When the spores have matured, the cells in the annulus begin to dry out, causing the cells to collapse and pull the sporangium open, releasing the spores. Based on the orientation of the palisade and ...When the spores have matured, the cells in the annulus begin to dry out, causing the cells to collapse and pull the sporangium open, releasing the spores. Based on the orientation of the palisade and spongy mesophylls in the leaf cross section above, is this sorus on the upper or lower surface of the leaf?
