190 PLANT PHYSIOLOGY 



Methods oe Testing Occurrence of Transpiration. There are 

 several of these, some of them crudely quantitative. Most important of 

 all is the cobalt-chloride test whose value was first shown by Stahl (Botanische 

 Zeitung, 52, 1894, 118). Cobalt chloride is an inexpensive salt of a red 

 color, very soluble in water; if discs of filter-paper are dipped into an 

 aqueous solution of from 1 to 5% strength (the weaker giving the quicker, 

 and the stronger the more conspicuous, test), and then are dried over a flame, 

 they turn bright blue, returning gradually to redness on access of any mois- 

 ture. The discs are most convenient for use when kept, impregnated with 

 the salt and dried, in stoppered bottles, being given a final drying over the 

 flame just before use; they keep good indefinitely. Of course the student 

 will make himself familiar with the action of the material and method tefore 

 applying it to the present subject. The discs may be applied to the leaf 

 by some make-shift arrangement of pieces of mica held in place by a clairp, 

 but can be applied much more conveniently and efficiently by some such 

 leaf clasp as is described below. The method may be made crudely quan- 

 titative by noting the comparative times requisite for the change of color. 

 Second in importance of these methods is the use of small hygroscopes, cr 

 hygrometers, contained in chambers which may be applied to the two sides 

 of the leaf. A hygroscope is a small instrument using the hygroscopic swell- 

 ing of horn, gelatin, etc., to move a pointer; an excellent form, of horn, is 

 F. Darwin's, described in the Philosophical Transactions, 190, 1898, 

 531 (figured in Burgerstein, 33); another is MacDotjgal's, using a cellu- 

 loid and gelatin (photographers') film, described in Torreya, 1, 1901, 16, 

 and his "Physiology," 200. Most useful of all, however, is F. Darwin's 

 awn hygrometer, utilizing the hygroscopic twisting of Erodium or Stipa 

 awns to carry a revolving pointer, as described in Darwin and Acton, 103, 

 while the instrument is supplied among the Arthur apparatus (pa»e 54). 

 A very fair substitute can be adapted from Erodium awns attached hy seal- 

 ing-wax to corks placed inside of cut-off vials. Another form, invented ! y 

 J. Aitkin, composed of a petal of an Everlasting flower with a hair, in a 

 metal case, is mentioned in Science, 27, 1908, 475. As a rule it is not 

 necessary to seal the chamber to the leaves, but this can be done either by 

 vaseline-wax or by strips of thin rubber sheeting bound around, and pro- 

 jecting beyond, the rim of the chamber. Third of these methods is the use 

 pf weighed water-absorbing chemicals in the chambers on the two sides 

 of the leaf, obviously a quantitative method, introduced by Garreau and 

 figured by Detmer, 215. Fourth is another quantitative method, depend- 

 ing upon loss of weight with stomata variously closed in severed leaves, 

 as described by Darwin and Acton, 102. Fifth is the method recently 

 described by F. Darwin in the Botanical Gazette, 37, 1904, 81, in which 

 the cooling effect of transpiration is utilized, by employing two small plati- 

 num resistance thermometers on opposite sides of the leaf, recording upon 

 a drum. Sixth, there is a simple method, of some value for demonstration, 

 in which watch-crystals are applied to the two surfaces of the leaf, being 

 sealed on if the leaf is rough, the collection of moisture on the glass giving 

 some idea as to the transpiration. 



