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.