Biological Units Barriers to Water Movement

Water can enter a BU due to a concentration difference (diffusion) or because of pumping (convection). Because the water concentration of the atmosphere is much less than the water concentration internal to a BU, there must be an effective water barrier between the organismal BU and its atmospheric environment. Plants have their cuticle and animals have their skin. Water can enter through 

Surface lipids in the form of waxes have much to do with limiting drying rates to values below lethality. Plants have a water barrier composed of cutin, embedded waxes and pectin. Insects and other arthropods have embedded wax in their outer chitin layer. Amphibians have skin that is very permeable to water movement in order to absorb water from their environments. When spending time in the sun or in the ground, however, they either spread exuded lipids on their skins, or else form an impermeable skin layer. Reptiles use keratin as their principal water barrier. Mammals have a thin lipid film that covers their outer skin layer, the stratum corneum (Hadley, 1980). 

Example 6.1.1 Monitoring Almond Tree Trunk Diameter 

FIGURE 6.1.1 Linear variable differential transformers continuously monitored the diameters of tree trunks. (From Goldhamer, D.A. et al., Calif. Agric., 57, 138, October-December 2003. With permission.) 

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