BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.3. Transpiration and Water Movement Through Leaves

Water typically leaves the plant in the form of vapor. The transition from liquid to gas requires additional energy, which is provided by the sun. It is this energy that drives the overall flow of water through the plant. Three main structures are involved in Transpiration.

1. Stomata — pores through which water evaporating from The Cell surface diffuses. Such pores are present in the epidermis of leaves and green stems (approximately 90% of water is lost through the stomata).

2. Cuticle — a wax-like layer covering the epidermis of leaves and stems; water evaporating from the outer walls of epidermal Cells passes through it (about 10% of water loss, depending on cuticle thickness).

3. Lenticels — small openings (areas of loosely arranged tissue) in the periderm of trees and shrubs that serve for gas exchange (the proportion of water loss is minimal; however, in deciduous trees after leaf fall, the bulk of water is lost through lenticels).

The amount of water lost by a plant through transpiration can be very large. Herbaceous plants, such as cotton or sunflower, lose 1—2 liters of water this way per day, while a mature oak tree loses over 600 liters.

Water enters the leaves via xylem vessels. The Structure of these vessels is described in section 6.2.1. The xylem forms part of the vascular bundles that permeate the entire leaf, forming a network of fine Veins within it. These bundles terminate in one or a few lightly lignified xylem vessels, through which water easily passes into the surrounding mesophyll cells. Fig. 13.8 illustrates three pathways for its further movement: apoplastic (through cell walls), symplastic (through Cytoplasm and plasmodesmata), and vacuolar (through vacuoles).

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Fig. 13.8. A. Diagram of water movement pathways in a leaf. There are three possible pathways: the Symplastic and vacuolar are shown on the left, and the apoplastic on the right. Cells 1, 2, and 3 are discussed in the text. The relative thickness of the cell walls in the diagram is intentionally exaggerated. B. Schematic representation of a group of plant cells summarizing all possible pathways of water (solution) movement. Several pathways can operate simultaneously. Such pathways can function in both the leaf and the ROOT cortex. Vacuolar transfer of ions necessarily involves Active Transport. The apoplastic pathway plays the most crucial role, while the vacuolar plays the least.

13.13. Why does transpiration occur primarily through the stomata rather than the cuticle and lenticels?

13.3.1. Apoplastic Transport

The apoplast is The system of interconnected cell walls that forms a continuous network throughout the plant. Up to 50% of this cellulosic framework acts as a "free space" that can be occupied by water. As water evaporates into the intercellular spaces from The surface of mesophyll cells, a tension develops within the continuous apoplastic water layer, and the entire Column is pulled toward the site of depletion by the bulk flow mechanism, driven by the cohesion ("stickiness") of water molecules (see section 13.4). Water enters the apoplast from the xylem.