i82 PLANT PHYSIOLOGY 



and release moisfcre from the air, and therety alter length, or twist; pieces 

 of these, suitably mounted and provided with a pointer moving over a grad- 

 uated scale, form hygrometers. All of them, however, are variable and unsta- 

 ble, needing frequent standardization. One of the most useful is the Mitt- 

 hof form, sold by supply companies for about $2.00. One of the best of 

 the hair forms is called the polymeter. Small forms of these are often called 

 hygroscopes, which will later be described. Second, advantage is taken of 

 the fact that evaporation is a cooling process, and, further, that its rapidity 

 is inversely proportional to the moisture in the air; hence by comparing 

 the temperature of two thermometers, one with its bulb dry and the other 

 with its bulb exposed to evaporation from a thin water-soaked muslin cover- 

 ing, it is possible, by aid of tables, to calculate the exact humidity of the air. 

 Thus is made the standard instrument, the wet- and dry-bulb thermometer, 

 or psychrometer, the standard form of which, in this country, is the sling psy- 

 chrometer of the United States Weather Bureau, obtainable from supply com- 

 panies and costing about $12.00. A modification of the swinging mechanism, 

 the cog psychrometer, has been introduced by Clements, as described in his 

 "Plant Physiology and Ecology," 28. The registering hygrometer, or hygro- 

 graph, as constructed by Richard Freres, has outside the case a band of 

 hairs which lengthen or shorten as they absorb or release moisture from the 

 air. This motion is communicated to an arm which records precisely as for 

 the thermograph, the record paper being ruled for percentages of saturation. 

 It gives a record of relative humidity, which is that of most importance to the 

 physiologist. For some physiological purposes, notably transpiration, it is 

 best to invert the curve, thus transforming it into a curve of dryness, the advan- 

 tage of which is this, that it makes the condition favoring transpiration show 

 in a rising curve as in the case of temperature and light. Richard Freres 

 also makes a registering psychrometer. The Draper hygrograph resembles 

 outwardly the Draper thermograph. A new instrument, which measures 

 the evaporation from a standard porous surface, is the vaporimeter invented 

 by Livingston (1906, see Literature telow); its application to physiological 

 studies is illustrated ry Transeaxt in the Eotanical Gazette, 45, 1908, 217. 



Barometers and Barographs. These are of minor importance to the physi- 

 ologist working in a laboratory, since plants are only inappreciably affected 

 by the comparatively slight atmospheric fluctuations in one locality. Still, 

 for some purposes they are needed, especially for making corrections in the 

 accurate use of pressure-gauges. The most accurate instrument is the mer- 

 curial barometer, too well known to need comment. It is obtainable self- 

 registering, but at very large expense, and the commonly used autographic 

 form is the barograph. The Richard form (Fig. 47) has inside the case 

 eight aneroid barometer boxes, viz., elastic-iron vacuum boxes; these swell 

 and shrink with variations in pressure, and communicate their motion to an 

 arm which records precisely as for the thermograph, the record paper being 

 ruled to correspond with millimeters of mercury of the mercurial barometer. 



The Richard instruments are made by Richard Freres of Paris and 

 cost from about $35.00 to $45.00 each, without duty. They are made in 

 substantially the same form, but at higher prices, by JtrLiEN P. Friez of 



