Both pictures show primary cell walls. Although each cell appears encased within a box, in fact primary cell walls are perforated permitting plasmodesmata to connect adjacent cells. The cells of sclerenchyma collenchyma xylem have secondary deposits of lignified cellulose which provide mechanical strength to the tissue. Vacuoles Vacuoles are bounded by a single membrane. Young plant cells often contain many small vacuoles, but as the cells mature, these unite to form a large central vacuole .
Plant cells avoid bursting in hypotonic surroundings by their strong cell walls.
These allow the build-up of turgor within the cell. Loss of turgor causes wilting. This is because sea water is hypertonic to the cytoplasm. As water diffuses from the cytoplasm into the sea water, the cells shrink - drawing their plasma membrane away from the cell wall. The photomicrograph shows plasmolyzed cells in the freshwater plant Elodea which has been placed in sea water.