I24 PLANT PHYSIOLOGY 



ical Gazette, 27, 1899, 263) and used with the open arm in a tall bottle of 

 mercury, is perhaps the most practicable for general work. In H the seeds 

 must be supported upon a wad of loose glass wool or wire netting. In F, G, H 

 the seeds must be inserted through the tube, which must then be dried by 

 a mop if potash is to be used, or this will diffuse up the lines of moisture and 

 kill the seeds. Another form in which chamber and tube are detachable 

 by a ground-glass joint is described by RICHARDS in Torreya, i, 1901, 28. 



To these tubes the carbon -dioxide test may be applied either by the 

 reagent tubes (page 105), or by introducing, through a curved pipette, a few 

 drops of caustic-potash solution, or by inserting a small lump of solid caustic 

 potash. For make-shift purposes the mercury may be replaced by water, 

 whose absorption of carbon dioxide is not too great for its use in roughly 

 qualitative experimenting. 



DEMONSTRATION METHODS. Since the test for carbon dioxide is always 

 the first to be applied, it is convenient and effective to apply this absorbent 

 at the start; for thus the gradual progress of the gas release may be watched, 

 while there is also this additional advantage, that the seeds grow somewhat 

 better if the carbon dioxide is removed as formed. The caustic potash may 

 be introduced over the mercury, or in solution may replace it, its absorption 

 from the atmosphere not mattering, especially if the solution is in consider- 

 able bulk and small surface. This method, however, involves a logical 

 error, which is commonly overlooked in this kind of experimenting, viz., 

 the rise of the carbon -dioxide absorbent does not of itself prove that any 

 carbon dioxide has been formed, because the same result would follow if 

 gas were absorbed by the seeds without giving off any, i.e., if the seeds 

 absorbed the oxygen in the tube without giving off any carbon dioxide. 

 Hence it is necessary to add another tube similar in all ways except that 

 it contains mercury, the approximately constant level of which shows that 

 gas is released as well as absorbed, the two together proving that this 

 released gas can be only carbon dioxide. Another method, a modification of 

 the preceding, consists in placing a vial of caustic potash with the seeds in 

 the chamber (which may be a flask or bottle), and supplying mercury to the 

 open tube (compare DARWIN and ACTON, 2, and MACDOUGAL, 253). This 

 saves the exposure of the caustic potash to the action of the air outside, but 

 has the demerit that the weight of the mercury prevents a fully effective 

 demonstration of the absorption. 



For elementary and lecture purposes the demonstration is very effective 

 with either of the arrangements of Fig. 31. A is a cylindrical bottle holding 

 a quantity of clear lime-water, over which is suspended a diaphragm of wire 

 netting supporting wet filter-paper on which are germinating seeds. When 

 tightly stoppered and placed under good conditions for growth, the gradual 

 whitening of the lime-water is striking, especially if it is well shaken at times, 

 as this arrangement permits, and if it be tried beside a control lacking the 

 seeds. Or the lime-water may be in a vial surrounded by the seeds. B, espe- 

 cially good for lecture demonstration, is a wide bottle, tightly closed by stop- 

 pers and clamp, on whose bottom is wet filter-paper containing germinating 

 seeds. If at the end of two or three days water be poured down the thistle- 



