Fig. 9.1-3. Transverse section of a stem of cactus (Opuntia verschaffeltii, no common name). The cacti have two different types of mucilage cells: one in which the mucilage is trapped within the confines of the cell wall (as in the previous figure) and one in which the mucilage passes through the wall and accumulates in intercellular spaces, as shown here. This seems like a dangerous type of secretion because if too many intercellular spaces become filled with liquid, diffusion of gaseous carbon dioxide and oxygen will be prevented. Virtually nothing is known about the type of mucilage shown here -- presumably it is secreted by one or all of the cells surrounding the mucilage-filled space. Many species of cacti have both types of mucilage cell, many other species lack mucilage completely.