Transpiration from Wilting Leaves

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Abstract
Hygen's (1951) transpiration equation is re-examined and its arbitrary constants provided with physical meanings; the implications of his postulate (that a leaf can be treated as if the vapour pressure at the evaporating surface fell in direct proportion to the water-content) are thereby investigated, and the physical nature of his parameters determined. Particular attention is paid 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 within the constant-rate phase, during which transpiration is independent of water-content. This does not invalidate the concept of the ‘standard product’, whose calculation in such cases is greatly simplified. A similar situation is likely to obtain in other leaves, but Hygen's analysis, representing a linear falling-rate phase of drying, may be needed for more hygrophytic leaves. Improvements in the methods of calculating his parameters are suggested which would be applicable in such cases.

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