This from our friends at optemistworld.com:-
A plant compound in watercress may have the ability to suppress breast cancer cell development by ‘turning off’ a signal in the body, thereby starving a growing tumour of essential blood and oxygen.
The research suggests that the watercress compound is able to interfere with the function of a protein which plays a critical role in cancer development. As tumours develop they rapidly outgrow their existing blood supply. So they send out signals which make surrounding normal tissues grow new blood vessels into the tumour which feed them oxygen and nutrients.