Fig.
10.4-1. Transverse section of milkweed petal (Asclepias
tuberosa). Compare the upper and lower epidermises of this petal – the
epidermis cells on the lower side have a rather ordinary shape, but those
on the upper side each bulge outward as a large papilla. You can see
that the papilla is formed as the outer wall and the protoplast bulge outward;
this same surface texture could also be created by having a round protoplast
with a thickened wall or an ordinary protoplast and wall with a thickened
cuticle. Papillose epidermis cells are quite common on petals, giving them a
velvety appearance.