Chinese Cabbage

I have always wanted to ask someone this - why are there always black dots on chinese cabbage, especially on the whiter part of the leaves. Though the chinese cabbage look fresh and crisp, there are still black dots on the leaves. Does anyone know if the black dots, if eaten will cause any harm? Are they fungus or are the dots a part of the composition of the leaves? My gals and I love chinese cabbage but each time I cook them, I throw almost a quarter to half of the leaves with black dots on them, which is really a waste.