Hi there.....it's a tradition. I don't know where it comes from. I am providing some pictures which will show various examples:
At the very top, middle is the Sigma Gamma Rho tree at Howard; to the far right is the Gamma Sigma Sigma tree, which I didn't even notice at first. (Those are Deltas in the foreground)
Zeta Phi Beta tree
Alpha Kappa Alpha tree:
