Instant Notes: Plant Biology

important. The xylem and endodermis (a water-impermeable cell layer with

suberinized cell walls, through which water movement must be symplastic;

Topic C2) develops some distance behind the root tip. Water flows either

through the cell wall spaces (the apoplast) or through the cell contents (the

symplast). Water may be taken up from, or lost to, the apoplast by any cell in

the pathway. Movement of a water molecule across the root involving both

pathways is termed transcellular transport.

I1 – Plants and water 119

Water flow

Water-impermeable,

lignified cell wall

Perforation plates

Gas

bubble

Bordered pit

Border

Pit

membrane

Torus

Pit

cavity

Fig. 3. The action of bordered pits and perforation plates to contain a gas bubble (embolism)

formed in either a vessel element or tracheid. Note that the central torus acts as a valve to seal

the pit in the region of the bubble.