I was just wondering that, what are the differences between the spongy and palisade mesophyll cells?
Well I'm assuming that you already have a fair idea of the structure of a leaf (of course, why else would you ask this question), so I'll just come straight to the point.
The major difference between the two is that the palisade layer lies just above the spongy layer, and vice versa. That's understood.
So firstly, the palisade layer consists of closely packed cylindrical/rectangular cells, while the spongy layer has loosely packed cells with many air pockets.
Secondly, these cells mentioned above are cylindrical/rectangular in the case of palisade layer, and cells in the spongy layer are rounder in shape.
Thirdly, cells of the palisade layer contain more chloroplasts than the cells of the spongy mesophyll layer. A probable reason is that since the spongy layer is comparatively much further into the leaf, it doesn't get as much sunlight as the palisade, which is a lot closer to the surface.
Lastly, palisade layer helps in photosynthesis, (in fact palisade cells are the site of photosynthesis) while the air pockets in the spongy layer allow exchange of gases like CO2 required for photosynthesis. So there's a difference in function.
There are also other differences, since the spongy layer contains other components like vascular bundles, but I believe these are the major ones.