Fig. 11.3-9. Transverse section of a cactus stem (Borzicactus humboldtii; no common name). Many, but not all, cacti have what is known as collapsible cortex: part of the cortex has walls that can flex and wrinkle, allowing the cells to collapse to a very small size without damaging themselves or their walls. The arrows in the lower micrograph point out just a few areas where the walls are extremely plicate. Although the cells in the lower micrograph look terrible, as if fixed and processed improperly perhaps, or maybe taken from a dead plant, this is the way the cells of the collapsible cortex look when they are collapsed. If the plant had been given water, or if the sample had been collected in the rainy season, the cells of the lower micrograph would have been full of water, swollen again and looking much like the cells of the upper micrograph; collapsible cells can go through this cycle many times without being harmed. In many other parts of the cortex, the cells are not like this, their walls will not fold easily (upper micrograph). Consequently, the cortex has several regions: a region of collapsible cells that give up their water very easily and a region of more resistant cells. As the stem begins to lose water in a drought, it is the collapsible cells that give up water first, shrinking as they do so, and remaining healthy. The resistant cells will absorb that water and remain fully hydrated: even though the plant is losing water, the resistant cells are not losing any. You can guess which cortex cells are resistant: the photosynthetic outer cortex chlorenchyma cells and the cortex cells immediately adjacent to the phloem (the cells in the upper micrograph are only a few micrometers away from the phloem).
This phenomenon is newly discovered and is not in any current anatomy book; for more information, see:
Mauseth, J. D. 1995. Collapsible water-storage cells in cacti. Bulletin of the Torrey Botanical Club 122: 145-151.