| Abstract: | Hygen's (1951) transpiration equation is re-examined and itsarbitrary constants provided with physical meanings; the implicationsof his postulate (that a leaf can be treated as if the vapourpressure at the evaporating surface fell in direct proportionto the water-content) are thereby investigated, and the physicalnature of his parameters determined. Particular attention ispaid to his successful use of the ‘standard product’as an index of xerophytism. Experiments show that the postulate is invalid for Pelargonium,since the whole course of a 12-hour Hygen curve lies withinthe constant-rate phase, during which transpiration is independentof water-content. This does not invalidate the concept of the‘standard product’, whose calculation in such casesis greatly simplified. A similar situation is likely to obtainin other leaves, but Hygen's analysis, representing a linearfalling-rate phase of drying, may be needed for more hygrophyticleaves. Improvements in the methods of calculating his parametersare suggested which would be applicable in such cases. |
