It has also been observed that when the pollen of one species is placed on the stigma of a distantly allied species, though the pollen-tubes protrude, they do not penetrate the stigmatic surface.

还观察到， 当一个物种的花粉放在远缘物种的柱头上时，尽管花粉管突出， 但它们不会穿透柱头表面。