Fig. 10.2-15. Transverse section of onion leaf (Allium cepa). This leaf transverse section is different from most in this chapter: there is an epidermis on the lower side of the micrograph (vertical arrow points to a stoma), but there is no upper epidermis (diagonal arrow points to cell debris). This is from the green part of an onion leaf, not the white part. The green part of the leaf is hollow because the leaf tissues have torn apart, and that is what we are seeing on the top part of this micrograph. You can tell that the upper layer here is not an epidermis because the cells have irregular sizes and shapes. Although this layer lacks stomata, that is not an absolute criterion for saying that this is not epidermis: many true epidermises lack stomata (many petals and stamens, for example).

            The white part of on onion the part that we slice up to put on hamburgers or to make into onion rings that part is also a hollow leaf, but it is hollow because it is tubular, not because it is torn apart. For details, see Ontogeny of unifacial leaves (page 265) and fig. 12.41 (page 264) in Plant Anatomy (Mauseth).