Big Idea 2, #30, 33. Phloem, Stomata. Stomata are microscopic holes in the leaves and stems of plants that allow gases to pass through (essential for photosynthesis). These leaves are covered in stomata so they can take in carbon dioxide and release oxygen. A phloem is the vascular tissue in plants that takes sugars downward from the leaves. In this tree, the phloem is on the innermost layer of bark.