When hen I looked at some plants in my back yard I wondered why leaves a dark shade of green on the topside and a light shade of green on the bottom side. I found out that the palisade mesophyll is packed tightly together, witch makes the darker shade of green and on the underside of the leaf, the spongy mesophyll is loosely packed so it can allow room for the things that make photosynthesis possible, and this creates the lighter shade of green.