Question

1. What is the driving force of water movement in plants and describe how water moves through vascular plants.

Answer 1

All of the water used by terrestrial plants is absorbed from the soil by roots. The root system consists of complex network of individual roots that vary in age along their length. Roots grow from their tips and initially produce thin and non-woody fine roots. Fine roots are the most permeable portion of a root system, and have the greatest ability to absorb water. Fine roots can also be covered in root hairs that significantly increase the surface area for absorption and improve contact between roots and the soil. Upon absorption by the root, water first crosses the epidermis and then makes its way toward the center of the root crossing the cortex and endodermis before arriving at the xylem. Along the way, water travels in cell walls (apoplastic pathway) and/or through the inside of cells (cell to cell pathway, C-C). Some cell layers present a barrier to the transport of water as opposed to xylem where transport occurs in open tube. There are two types of tubes or conducting elements found in the xylem- the tracheids and the vessels. Water movement is driven by pressure (Cohesion-Tension mechanism) and chemical gradients.

After traveling from the roots to stems through the xylem, water enters leaves via the petiole. Petiole xylem leads into the mid-rib which branches into progressively smaller veins that contain tracheids and are embedded in the leaf mesophyll.