GENERAL. 



the expanded end of a calibrated tube r, and by means of an india-rubber pipe 

 air is sucked out of r until the mercury is at the required height. A measured 

 quantity of pure carbon dioxide is then introduced. After exposure to light the 

 leaf is withdrawn by means of the platinum thread, and by analyzing the residual 

 gas it can be found how much carbon dioxide has been decomposed and how 

 much oxygen produced. By a control experiment carried on in darkness the 

 amount of carbon dioxide produced by respiration can be determined. 



Various other methods may be employed, but no account can be given here of 

 these nor of the methods of gas analysis. Large volumes of gas may be analyzed 

 rapidly and with sufficient accuracy by Hempel's method, while for small samples 

 of gas Bonnier and Mangin's apparatus may be used ; the latter in its newest form 

 is extremely serviceable. Kreusler has described an apparatus by means of which 

 a small but almost constant quantity of carbon dioxide may be maintained in 

 an enclosed volume of air surrounding an assimilating plant '. 



Engelmann introduced an admirable and irreplaceable physiological method 

 by employing the movement of certain aerobic 

 bacteria as a test for the presence of oxygen 2 . 

 Single cells, algal filaments, sections of leaves, &c. 

 are placed in a fluid containing aerobic bacteria, and 

 after covering the preparation with a coverslip, the 

 latter is ringed with vaselin-paraffin or paraffin-wax. 

 In darkness the enclosed oxygen is soon exhausted 

 and the bacteria come to rest ; as soon as light is 

 admitted the bacteria in the neighbourhood of the 

 assimilating cells begin to move, and are chemotacti- 

 cally attracted in the same manner as they would be 

 by a bubble of air or oxygen (Fig. 43). By this 

 excessively delicate reaction the billionth part of a 

 milligramme of oxygen can be detected, and hence 

 also the mere trace of free oxygen which escapes 



from the objects examined, so that the assimilatory activity of single cells, or parts 

 of cells, may be tested. This physiological method can only be used as a test for 

 the evolution or non-evolution of oxygen, though it may be applied in a variety of 

 ways 5 (cf. Sects. 58, 60). The employment of appropriate bacteria is obviously 

 essential, and these are to be found among the forms usually grouped under the 

 general term ' Bacterium termo! Bacillus liqitefaciens v. vnlgaris, Beyerinck, is 

 a suitable form, and may be isolated from the integuments of peas, and cultivated 

 on Agar-agar. Material from young cultures may be added either to water, sugar 



FlG. 43. In the centre is an assimi- 

 lating alga. 



1 Hempel, Gasanalytische Methoden, 2. Aufl., 1890; Bonnier et Mangin, Rev. generale. 1891, 

 T. Ill, p. 97. Cf. also Stich, Flora, 1891, p. 7. A method of obtaining small samples of the air 

 from a closed receiver is given by Richards, Annals of Botany, 1896, Vol. X, p. 534; Kreusler, 

 Landw. Jahrb., 1885, Bd. xiv, p. 913. 



2 Engelmann, Bot. Zeitung, 1881, p. 442; 1883, p. 4; iSS6, p. 49; 1887, p. 102; Die Ent- 

 stehungsweise d. Sauerstoffausscheidung, 1894 Sep.-abdr. a. d. Verh. d. Amsterd. Akad.). 



3 On the macroscopically visible results, cf. Beyerinck, Bot. Zeitung, 1890, p. 743. 



