d) anaphase II, tripolar spindle; (e) telophase II, fused spindle resulted in fusion of adjacent polar nuclei; (f) metaphase II, prematural cytokinesis; (g) aborted pollen before mature; (h) withered anther wall before matural opening.