This cells, with small and few chloroplasts,
have often undulating flexible, scarcely lignified, and elastic
wall, that permit the cell volume to change significantly
as water is absorbed or released.
The selective advantage of storing water is
not just that it keeps water-storage cells alive but rather that
the water can be made available to photosynthetic cells,
to the apical and axillary buds, any flower buds or developing
fruits and so on. Consequently, water-storage cells should have
thin, flexible walls that can contract or shrink readily such
that the cells volume diminishes as water is transferred out.
On the other hand, the cells that need the water should be more
resistant to shrinkage: if all cell walls were equally flexible,
all parts of the plant would suffer water stress equally, but
that is not adaptive. Instead, water-storage tissues should give
up water so easily that the more active cells do not experience
water stress unless drought is extremely prolonged. In all
cacti, cell walls of the inner cortex are especially thin and
flexible, but in many cacti there is an additional modification:
the walls are folded or undulate, even when young and recently
produced by the shoot apical meristem. Because the walls already
have folds in them, the cells are presumably set to shrink very
easily. These cells have been called collapsible cortex cells.
In contrast, wall of palisade cortex cells the photosynthetic
cells are thicker and lack folds, so these cannot shrink as
easily. Consequently, as the volume of water within a cactus
shoot diminishes, cells of the inner cortex give up water
preferentially, transferring it to other cells, minimizing water
stress and thus allowing photosynthesis to continue.