Long-Distance Transport of Assimilates

Abstract

The evolution of cell walls allowed plants to solve the problem of osmoregulation in freshwater environments; however, cell walls restrict motility and place constraints on the evolution of long-distance transport systems. Tissues are too rigid for a heart-pump mechanism; instead, higher plants have two systems for long-distance transport. The dead elements of the xylem allow transport of water and solutes between sites of different water potentials. That transport system is dealt with in Chapter 3 on plant water relations. The other transport system, the phloem, allows the mass flow of carbohydrates and other solutes from a source region, where the hydrostatic pressure in the phloem is relatively high, to a sink region with lower pressure.