This is known to almost all children. Let me see whether you answer this. தாவரங்கள் , இலைகள் ஏன் பச்சையாக வளர்கின்றன ? "WISH YOU ALL A HAPPY DEEPAVALI."
Actually giving a guess! Maybe because we give pachai thaneer to them. Wish you and your family Happy Deepavali! Vaidehi.