Apical buds produce a growth hormone called auxin.

VOA: special.2009.10.06

And this particular hormone stimulates and feeds the cancer growth,essentially.

VOA: standard.2010.07.14

It took several years of further experiments and analysis, but During and his colleagues ultimately concluded that the mild stress of all that activity and interaction activated a communication channel that prompts fat cells to stop releasing a hormone called leptin, which accelerates cancer growth.

VOA: standard.2010.07.14