184 PLANT PHYSIOLOGY 



ent problem in a general way, it does not, nevertheless, permit 

 the exact determination of the effects of each influence singly. 

 This can be effected only by use of a method by w^ieh^a-sjagle 

 Jnfluence can be varied at a time, leaving all the others constant. 

 For precision studies, using potted plants, this would necessi- 

 tate the use of a meteorostat, somewhat upon the plan earlier 

 described (page 37). But using the less accurate material 

 of cut shoots, it can be effected by use of potometers, in conjunc- 

 tion with a supported bell jar in which the influences can be 

 varied one at a time. -i£ 



■ Potometers. The principle of these is this: the plant, either a shoot 

 cut under water, a slip rooted in water, or a water-culture specimen, 

 is sealed air-tight, by use of a bored-and-split rubber stopper and wax, into 

 a small chamber of water, which is in communication with a small-bore, 

 almost capillary, record tube; along this tube, which is graduated or calibrated, 

 an air-bubble is driven by atmospheric pressure as the plant absorbs water, 

 and the movement of this bubble may be timed. The simplicity of the 

 mechanical problem, in conjunction with the visible effectiveness of the result, 

 has proven a great temptation to ingenuity, and divers and protean forms 

 have been described, — by Kohl (figured in Burgerstein, 16), by Sachs 

 (in his "Lectures," 252), by Pfeffer ("Physiology," 1, 242), by Det- 

 mer, 216 (and "Kleines Praktikum," 112), by Hall (Annals of Eotany, 

 15, 1890, 558), by F. Darwin (Darwin and Acton, 80, 83, 99), by Green, 

 89, by MacDougal (Botanical Gazette, 24, 1897, no, and "Physiology," 

 207, 210), by Farmer ("New Phytologist," 2, 1903, 53), by Reed (Jour- 

 nal of Applied Microscopy, 5, 1902, 2047), by Pethybridge (Scientific 

 Proceedings of the Royal Dublin Society, 10, 1904, 149), by Osterhotjt, 206 

 (a make-shift form), by myself (first edition of this book, 80), and by sev- 

 eral others. -The requisites of a good potometer are these: (a) the smallest 

 practicable capacity in order that the water rray more quickly take the tem- 

 perature of a new situation, and hence not vitiate the readings by volume 

 changes; (b) a horizontal record tube, which prevents buoyant rise of the 

 air-bubble, if that is used, or a varying weight of water if an air column is 

 used; (c) an outer reservoir of water which can be used either to supply a 

 reserve to the plant, or to drive back the air-bubble to the starting-point; 

 (d) a calibrated as well as graduated record tube, so that the transpira- 

 tion may be determined in absolute units as well as relatively. All of these 

 merits I have incorporated in the instrument which is among my normal 

 apparatus (page 46) and which is constructed as follows: 



It has four parts. First (Fig. 48) is the shoot chamber, made small that 

 its water may more quickly take a new temperature and hence not affect 

 the record while changing volume. Second is the small-bore record tube, 

 calibrated to cubic centimeters; it is bent down at its distal end, which 



