422 THE POPULAR SCIENCE MONTHLY 



qualitative character and most of the quantitative measurements have 

 been meaningless, either by reason of the use of fragments of plants or 

 because the calibrations were of a plausible rather than of an analytical 

 character. As a matter of fact, this loss of water from the plant, like 

 many organic activities, is complicated by indirect and accessory func- 

 tions in such manner that the main processes are difficult to evaluate. 

 It is desirable in all such cases to construct a physical analogue which 

 will reproduce the essential feature of the process to be measured. 

 Notable success in this case has been attained by Professor Livingston 

 who has devised and perfected an evaporimeter consisting of a porous 

 clay cylinder closed at one end, which is kept filled with water (see 

 Fig. 3). The liquid saturates the walls of this vessel and evaporates 

 after the manner in which it would in the plant, excepting for the modi- 

 fications induced by light and by the incidental structural features of 

 the plant. The exposure of the instrument during any given set of con- 

 ditions for any period gives data from which the actual evaporation 

 may be calculated. Comparison with measured areas of leaf-surface 

 shows that departures from the normal evaporation are made by the 

 plant, and the departure may be expressed as the relative transpiration. 

 This relative transpiration is never more than seven tenths of the evap- 

 oration from the instrument, and is generally much less. The 

 evaporimeter has given us a standard and means of measurement by 

 which all of the phases of water-loss with reference to diverse environ- 

 ment, in widely separated localities, and in the different stages of 

 development of the individual may he measured with exactness. 



As was fully expected, the exact calibrations have yielded data upon 

 which new conceptions have been erected and new generalizations formu- 

 lated. Among these may be mentioned that of " incipient desiccation." 

 When the water-loss from an evaporimeter and from a plant is followed 

 throughout the course of a June day at the Desert Laboratory, it is 

 found under certain conditions that the midday maximum of tempera- 

 ture is accompanied by a maximum loss of water from the instrument. 

 When this was compared with the loss from a plant it was seen that in 

 certain cases the increased loss of water from the latter toward the 

 middle of the day was checked. 



All other means of interpretation of this lessened water-loss, includ- 

 ing a consideration of the partial closure of the stomata, as determined 

 by Lloyd, failing to explain the fluctuation in the middle of the day, 

 recourse was had to determining the amount of water actually present 

 in the leaves at such times. This revealed a deficiency. Briefly put, 

 water was being lost from the membranes of the plants faster than it 

 was being supplied to them with the result that vaporization slackened. 

 This condition was designated as incipient desiccation and, as it is 

 not accompanied by any external indications, its discovery was taken to 

 be of great importance, both scientifically and economically, since the 



