BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.3. Transpiration and Water Movement Through Leaves

13.3.4. Water Loss via Stomata

All three pathways of Water transport culminate in its evaporation into the internal air spaces of the leaf, namely the intercellular spaces. From there, water vapor diffuses through the Stomata along the path of least resistance: from a higher water potential inside the leaf to a lower one outside. In dicotyledons, stomata are located either exclusively on the lower leaf surface or are predominantly found there. The regulation of stomatal opening and closing is discussed in Section 13.3.9.

A leaf is directly surrounded by a boundary layer of stationary air, the thickness of which is determined by leaf size, structural features such as pubescence (hairiness), and wind speed. Water vapor first diffuses through this layer and is only subsequently carried away by air currents (via bulk flow). The thinner the boundary layer, the higher The rate of Transpiration. A diffusion gradient exists here, decreasing with distance from the mesophyll Cells. Theoretically, each stoma maintains its own diffusion gradient, forming a 'diffusion dome' (Fig. 13.9). In reality, however, when the air is still, the diffusion 'domes' of adjacent stomata overlap, creating a unified diffusion layer around the leaf.

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Fig. 13.9. Diffusion of Water molecules from a stoma.